Bacterial cells.pdf

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Bacterial CellsTerry J Beveridge, University of Guelph, Guelph, Ontario, Canada

Bacteria are prokaryotes andtherefore have a simple cell structure. There is no nucleus and

genetic material is free within the cytoplasm. The two fundamental types of bacteria

Gram-positive and Gram-negative

have different cell wall structures.

Introduction

Bacteria (previously termed eubacteria) are a lineage ofprokaryotic cells that split off from the ancestral lineagerelatively early in the evolution of life. After this time, twoprokaryote domains existed, the Bacteria and the Archaea.Eukaryotic life emerged at a later date from the archaeallineage.

The general structural imprint for bacterial life seems tohave been well established during the development of the

ancestral lineage since the fossilized remains (microfos-sils) of such ancestors have been found preserved inorganic-rich cherts and shales dating back to about 30003600 million years ago. These remarkable imprints ofsimple lifeforms do not reveal much structural detail(Figure 1), but they do demonstrate that prokaryotic cells(atthatearly time) were similar inshape and size to present-day bacteria. The common features that are retained todayare: (1) small size, (2) simple cellular organization, (3)robust cell envelopes and (4) binary fission for reproduc-tion. Clearly, this is a design strategy that has been highlysuccessful for the last 3600 million years and it has beenmalleable enough to change with (or withstand) the

stressful influences (some of which have been globallycatastrophic for long time periods) on bacteria over eons.

General Description of Cell Structure

Bacteria are small ($ 1.52.5mm3) cells of relatively simpleconstruction (Figure 2) as compared to eukaryotic cells.Most have a single, circular chromosome that entwinesitself throughout the cytoplasmic matrix. (Recent studiessuggest that some bacteria, such as Borrelia and Agrobac-terium have a linear chromosome and that others, such as

Rhodobacter, Leptothrix, Brucella, Burkholderia and Rhi-zobium, have multiple nuclear elements some of which aretoo small to be considered true chromosomes.) Unlikeeukaryotic cells, bacteria do not compartmentalize theircytoplasm into separate functional organelles. For thisreason, the bacterial chromosome is not bound by anuclear envelope and the cytoplasmic space it occupies isreferred to as the nucleoid (Figure 2). As the cells grow insize, the chromosome is constantly replicating so that, bythe time of division, each daughter cell obtains an equal

chromosomal complement. Sometimes, under optimgrowth conditions, bacteria can have remarkably shorcell-doubling times; for example, the doubling time o

Escherichia coliK12 can be 20 minutes. For these growtrates, replication of the chromosome can barely keep uwith cell division and multiple replication forks in thchromosome are necessary.

Most bacteria also contain small, additional nucleaelements called plasmids or extrachromosomal elementThese can be transferred from one cell to another bprocesses called transformation and conjugation so thaadditional genes are constantly being exported betweebacteria. Through this mechanism, genes conferring traitsuch as antibiotic- and heavy metal-resistance factors arfrequently conveyed amongst bacteria. Remarkably, thihappens not only between similar strains of bacteria bu

also between dissimilar genera. Transfer by conjugatiooften requires specialized surface structures, called F-pilthrough which the extrachromosomal DNA is thought tpass. Special viruses, called bacteriophages or phages, caalso transfer new genes to bacteria. Certain bacteriophageare not always lytic and their genomes (after injection intthe bacterium) can lie dormant for long periods of timeThese are called lysogenic bacteriophage and, frequentlytheir DNA can integrate into the bacterial chromosomwhere it replicates and is transferred to daughter celduring binary fission.

Bacterial ribosomes are smaller than the eukaryotvariety (70S versus 80S) and consist of a small 30S subun

attached to a larger 50S subunit. Both subunits consist ofnumber of separate proteins which are integrated witribonucleic acid (rRNA) to produce a particle that can bvisualized by transmission electron microscopy (Figure 2Ribosomesare scattered throughoutthe cytoplasmand arfrequently aligned on the inner face of the plasm(cytoplasmic) membrane where they are called polysomeBoth ribosomal varieties are in close contact with thnucleoid (this ensures quick, efficient transcription!translation! protein synthesis) so that rapid metaboli

Article Contents

Introductory article

. Introduction

. General Description of Cell Structure

. Specialized Internal Structures

. Shape and Form

. Biofilms

. Enveloping Structures

. Gram-staining Properties

. Motility Structures

. Specialized Structures for Survival

. Concluding Remarks

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rates are possible. Polysomes are attached to the plasmamembrane so that protein translation (to the periplasm)and secretion (to the outside) can occur.

The cytoplasm is bounded by a plasma membrane(Figure 2; this is similar to the cytoplasmic membrane ineukaryotic cells but, by convention, is usually called theplasma membrane as coined by early light microscopists).

This membrane is a lipid/protein bilayer which semipermeable and which contains a large number ofunctional enzymes. Onecommon trait of this membranethat it is energized; there is an electron flow through it anthe membrane (usually) pumps protons (H1 ) from thcytoplasm to the periplasm. This membranes semipermeability enriches the cytoplasm with a relatively hig

Figure1 (a)Photographof a pieceof thegunflint chertfrom thenorthern shoreof Lake Superior,Ontario, Canada. Therockmeasuresabout 20cm alonthelong axis andthe waveystriations of thelayers of an ancient $3000 million-year-old stromatolitecan be seen in themiddle of therock.(b) Bright-fielight micrograph of a thin section of the gunflint chert showing the mineralized remains of prokaryotic cells. (c) Light micrograph using phase optics of

living modern-day biofilm from a stream in southern Ontario for comparison with (b). These figures were originally supplied by F.G. Ferris, University oToronto and are reprinted from Beveridge (1988) with permission of the Canadian Journal of Microbiology.

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concentration of organic and inorganic ions so that, bydiffusion,a water gradient develops between the cytoplasm(high) and the outside (low). Consequently, a large turgorpressure (usually between 3 and 25 atmospheres) pushesagainst the plasmamembrane;this is so formidable thatthebilayer would burst unless additional boundary layers (e.g.a cell wall) were added for additional support. Theseadditional layers are discussed in more detail below. Thefluid of the cytoplasm, in which particulate matter floats(such as the ribosomes and chromosome), is called thecytosol.

Specialized Internal Structures

Usually, there are few cytoplasmic structures in bacteria.When they are found, it is because they possess a distinctfunctional attribute that is necessary for that particularcell. For example, some bacteria possess small, single-domain granules of magnetite (Fe3O4) aligned to the long

axis of the cell (Figure 3). These membrane-bound particleare called magnetosomes and are the smallest possiblform of magnetite that has a magnetic moment. Theilinear alignