Antimicrobial resistance and gene transfer in Staphylococcus aureus

Journal of Applied Bacteriology 1991, 70, 279-290 ADONIS 0021884791000431

A REVIEW

Antimicrobial resistance and gene transfer in Staphy/ococcus aureus S.B. Al-Masaudil, M.J. Day' and A.D. Russell' 'School of Pure and Applied Biology and 2Welsh School of Pharmacy, University of Wales College of Cardiff, UK

Accepted 4 October 1990

Paper number 1 3425/06/90

1. 2.

3.

4. 5.

Introduction, 279 History of multiple-antimicrobial-resistant Staphylococcus aureus, 279 Methicillin-resistant Staphylococcus aureus (MRSA), 280 3.1 History of MRSA, 280 3.2 Properties of MRSA, 280 3.3 Mechanism of methicillin resistance, 280 Genetic basis for antimicrobial resistance, 280 Gene transfer systems in staphylococci, 281 5.1 Transduction, 281 5.2 Phage-mediated conjugation, 282 5.3 Transformation, 282

5.4 Conjugation, 282

Staphylococcus aureus, 283 6.1 Antibiotic resistance, 283 6.2 Resistance to metal ions, 286 6.3 Resistance to antiseptics and disinfectants,

6. Mechanism of antimicrobial resistance in

286 7. Origin of Staphylococcus aureus resistance

determinants, 287 8. Conclusions, 287 9. Acknowledgements, 287

10. References, 287

1. INTRODUCTION

Since the discovery of transferable antibiotic resistance in 1959, both the genetic basis and mechanisms of antimicrobial resistance have been studied intensively in Gram- negative bacteria. The development of molecular genetic techniques in the last decade has brought a dramatic increase in the information about resistance in staphylococci. It is only recently that conjugation has been demonstrated in staphylococci but there are now many reports of conjugative plasmids capable of transferring and mobilizing non-transmissible plasmids between Staphylococcus aureus and Staph. epidermidis. Transposable elements have also been found in Staph. aureus and there is increasing evidence for a high degree of conservation in the DNA sequences of resistance determinants found in Staph. aureus and Staph. epidermidis. These findings and others may give some explanation for the gain of antibiotic resistance in staphylococci and support the belief that gene exchange occurred between them. In this review we shall discuss the genetic basis of antimicrobial resistance and the mechanism by which this organism becomes resistant to certain agents.

Correspondence to: Dr A.D. Russell, Welsh School of Pharmacy, University of Wales College of Cardrff, Cardrff CFI 3XF, UK.

2. HISTORY O F MULTIPLE- ANTIMICROBIAL-RESISTANT STAPHYLOCOCCUS AUREUS

Novel resistant Staph. aureus strains have appeared follow- ing the introduction of new antibiotics. At the time of the introduction of penicillin for therapeutic use in the early 1940s, less than 1% of Staph. aureus strains isolated showed resistance. A few years later, the incidence of penicillin resistance in Staph. aureus had increased, such that by 1946 about 60% of hospital strains isolated in the UK were penicillin-resistant (Barber & Rozwadowska-Dowzenko 1948). The introduction of streptomycin, tetracycline, chloramphenicol and erythromycin for the treatment of infections caused by penicillin-resistant staphylococci was similarly followed by the emergence of strains resistant to these antibiotics (Shanson 1981). The decade of the 1950s was characterized by an increased prevalence of both virulent and multiple-antibiotic-resistant strains throughout the world. In one hospital in the US during 1959, more than 40% of Staph. aureus strains isolated from in-patients were resistant to four or more antibiotics (Bulger & Sherris 1968). The successive introduction during the 1960s of methicillin and other isoxazolyl penicillins stable to staphylococcal p-lactamase brought a general decline in the inci-

280 S.B. AL-MASAUDI ET A L .

dence of the multiple-antibiotic-resistant Staph. aweus (Shanson 1981). Unlike all other antibiotics previously introduced, methicillin-resistant strains did not imme- diately pose a serious problem in spite of their extensive usage in clinical practice. However, in the late 1960s and early 1970s the isolation of strains showing resistance to methicillin and to many other non-8-lactam antibiotics was reported with increasing frequency in many countries (Rountree & Beard 1968; O’Toole et al. 1970; Parker & Hewitt 1970). Resistance to the aminoglycoside gentamicin emerged after some 10 years use and was associated with its extensive use as a topical antibiotic (Noble & Naidoo 1978).

Early in the 1980s methicillin-resistant Staph . aureus (MRSA) strains re-emerged and many showed additional resistances to many antimicrobial agents including gentamicin and unrelated agents such as antiseptics. Indeed, nowa- days, some strains of Staph. aureus have shown resistance to as many as 20 antimicrobial agents, including antiseptics and heavy-metal ions (Lyon & Skurray 1987). These new MRSA strains were responsible for outbreaks in many hospitals throughout the world (Schaefler et al. 1984; Marple & Cooke 1988). Although these types of strains are generally susceptible to teicoplanin, rifampicin and vancomycin, they have the ability to survive challenges by almost all the anti- staphylococcal agents available today. Vancomycin remains the drug of choice for the treatment of many life- threatening infections caused by these MRSA strains.

3. METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS (MRSA)

3.1 History of MRSA

Methicillin was first available for the treatment of infection caused by penicillin-resistant staphylococci in 1959. I t proved clinically effective and brought a general decline in the prevalence of the multi-antibiotic-resistant Staph. uureus. Methicillin-resistant strains emerged later such that between 1968 and 1971 the number of resistant strains isolated had reached 16%) in Zurich, 18% in Australia and 46% in Denmark. British hospitals also witnessed an increase in MRSA during the 1960s from 1% in 1965 to 5% in 1969 in the London area (Shanson 1981). At that time MRSA were uncommon in USA hospitals and the problem became serious in the late 1970s (Haley et al. 1982). The first methicillin-resistant gentamicin-resistant outbreak in London was described by Shanson et al. (1976), which was followed by anothcr in the early 1980s; strains of Staph. aureus resistant to both gentamicin and methicillin were subsequently reported throughout the world.

Novel MRSA strains emerged in Australia during the late 1970s and early 1980s. These strains could be dis- tinguished from other MRSA strains by their antibiotic resistance profile, location of resistance and by their phage type (Cookson & Phillips 1988, 1990). A similar strain caused a major outbreak in the London Hospital in early 1982. Recently, this strain has been termed epidemic methicillin-resistant Staph. aureus (EMRSA) (Duckworth et al. 1988).

MRSA are now common in many parts of the world, they cause serious infections as well as colonization and the successful choice of antimicrobial therapy is sometimes difficult. Experience of recent outbreaks suggests that the main risks of serious MRSA infections are to patients in special units, e.g. intensive care, cardiothoracic and surgical neonatal, and in patients undergoing major or prosthetic surgery (Shanson 1986).

3.2 Propertles of MRSA

MRSA strains isolated from different parts of the world were found to have many properties in common. These are summarized in Table 1.

3.3 Mechanlsrn of rnethlclllin resistance

Resistance to, methicillin in clinical strains results from the production of penicillin-binding protein (PBP), designated PBP2’ or PBP2a (Reynolds & Fuller 1986). When methicillin resistance is selected in vitro by serial passage of sensitive strains on increasing concentrations of methicillin there are changes in the binding characteristic of PBP2 and 4. A comparison showed there was no immunological or genetic relationship between the methicillin-determined low affinity PBP2’ and the tn vitro selected mutants (Berger-Bachi et al. 1989).

Mec is the name given to the structural gene for PBP2’. The gene for mec-specific PBP2’ might be of foreign origin (Reynolds 1985) and may be a site-specific transposon, Tn4921 (Skurray et al. 1988). Recently, other workers have cloned different fragment sizes of chromosomal DNA from a clinical strain of MRSA. Some of the fragments direct the synthesis of a new PBP2‘ and mediate an increased level of methicillin resistance into the recipient strain (Matsuhashi et al. 1986; Beck et al. 1986; Inglis el al. 1988).

4. GENETIC BASIS FOR ANTIMICROBIAL RESISTANCE

When a drug is able to inhibit or inactivate its target it is considered to be effective and the cell is regarded as being sensitive. A cell which is not inhibited or inactivated by the

RESISTANCE AND GENE TRANSFER 281

Table 1 Properties common to methicillin-resistant strains of Staphyfococcus aureus

Properties Reference

1. Most MRSA strains are resistant to many antibiotics including erythromycin, tetracycline, streptomycin and sometimes gentamicin and chloramphenicol. Vancomycin is the only drug to which the resistance is rarely found.

2. Almost all MRSA isolates produce 8-lactamase (resistance to methicillin is not due to 8-lactamase production).

3. All MRSA strains show phenotypically heterogeneous resistance. The degree of heterogeneity is influenced by many factors such as temperature, osmolarity and pH.

4. Resistance to methicillin can be rapidly lost by growth at higher temperature.

5. Recently isolated MRSA strains are difficult to type with phages.

6. Most MRSA strains harbour phage(s) and carry a transmissible determinant which could be located either on a plasmid or on the chromosome.

Lyon & Skurray 1987

Lacey 1975

Matthews & Stewart 1984

Brown & Reynolds 1980

Archer & Mayhal 1983

Lyon & Skurray 1987

drug is described as resistant or tolerant. Differences between ‘resistance’ and ‘tolerance’ are not always clear. However, ‘resistance’ should be used primarily where the drug is degraded by a cellular mechanism or where it is unable to penetrate the cell but it is not able to inhibit or reach its target. ‘Tolerance’ should be used when the drug is not able to penetrate through the cell wall but is not inactivated and insensitivity results primarily from an alteration in the target area.

Spontaneous chromosomal mutants resistant to drugs generally arise at a frequency of to lo-’ per bac- terium per generation. Resistance to certain antibiotics, including streptomycin, fusidic acid, rifampicin and novo- biocin can arise by chromosomal mutation, although chromosomal mutants of this sort may be less virulent (Saunders 1984). The types of natural mechanisms responsible for resistance are very different from those found in mutants. Resistance to streptomycin produced by a chromosomal mutation is due to an alteration in the ribosome so that streptomycin could no longer bind to the ribosome (Lacey & Chopra 1972). Plasmid-borne resistance to Streptomycin is achieved by a different mechanism, it encodes an enzyme which modifies streptomycin so that it can no longer bind to the ribosome (Grinsted & Lacey 1973). The acquisition of resistance genes which have little or no effect on fitness of the bacteria would therefore be expected to occur in the natural environments. Indeed it is now generally accepted that plasmids play a major role in the mediation and transfer of antimicrobial resistance among the staphylococcal population (Lacey 1975). The presence of the plasmid is suspected if the resistance is easily lost and not regained. Plasmids may be the vectors of the resistance genes, or the genes may themselves be located on discrete movable DNA elemenis’, called trans-

posons. Antimicrobial resistance genes carried by transposons are genomically mobile and can be transposed from one DNA molecule to another; this can include bacteriophage and plasmid DNA. Since most of the mechanisms for gene flow have been shown to occur in Staph. aureus it seems logical to suppose that they have contributed to the rapid spread of resistance genes throughout the staphylococcal population. I t also provides an explanation for the emergence of multi-resistant Staph. aureus.

5. GENE TRANSFER SYSTEMS IN STAPHYLOCOCCI

In the laboratory, genes conferring resistance can be transferred between staphylococci via four basic mechanisms. These are transduction, phage-mediated conjugation, transformation and conjugation. The exact qualitative signifi- cance of these mechanisms in vivo is unknown.

5.1 Transduction

Phages are widely distributed in staphylococcal populations. Indeed most clinical strains of Staph. aureus harbour at least one prophage, the presence of which may also affect the ability of the strain to engage in gene transfer (Cohen et al. 1977). Until the 1980s, phage-mediated transduction was thought to be the only natural mechanism of gene transfer in staphylococci (Lacey 1975). However, the apparent requirement for the death of the donor cell, and the need to protect the recipient from destruction by normal phage particles, as well as the artificial conditions needed for the transduction process in vitro, raised doubts about whether it could be a significant process in nature (Lacey 1984).

282 S.B. AL-MASAUDI ET A L .

Table 2 Some conjugative plasmids present in Staphylococci spp.

Plasmid Size Resistance phenotype Ability to mobilize Reference

pAM899-1

pSH8 pSH9

pCRGl800

pUW3626

pWG613

pWG14

pG05

pSK41

pWRG637

PJE 1

28 MDa

30 MDa 38 MDa

37 MDa

54.4 kb

41.6 kb

45-5 1 kb

55.2 kb

47.8 kb

34.5 kb

50 kb

Gm

Gm, Km, EB, Tm Gm, Km, Pen, Tm

Gm, Pen

Gm, T m , Km, Nm, EB, QAC, Pen

Gm, Km, Nm, EB, Cet

Km, Nm, Sm, Em, Lin

Gm, QAC, Pen, Tri, EB

Gm, Tm, Km, Nm, EB, QAC

Cryptic plasmid

Gm, EB

Tetracycline-resistance plasmid pAM899-2 and erythromycin-resistance plasmid pAM899-3

Tetracycline-resistance plasmid pSH5 (-3 MDa) and chloramphenicol- resistance plasmid pC22.1 (3 MDa)

ND

ND

Tetracycline-resistance plasmid pWG3 (2.7 Md)

NM

A cryptic plasmid of 30 kb was mobilized

ND

Streptomycin-resistance

ND plasmid pWBG633 (4.2 kb)

Forbes & Schaberg 1983

McDonnell et al. 1983

Goering & Ruff 1983

Goering & Ruff 1983

Archer &Johnstone 1983

Townsend et al. 1985a

Archer et af. 1986

Lyon et al. 1987

Udo et al. 1987

Evans & Dyke 1988

Cet, Cetrimide ; EB, ethidium bromide ; Em, erythromycin ; Gm, gentamicin ; Km, kanamycin ; Lin, lincomycin ; Nm, neomycin ; Pen, penicillin ; Sm, streptomycin; Tm, tobramycin ; Tri, trimethoprim; QAC, quaternary ammonium compounds; ND, not determined; NM, no tested plasmid mobilized. kb, Kilobases; MDa, megadaltons.

5.2 Phage-mediated conjugatlon

Unlike generalized transduction, phage-mediated conjugation needs viable cell-to-cell contact as well as the presence of lysogenic phage in either the donor or recipient cell (Lacey 1980). Gene transfer was achieved without the death of the donor or the risk of the recipient being destroyed by ‘normal’ phage particles. The precise sequence of events in this transfer are not known; but high cell densities and calcium ions are both required for high rates of gene transfer (Witte 1981). Under optimal conditions, transfer can be very efficient, up to 1 in 10 recipients (Lacey 1980) and this high frequency of transfer suggests that transfer of small plasmids and chromosomal genes between staphylococci is likely to occur under natural conditions by this mechanism (Lacey 1984).

5.3 Transformation

The transfer of ‘naked’ DNA either plasmid or chromosomal can be performed in nitro under certain conditions (Lindberg et al. 1972; Sjostron et al. 1975). The require-

ment for a non-physiological calcium concentration (0.1 mol/l) and the short period of competence (when nucleases are absent) will reduce the likelihood of transformation in nature (Lacey 1984).

5.4 Conjugation

Two independent reports by Forbes & Schaberg (1983) and McDonnell et al. (1983) demonstrated conjugation transfer of a gentamicin-resistance plasmid from Staph. epidermidis to Staph. aureus. There have since been many reports of conjugation-like transfer (Townsend et al. 1986). The transfer mechanism has been concluded to be conjugation since viable cell-to-cell contact is an absolute requirement and the system is not affected by DNAase, chelating agents and human serum (Forbes & Schaberg 1983; McDonnell et al. 1983). Until recently most self-transmissible staphylococcal plasmids isolated encoded resistance to gentamicin and other aminoglycosides, ethidium bromide, and quaternary ammonium compounds. Some of these plasmids also encoded resistance to penicillin and trimethoprim.


A new conjugative plasmid with no resistance phenotype has now been described in Staph. aureus (Udo et al. 1987). El-Solh et al. (1986) showed a chromosomal-resistance marker to be conjugally transferred in the absence of a con- jugating plasmid. Most of the conjugative plasmids when tested mobilized smaller plasmids (specifying tetracycline resistance and chloramphenicol resistance) in a non- lysogenic system (Forbes & Schaberg 1983; Townsend et al. 1986). Moreover, a self-transmissible gentamicin- resistance plasmid mediated its transfer over a wide range of pH values and temperatures (Al-Masaudi et al. 1990). These findings show that conjugation systems among staphylococci could also be a significant mechanism in the dissemination of resistance genes among staphylococcal populations. Selected conjugative plasmids in staphylococci are listed in Table 2.

6. MECHANISM OF ANTIMICROBIAL RESISTANCE IN STAPHYLOCOCCUS AUREUS

6.1 Antibiotic resistance

Antibiotics are specific inhibitors of biosynthetic processes in bacteria (Table 3). Various mechanisms of resistance to antibiotics have emerged among Staph. aureus populations. In several cases more than one type of resistance mechanism exists for a single group of antibiotics. Table 4 sum- marizes the mechanisms of resistance existing in Staph. aureus and the genetic location of this resistance.

Resistance to 8-lactam antibiotics was recognized in clinical strains in the early 1940s. Staphylococcal 8- lactamases are inducible enzymes and usually plasmid- borne (Novick & Bouanchaud 1971), although in some strains chromosomal resistance has been described (Sweeney & Cohen 1968). Plasmids encoding 8-lactamase usually also encode resistance to other antimicrobial agents, including inorganic ions such as mercury, arsenate, arsenite, cadmium and zinc ; antiseptics; disinfectants ; and dyes (Gillespie et al. 1986). There are four distinct groups of 8-lactamase plasmids, designated alpha, beta, gamma and delta (Shalita et a[. 1980). Naturally occurring recom- binants between plasmids of alpha and gamma groups have been described (Shalita et al. 1980). Evidence has shown that the B-lactamase regions of some B-lactamase plasmids are transposable elements. Four transposable elements have been described among staphylococcal-B-lactamase plasmids, viz. Tn552, Tn4002, Tn3852 and Tn4201 (Lyon & Skurray 1987). Some recent MRSA strains encode B- lactamase chromosomally (Gillespie et al. 1984); this might result from the integration of a complete 8-lactamase plasmid into the chromosome or by transposition of the

transposon encoding the B-lactamase into the chromosome. Occasionally these plasmids exhibit additional resistances such as erythromycin, fusidic acid, kanamycin and gentamicin (Gillespie & Skurray 1986).

Resistance to vancomycin among Staph. aureus is rare; it has been suggested that this is because of its multiple site of antibacterial action (Cooper & Given 1986). Some strains of Staph. aureus have been shown to exhibit low levels of tolerance to vancomycin. T o date vancomycin remains the drug of choice in life-threatening infection due to MRSA. In some cases vancomycin in combination with drugs such as rifampicin and gentamicin may be recommended.

Three mechanisms of resistance to aminoglycosides have been reported among bacteria (Shannon & Phillips 1982). Firstly, ribosomal resistance due to a mutation in genes coding for ribosomal protein, leads to the drug being unable to bind to ribosomes. Secondly, impermeability, which results in decreased accumulation of aminoglycosides due to impaired transport across the membrane, itself results from a defect in membrane energization. Thirdly, there is the presence of aminoglycoside-modifying enzymes which modify the antibiotics so that they no longer bind to ribosomes and cannot inhibit protein synthesis. There are three classes of these modifying enzymes: phos- phototransferases (APH), acetyltransferases (AAC) and adenylyltransferases (AAD) ; most of these enzymes have been shown to be plasmid encoded and several are transposable (Lyon & Skurray 1987). Streptomycin resistance in most clinical strains of Staph. aureus is due to chromosomal mutation, the result of which leads to alteration in the ribosome such that it no longer binds the antibiotics (Coleman et al. 1985). Plasmid-mediated enzymatic modification of streptomycin has been described in some strains of Staph. aureus (Grinsted & Lacey 1973). Neomycin and kanamycin resistances are always linked : resistance is due to modifying enzymes which alter the antibiotic molecules by phosphorylation, acetylation or nucleotidylation. The result of this is thought to block drug transport (Davies & Smith 1978). Resistance is commonly plasmid-mediated but in some strains resistance to both drugs cannot be associated with an extrachromosomal element. Linkage of neomycin/kanamycin resistance with a plasmid conferring 8-lactamase production has been reported. Gentamicin resistance in Staph. aureus is frequently mediated by AAC (6') and APH (2') activities. The determinants often seem to be plasmid-borne (Brunton 1984), but in some strains it is chromosomally encoded. This is not surprising since transposons which mediate resistance to gentamicin, tobramycin and kanamycin have been described (Lyon et al. 1984). Most of the plasmids coding for gentamicin resistance in Staph. aureus appear to be large (ranging in size from 18-57 kb) and express additional determinants for resistance to ethidium bromide, 8-lactamase production

284 S.B. AL-MASAUDI ET A L

Table 3 Mechanism of action of antibiotics

Antibiotics Target functions Mechanism of action

p-lactam Peptidoglycan biosynthesis Prevents transpeptidation (penicillins and between peptidoglycan strands cephalosporines)

Vancom yein

Aminogl ycosides (e.g. streptomycin, gentamicin)

Tetracyclines

Macrolides (MI S)

Chloramphenicol

Fusidic acid

Rifampicin

Sulphonamide and trimethoprim

Peptidoglycan subunit in transit to growing point of peptidoglycan

Protein synthesis

Protein synthesis

Protein synthesis

Protein synthesis

Protein synthesis

Nucleic acid synthesis (RNA synthesis)

Blocks transfer of subunit to growing point of peptidoglycan by binding to acetyl-Palanyl-Palanine

Bind to the 30s ribosomal subunit causing either inhibition of protein synthesis or the mis-incorporation of amino acids into peptide

Bind to the 30s ribosomal subunit and inhibit binding of aminoacyl-tRNA to the mRNA on its acceptor site

Inhibit the release of tRNA from the P-site by binding to 50s ribosomal subunit

Binds to the 50s ribosomal subunit and inhibits activity of the enzyme peptidyl transferase

Inhibits translocation by forming stable complex with EF-G, GTP and the ribosome

Binds specifically to the 8-subunit of bacterial DNA-dependent RNA polymerase

Nucleic acid synthesis (nucleotide synthesis)

The former inhibits dih ydropteroate synthetase (DHPS) and the latter inhibits dihydrofolate reductase (DHFR). Both enzymes are involved in the biosynthesis of folic acids

and quaternary ammonium compounds (Lyon & Skurray 1987). However, smaller plasmids coding for gentamicin resistance have also been isolated. Restriction endonuclease analysis of a number of gentamicin resistance plasmids have

revealed a high degree of structural relation (Jaffe et al. 1982). Several of these plasmids promote their own transfer, and mobilize non-conjugative plasmids, by a process which appears to be conjugation. However, the most


Table 4 Genetic location and mechanism of resistance to antibiotics in Sruphylococcus uureus

Gene location of resistance

Antibiotics Main resistance mechanism Chromosome Plasmid Transposon Comments

8-lactam Enzymatic hydrolysis of (penicillins and 8-lactam ring cep halosporines)

Methicillin Production of new PBP (PBP2’ or PBP2a) with reduced affinity to /?-lactam antibiotics

Recently Commonly Tn52 isolated found Tn4002 MRSA Tn3852

Tn4201

Exclusively Not found Probably

Always linked with other antimicrobial determinants

Strains resistant to methicillin always show resistance to other &lactam antibiotics

Streptomycin

Gentamicin

Alterations in the structure of the ribosome. Sometimes resistance is due to enzymatic modification of streptomycin

Enzymatic modification of the antibiotic frequently by the enzymes AAC(6) and APH (2’)

Common Infrequently

Often Frequently Tn4001 Tn3851 Tn4201

Tetracycline Emux Infrequent Commonly found

MLS antibiotics Enzymatic methylation of Common Commonly Tn551 23s rRNA found Tn554

Chloramphenicol Chloramphenicol acetyl Not found Exclusively transferase

They often encode resistance to other antimicrobial agents such as penicillin and quaternary ammonium compounds. Several of these plasmids are conjugative

common aminoglycoside-resistance plasmids in Australian strains are non-conjugative and transferred in vitro by a bacteriophage-mediated system to a lysogenic recipient (Townsend et al. 1985a, b).

Tetracycline resistance in Staph. aureus is commonly specified by a very homogeneous group of small plasmids, ranging in size from 4.0 to 4-5 kb (Lyon & Skurray 1987). Two major phenotypic patterns of tetracycline resistance have been described in Staph. aureus (Chopra et al. 1974). The first is inducible, conferring resistance to tetracycline but not to minocycline (MIC < 1 pg/ml). The second type is constitutive, conferring resistance to both. The former type of resistance is usually carried on a small plasmid, whereas the latter is chromosomally encoded.

Resistance to erythromycin is associated with resistance to other macrolides, the lincosamides and to streptogramin type B (MLS resistance). There are two main types of MLS-resistance in Staph. aureus, inducible resistance (erm

A) and Constitutive resistance (erm B), which are commonly mediated by small multicopy plasmids (Poston & Naidoo 1983). There is evidence to show that both can be carried by transposons. However, inducible and constitutive resistance to MLS antibiotics can also be chromosomally encoded. Recent MRSA strains from the UK and Australia exhibit chromosomal resistance to MLS antibiotic (Lyon & Skurray 1987).

Resistance to chloramphenicol in Staph. aureus is due to the production of an inducible chloramphenicol acetyltransferase (CAT) which is encoded by a heterogeneous group of small plasmids (ranging in size from 2.9 to 5.1 kb) (Poston & Naidoo 1983). Because of these differences in plasmid size, incompatibility, restriction maps and relax- ation complexes and the similarity of the enzyme chloramphenicol acetyltransferase, it is suggested that the chloramphenicol resistance locus is a transposable element (Iordanescu et al. 1978).

286 S .B . A L - M A S A U D I E r A L .

Resistance to fusidic acid in clinical strains may be chromosomally- or plasmid-borne. Mutational resistance (chromosomal) is due to decreased affinity of the G factor for the antibiotic. Plasmid-specified resistance to fusidic acid is probably due to decreased permeability because a change has been shown in the ratio of phosphatidylglycerol to 1 ysylphosphatidylglycerol in resistance strains (Bryan 1982). Co-transfer of fusidic acid resistance with other plasmid markers such as /I-lactamase and aminoglycoside resistance has been reported (Lacey & Grinsted 1972).

Resistance to rifampicin involves an alteration to the target site, the /I-subunit of DNA-dependent RNA polymerase, which reduces the affinity of the antibiotic to the enzyme. The resistance is chromosomally encoded and there is no evidence to suggest it is plasmid-mediated.

The mechanism of resistance to sulphonamide has not been fully elucidated in staphylococci. One possible mechanism involves increased production of p-aminobenzoic acid (Landy et al. 1943; Russell & Chopra 1990) and is probably due to a chromosomal mutation. Plasmid- mediated resistance to trimethoprim in Staph. aureus is associated with the synthesis of a DHFR, with markedly decreased affinity for trimethoprim (Lyon & Skurray 1987).

6.2 Resistance to metal Ions

Heavy metal ions such as mercury, cadmium, arsenate, arsenite, antimony, lead, zinc and bismuth are highly toxic to most forms of life. In many staphylococci metal resistance is plasmid-encoded, and frequently the plasmids also carry antibiotic resistances as well.

Mercury and organomercurial compounds are inhibitors of membrane enzymes which contain sulphydryl ( - SH) groups. Resistance to mercurials is due to mercury reductase which converts Hg2 ' to volatile mercury (Hg'). Hydrolysis of organomercurials involves first, the detoxifi- cation of the organomercurials by organomercurial lyase to produce Hg2' which is then volatilized by the mercuric reductase. Resistance to mercurials is a common property of clinical strains of Staph. aureus with /?-lactamase plasmids (Richmond & Madeleine 1964). In some recent MRSA strains, mercury resistance is chromosomally encoded, suggesting transposition or integration of the heavy metal resistance plasmid into the chromosome (Witte el al. 1986).

The lethal effect of cadmium results from the rapid cess- ation in respiration as it binds to sulphydryl groups in pro- teins. Resistance to cadmium is commonly plasmid-borne in Staph. aureus and there are two distinct resistance mechanisms encoded by two separate genes, cad A and cad B (Perry & Silver 1982). cad A codes for an energy- dependent efflux mechanism which prevents an internal accumulation of cadmium ions (Witte et al . 1986) and the

cad B gene product may bind to cadmium ions (Perry & Silver 1982). These determinants also confer resistance to zinc ions. Witte et al. (1986) report a third type of cadmium resistance which is chromosomally located and is associated with MRSA strains.

6.3 Resistance to antiseptics and disinfectants

Staphylococci are generally considered to be sensitive to antiseptics and disinfectants. However, strains resistant to antiseptics and disinfectants, including ethidium bromide (Eb), acriflavine (AC), quaternary ammonium compounds (QACs) such as cetrimide and benzalkonium chloride (BC), and diamidines such as propamidine isethionate (PI) and diamidinodiphenylamine dihydrochloride (Dd) have been isolated (Townsend et al. 1984; Lyon & Skurray 1987). Most of these compounds bind to nucleic acid and resistance to these compounds is sometimes called resistance to nucleic acid-binding (NAB) compounds. The association of resistance to E b with /I-lactamase plasmid was reported over 20 years ago and more recently it has become associated with plasmids encoding gentamicin resistance (Archer & Mayall 1983; Townsend et al. 1985b). A large survey carried out by Townsend et al. (1985b) has demonstrated that 60% of MRSA strains isolated from around the world show that resistance to NAB compounds is global.

I t has been suggested that the extensive use of these and related compounds in the hospital environment may have selected for the emergence of NAB-resistant strains (Tennent et al. 1985). The NAB-resistance determinants are usually found in large plasmids in association with other phenotypic markers, the most common of which is gentamicin (Townsend et a f . 1984; Emslie et al. 1985). However, they have also been found on the chromosome, associated with other plasmid markers and on small plasmids which carry no other known phenotypic markers (Emslie et al. 1986). Genetic analysis of Staph. aureus plasmids encoding resistance to antiseptics and disinfectants has identified five distinct genes designated as qac A-E (Tennent et al. 1989). This might reflect the existence of several genes coding resistance to NAB compounds, or it may arise from varia- tions in phenotypic expression. Both qac A and qac B share restriction-site identity and DNA sequence homology, and encode resistance to Eb, AC, QACs and PI. However, the distinguishing feature between these two determinants is the level of resistance conferred to these compounds. Recently Tennent et al. (1989) have found that qac A also confers resistance to a number of compounds, including chlorhexidine diacetate, cetylpyridinium chloride, crystal violet, pentamidine isethionate, pyronin Y, quinaldine red, rhodamine 6G, and safranine. A third gene qac C has been detected on a small plasmid in strains of Staph. aureus isolated in Australia, Italy and the USA (Lyon & Skurray


1987); it also encodes resistance at a low level to Eb. Genes qac D and 4ac E are both carried on conjugative aminoglycoside resistance plasmids present in strains isolated in the USA (Lyon et al. 1987) and West Germany (Evans & Dyke 1988), respectively. They both share some similarity with qac C. Despite the phenotypic and genetic differences between the various 4ac genes the resistance mechanism is believed to be due to an efflux system, similar to that responsible for resistance to cadmium, tetracycline and arsenate (Tennent et al. 1989). Resistance levels of MRSA strains to chlorhexidines are however low and the clinical relevance is doubtful (Al-Masaudi et al. 1988; Cookson & Phillips 1990; Russell & Chopra 1990).

Haley et al . (1985) found MRSA and methicillin- sensitive strains to be equally sensitive to the lethal action of antiseptics.

7. ORIGIN OF STAPHYLOCOCCUS AUREUS RESISTANCE DETERMINANTS

Both intergenic and interspecific transfer of antimicrobial resistance genes have been proposed to occur among and between staphylococcal and other bacterial populations. Some evidence supports this hypothesis, including the demonstration of a close similarity between resistance determinants in staphylococci and other bacteria, e.g. soil bacteria (Bacillus spp.), antibiotic-producing organisms (streptomyces) and streptococci (Polak & Novick 1982). Moreover, several plasmids of staphylococcal origin have been transferred into B. subtilis and shown to replicate in, and confer antibiotic resistance upon, the new host. The transfer of resistance genes between staphylococci and streptococci (Guild et a/. 1982) has also been described.

Interspecific transfer of resistance would be expected to occur between Staph. aureus and Staph. epidermidis since they share a common ecological niche. A number of reports have shown close similarity between some plasmids from the two species on the basis of plasmid size, DNA homology and restriction enzymes (Groves 1979; Naidoo 1984). Exchange of resistance determinants has been described in the laboratory by transduction, transformation, in mixed culture, and also on the skin (Naidoo & Noble 1981). Inter- specific transfer of a conjugative gentamicin resistance plasmid, which has shown the ability to mobilize smaller plasmids from Staph. epidermidis to Staph. aureus (Naidoo 1984) strongly suggests a possible mechanism for the interspecific spread of other resistance determinants.

Can these findings be taken as proof that intergenic and interspecific gene transfer does occur among staphylococcal populations under natural conditions ? Lacey (1984) stated that transfer of plasmids between cultures in nature is most likely to have occurred where the frequencies of transfer in vitro are high. If this is true then the answer is yes.

8. CONCLUSIONS

If we can reasonably extrapolate from the evolutionary processes which occur then gene transfer between Staph. aureus and other Gram-positive species is clearly possible. Resistance determinants can be chromosomally, plasmid and conjugatively borne. Both transposition and site- specific recombination processes occur and these provide evolutionary mechanisms needed by an organism to gain multi-resistance phenotypes.

From this we can conclude that selection was and is a primary force behind the frequent isolation and evolution of multi-resistant Staph. aureus strains that are formed through the processes of mutation and gene transfer.

What is needed now? These various genetic mechanisms must be examined in detail to find out what role they play, both individually and collectively, in the evolution of novel MRSA strains. This means that studies must be done in viva and in vitro. We can then apply this knowledge to design clinical procedures and drugs to minimize the risks to future patients through infection from newly evolved strains.

9. ACKNOWLEDGEMENTS

The authors gratefully acknowledge support from the Biology Department, King Abdulaziz University, and the Ministry of Higher Education, Saudi Arabia.

10. REFERENCES

AL-MASAUDI, S.B., DAY, M.J. & RUSSELL, A.D. (1988) Sensitivity of methicillin-resistant Staphylococcus aureus strains to some antibiotics, antiseptics and disinfectants. Journal of Applied Bacteriology 65, 329-337.

AL-MASAUDI, S.B., RUSSELL, A.D. & DAY, M.J. (1990) Factors affecting conjugative transfer of plasmid borne gentamicin resistance in Staphylococcus aureus. Journal of Medical Microbiology (in press).

ARCHER, G.L. & JOHNSTON, J . L . (1983) Self-transmissable plasmids in staphylococci that encode resistance to aminoglycosides. Antimicrobial Agents and Chemotherapy 24, W 7 7 .

ARCHER, G.L. & MAYALL, C.G. (1983) Comparison of epi- demiological markers used in the investigation of an outbreak of methicillin-resistant Stapkylococcus aureus infection. Journal of Clnical Microbiology 18, 395-399.

ARCHER, G.L., COUGHTER, J.P. & JOHNSTON, J .L . (1986) Plasmid encoded trimethoprim resistance in staphylococci. Antimicrobial Agents and Chemotherapy 29, 735-740.

BARBER, M. & ROZWADOWSKA-DOWZENDO, M. (1948) Infection by penicillin-resistant staphylococci. Lancet ii, 641- 644.

BECK, W.D., BERGER-BACHI, B. & KAYSER, F .H . (1986) Additional DNA in methicillin-resistant Staphylococcus aureus and molecular cloning of mec-specific DNA. Journal of Bac- teriology 165, 373-378.

288 S . B . AL-MASAUDI E T A L .

BERGER-BACHI, B., STRASSLE, A. & KAYSER, F . (1989) Natural methicillin resistance in comparison with that selected by in-vitro drug exposure in Staphylococcus aureus. Journal of Antimicrobial Chemotherapy 23, 179-188.

BROWN, D.F .J . & REYNOLDS, P .E . (1980) Intrinsic resistance to B-lactam antibiotics in Staphylococcus aureus. FEES Letters 122, 275-278.

BRUNTON, J . (1984) Antibiotic resistance plasmid of streptococci, staphylococci, and bacteroides. In Antimicrobial Drug Resistance ed. Bryan, L.E. Orlando, Florida : Academic Press.

BUI.GER, R . & SHERRIS, J .C. (1968) Decreased incidence of antibiotic resistance among Staphylococcus aureus study in a university hospital over a nine year period. Annals of Internal Medicine 69, 1099-1 108.

B R Y A N , L . E . ( 1982) Bacterial Resistance And Susceptibility To Chemotherapeutic Agents. Cambridge : University Press.

CHOPRA, I . , L A C E Y , R.W. & C O N N E L L Y , J . (1974) Bio- chemical and genetic basis of tetracycline resistance in Staphy- lococcus aureus. Antimicrobial Agenrs and Chemotherapy 6, 397-404.

COHEN, S., SWEENEY, H . M . & BASU, S . K . (1977) Muta- tions in prophage P11 that impair the transducibility of their Staphylococcus aureus lysogens for methicillin resistance. Journal of Bacteriology 129,237-245.

C O L E M A N , D.C. , POMEROY, H., ESTRIDGE, J . K . , K E A N E , C.T., CAFFERKEY, M.T. , H O N E , R . & FOSTER, T . J . (1985) Susceptibility to antimicrobial agents and analysis of plasmids in gentamicin and methicillin resistant Staphylococcus aureus from Dublin hospitals. Journal of Medical Microbiology

COOKSON, B.D. & P H I L L I P S , I . (1988) Epidemic methicillin- resistant Staphylococcus aureus. Journal of Antimicrobial Chemo- therapy 21, Suppl. C 57-65.

COOKSON, B.D. & P H I L L I P S , I . (1990) Methicillin-resistant staphylococci. Journal of Applied Bacteriology Symposium Sup- plement 69, 55s-70s.

COOPER, G . L . & G I V E N , D.B. (1986) Vancomycin: A Com- prehensive Review of 30 Years of Clinical Experience. Chichester : John Wiley.

D A V I E S , J . & SMITH, D . I . (1978) Plasmid-determined resistance to antimicrobial agents. Annual Review of Microbiology 32,

DUCKWORTH, G.J . , LOTHIAN, J . L . E . & WILLIAMS, J . D . (1988) Methicillin-resistant Staphylococcus aureus: report of an outbreak in a London teaching hospital. Journal of Hospital Infection 11, 1-15.

E L SoLti, N . , ALLGNET, J . , BISMUTH, R . , BURET, B. & FOU ACE, J . (1986) Conjugative transfer of staphylococcal antibiotic resistance markers in the absence of detectable plasmid DNA. Antimicrobial Agents and Chemotherapy 30, 161-1 69.

EMSLIE, K . R . , TOWNSEND, D . E . , BOLTON, S. & G R U B B , W . B . (1985) Two distinct resistance determinants to nucleic acid binding compounds in Staphylococcus aureus. F E M S Microbiology Letters 27, 61-64.

EMSLIE, K . R . , T O W N S E N D , D . E . & G R U B B , W.B. (1986) Isolation and characterization of a family of small plasmids encoding resistance to nucleic acid-binding compounds in

20, 157-167.

469-518.

Staphylococcus aureus. Journal of Medical Microbiology 22,9-15. EVANS, J . & DYKE, K.G.H. (1988) Characterization of the

conjugation system associated with Staphylococcus aureus plasmid pJE1. Journal of General Microbiology 134, 1-8.

FORBES, B.A. & SCHABERG, D . R . (1983) Transfer of resistance plasmid from Staph,ylococcus epidermidis to Staphylococcus aureus : evidence for conjugative exchange of resistance. Journal of Bacteriology 153, 627-634.

GILLESPIE, M . T . & SKURRAY, R.A. (1986) Plasmids in multiresistant Staphylococcus aureus. Microbiological Science 3,

G I L L E S P I E , M.T. , M A Y , J . W . & S K U R R A Y , R . A . (1984) Antibiotic susceptibilities and plasmid profiles of nosocomial methicillin-resistant Sfaphyylococcus aureus: a retrospective study. Journal af Medical Microbiology 17, 295-310.

GILLESPIE, M . T . , M A Y , J .W. & SKURRAY, R.A. (1986) Plasmid encoded resistance to acriflavine and quaternary ammonium compounds in methicillin-resistant Staphylococcus aureus. F E M S Microbiology Letters 34,47-51.

GOERING, R.V. & RUFF, E .A. (1983) Comparative analysis of conjugative plasmids mediating gentamicin resistance in Staphylococcus aureus. Antimicrobial Agents and Chemotherapy 24,45&452.

GRINSTED, J . & LACEY, R.W. (1973) Genetic variation of streptomycin resistance in clinical strains of Staphylococcus aureus. Journal of Medical Microbiology 6, 351-361.

GROVES, D . J . (1979) Interspecific relationships of antibiotic resistance in Staphylococcus spp. : isolation and comparison of plasmid determining tetracycline resistance in StuphyEococcus aureus and Staphylococcus epidermidis. Canadian Journal of Microbiology 25, 1468- 147 5.

G U I L D , W . R . , SMITH, M . D . & SHOEMAKER, N . B . (1982) Conjugative transfer of chromosomal R determinants in Strep- tococcus pneumoniae. In Microbiology 1982 ed. Schlessinger, D. pp. 88-92. Washington DC: American Society for Micro- biology. A L E Y , R.W., H I G H T O W E R , A.W., KHABBAZ, R . F . , THORNSBERRY, C. , MARTONE, W.J. , A L L E N , J . R . & HUGHES, J . M . (1982) The emergence of methicillin-resistant Sraphylococcus aureus infection in United States hospitals. Annals of Internal Medicine 97, 297-308. ALEY, C . E . , M A R L I N G - C A S O N , M . , SMITH, J .W., LUBY, J . P . & M A C K O W I A K , P .A. (1985) Bacteriocidal activity of antiseptics against methicillin-resistant Staphylo- coccus aureus. Journal of Clinical Microbiology 21, 991- 992.

I N G L I S , B . , MATTHEWS, P . R . & STEWARTY, P .R. (1988) The expression in Staphylococcus aureus of cloned DNA encoding methicillin resistance. Journal of General Microbiology 134,

IORDANESCU, S. , S U R D E A N U , M. , D E L L A LATTA, P. & N O V I C K , R. (1978) Incompatibility and molecular relationships between small staphylococcal plasmids carrying the same resistance marker. Plasmid 1, 468-479.

JAFFE, H . W . , S W E E N E Y , H . M . , W E I N S T E I N , R.A., K A B I N S , S .A. , LATHAN, C. & COHEN, S. (1982) Struc- tural and phenotypic varieties of gentamicin resistance plasmids in hospital strains of Staphylococcus aureus and coagulase-

53-58.

1465-1 469.


negative staphylococci. Antimicrobial Agents and Chemotherapy

LACEY, R. W. (1975) Antibiotic resistance plasmids of Staphylo- coccus aureus and their clinical importance. Bacteriological Reviews 39, 1-32.

LACEY, R.W. (1980) Evidence for two mechanisms of plasmid transfer in mixed cultures of Staphylococcus aureus in vitro. Journal of General Microbiology 119,423435.

LACEY, R. W. (1984) Antibiotic resistance in Staphylococcus aureus and streptococci. British Medical Bulletin 40, 77-83.

LACEY, R.W. & CHOPRA, I . (1972) Evidence for mutation to streptomycin resistance in clinical strains of Staphylococcus aureus. Journal of General Microbiology 73, 175-180.

LACEY, R.W. & GRINSTED, J . (1972) Linkage of fusidic acid resistance to the penicillinase plasmid in Staphylococcus aureus. Journal of General Microbiology 73, 501-508.

L A N D Y , M. , L A R K U M , M.W., OSWALD, E.J. & STREIGHOFF, P. (1943) Increased synthesis of p-aminobenzoic acid associated with the development of sulfonamide resistance in Staphylococcus aureus. Science 97, 265-267.

LINDBERG, M., SJOSTROM, J.E. & JOHANSSON, T. (1972) Transformation of chromosomal and plasmid characters in Staphylococcus aureus. Journal of Bacteriology 109, 844-846.

LYON, B.R. & SKURRAY, R.A. (1987) Antimicrobial resistance of Staphylococcus aureus : Genetic basis. Microbiological Reviews 51, 88-135.

LYON, B.R., M A Y , J.W. 8i SKURRAY, R.A. (1984) Tn4001: a gentamicin and kanamycin resistance transposon in Staphylo- ccoccus aureus. Molecular and General Genetics 193, 554-556.

LYON, B.R., GILLISPIE, M.T. , BYRNE, M.E., MAY, J.W. & S K U R R A Y , R.A. (1987) Plasmid-mediated resistance to gentamicin in Staphylococcus aureus : the involvement of a transposon. Journal of Medical Microbiology 23, 101-1 10.

MCDONNELL, R.W., S W E E N E Y , H . M . & COHEN, S. (1983) Conjugational transfer of gentamicin resistance plasmid intra- and interspecifically in Staphylococcus aureus and Staphylococcus epidermidis. Antimicrobial Agents and Chemotherapy 23, 15 1- 160.

MARPLE, R.R. & COOKE, E.M. (1988) Current problems with methicillin-resistant Staphylococcus aureus. Journal of Hospital Infection 11, 381-392.

M. , D O I , M. , I N Q U E , M., U B U K A T A , K. , YAMASHITA, N. & KONNO, M . (1986) Molecular cloning of the gene of a penicillin-binding protein supposed to cause high resistance to 8-lactam antibiotics in Staphylococcus aureus. Journal of Bac- teriology 167, 975-980.

MATTHEWS, P.R. & STEWART, P.R. (1984) Resistance heterogeneity in methicillin-resistant Staphylococcus aureus. F E M S Microbiology Letters 22, 161-166.

NAIDOO, J. (1984) Interspecific co-transfer of antibiotics resistance plasmid in staphylococci in vivo. Journal of Hygiene, Cam- bridge 93, 59-66.

NAIDOO, J . & NOBLE, W.C. (1981) Transfer of gentamicin resistance between coagulase-negative and coagulase-positive staphylococci on skin. Journal of Hygiene, Cambridge 86, 183- 187.

21,773-779.

MATSUHASHI, M., SONG, M.D., ISHIMO, F., WACHI,

NOBLE, W.C. & NAIDOO, J . (1978) Evolution of antibiotic resistance in Staphylococcus aureus: the role of the skin. British Journal of Dermatology 98, 481-489.

NOVICK, R.P. & BOUANCHAUD, D . (1971) Extrachromosomal nature of drug resistance in Staphylococcus aureus. Annals of the N e w York Academy of Sciences 182, 279-294.

O’TOOLE, R . D . , DREW, W . L . , DAHLGREN, B.J. & BEATY, H . N . (1970) An outbreak of methicillin-resistant Staphylococcus aureus infection : observation in hospital and nursing homes. Journal of the American Medical Association

PARKER, M . T . & HEWITT, J . H . (1970) Methicillin resistance in Staphylococcus aureus. Lancet i, 800-804.

PERRY, R.D. & SILVER, S. (1982) Cadmium and manganese transport in Staphylococcus aureus membrane vesicles. Journal of Bacteriology 150, 973-976.

POLAK, J . & N O V I C K , R. (1982) Closely related plasmid from Staphylococcus aureus and soil bacilli. Plasmid 7, 152-162.

POSTON, S .M. & NAIDOO, J .L . (1983) Genetics and antimicrobial drug resistance in the staphylococci. In Staphylococci and Staphylococcal Infections, Vol. 1, Clinical and Epidemiologi- cal Aspects ed. Easmon, C.S.F. & Adlam, c. pp. 63-119. London : Academic Press.

REYNOLDS, P.E. (1985) Inhibition of bacterial cell wall synthesis. In The Scientific Basis of Antimicrobial Chemotherapy ed. Greenwood, D. & O’Grady, F. pp. 1340. Cambridge: Cam- bridge University Press.

REYNOLDS, P.E. & FULLER, C. (1986) Methicillin-resistant strains of Staphylococcus aureus ; presence of identical additional penicillin-binding protein in all strains examined. F E M S Microbiology Letters 33, 251-254.

RICHMOND, M.H. & MADELEINE, J. (1964) Co-transduction by staphylococcal phage of the gene responsible for penicillinase synthesis and resistance to mercury salts. Nature 202, 136& 1361.

ROUNTREE, P.M. & BEARD, M.A. (1968) Hospital strains of Staphylococcus aureus with particular reference to methicillin- resistant strain. Medical Journal of Australia 2, 1163-1 168.

RUSSELL, A.D. & CHOPRA, I. (1990) Understanding Anti- bacterial Action and Resistance. Chichester : Ellis Horwood.

SAUNDERS, J . R . (1984) Genetic and evolution of antibiotic resistance. British Medical Bulletin 40, 54-60.

SCHAEFLER, S., J O N E S , D. , P E R R Y , W., BARADET, T . , M A Y R , E. & RAMPERSAD, C. (1984) Methicillin-resistant Staphylococcus aureus in New York City hospitals : interhospital spread of resistant strain of type 88. Journal of Clinical Micro- biology 20, 536-538.

SHALITA, Z . , MURPHY, E. & NOVICK, R.P. (1980) Penicil- linase plasmid of Staphylococcus aureus : structural and evolutionary relationships. Plasmid 3, 291-31 1.

SHANNON, K . & PHILLIPS, I . (1982) Mechanism of resistance to aminoglycosides in clinical isolates. Journal of Antimicrobial Chemotherapy 9, 1-102.

SHANSON, D.C. (1981) Antibiotic-resistant Staphlococcus aureus. Journal of Hospital Infection 2, 11-36.

SHANSON, D.C. (1986) Staphylococcal infection in hospitals. British Journal of Hospital Medicine 35, 3 12-320.

213, 257-263.

290 S.B. AL-MASAUDI E r A L .

SHANSON, D.C., KENSIT, J.C. & DUKE, R. (1976) Out- break of hospital infection with a strain of Staphylococcus aureus resistant to gentamicin and methicillin. Lancet ii, 1347-1348.

SJOSTRON, J .E. , LOFDAH, S . & PHILIPSON, L . (1975) Transformation reveals a chromosomal locus of the gene@) for methicillin resistance in Staphylococcus aureus. Journal of Bac- teriology 123, 905-915.

S K U R R A Y , R.A., ROUCH, D.E., L Y O N , B.R. , GILLESPIE, M . T . , T E N N E N T , J . M . , B Y R N E , M . E . , MESSEROTTI, L . J . & M A Y , J . W. (1988) Multiresistant Staphylococcus aureus: genetic and evolution of Australian strains. Journal of Antimicrobial Chemotherapy 21, Suppl. C. 19-38.

S W E E N E Y , H . M . & COHEN, S . (1968) Wild-type strain of Staphylococcus aureus containing two genetic linkage groups for penicillinase production. Journal of Bacteriology 59, 92C924.

T E N N E N T , J . M . , L Y O N , B.R., G I L L E S P I E , M . T . , M A Y , J . W . & S K U R R A Y , R.A. (1985) Cloning and expression of Staphylococcus aureus plasmid-mediated quaternary ammonium resistance in Escherichia coli. Antimicrobial Agents and Chemo- therapy 21, 70-83.

T E N N E N T , J . M . , L Y O N , B.R. , M I D G L E Y , M. , J O N E S , I . G . , PUREWAL, A . S . & S K U R R A Y , R.A. (1988) Physical and biochemical characterisation of the yac A gene encoding antiseptic and disinfectant resistance in Staphylococcus aureus. Journal of General Microbiology 135, 1-10.

T O W N S E N D , D.E. , ASHDOWN, N . , GREED, L . C . 8z G R U B B , W. B. (1984) Transposition of gentamicin resistance

to staphylococcal plasmids encoding resistance to cationic agents. Journal of Antimicrobial Chemotherapy 14, 115-124.

T O W N S E N D , D . E . , BOLTON, S. , ASHDOWN, N. & GRUBB, W. B . (1985a) Transfer of plasmid-borne aminoglycoside- resistance determinants in staphylococci. Journal of Medical Microbiology 20, 169-185.

TOWNSEND, D.E. , ASHDOWN, N., MOMOTH, M. & GRUBB, W. B . (1985b) Distribution of plasmid-borne resistance to nucleic acid binding compounes in methicillin resistant Staphylococcus aureus. Journal of Antimicrobial Chemotherapy

TOWNSEND, D.E., BOLTON, S., ASHDOWN, N. , TAHERI, S. & GRUBB, W.B. (1986) Comparison of phage-mediated and conjugative transfer of staphylococcal plasmid in vitro and in vivo. Journal of Medical Microbiology 22, 107-1 14.

UDO, E. , T O W N S E N D , D.E. & G R U B , W.B. (1987) A conjugative staphylococcal plasmid with no resistance phenotype. FEMS Microbiology Letters 40, 279-238.

WITTE, W. (1981) Transfer of drug-resistance plasmid in mixed cultures of staphylococci. Zentrablatt f u r Bakteriologie, Parasi- tenkude, Infektionkrankheiten und Hygiene. Abreilung I Supple- ment 10, 543-545.

W I T T E , W . , G R E E N , L., MISRA, T . K . & SILVER, S. (1986) Resistance to mercury and to cadmium in chromosomally resistant Staphylococcus aureus. Antimicrobial Agents and Che- motherapy 29, 663-669.

15,417-434.

Antimicrobial resistance and gene transfer in Staphylococcus aureus

Documents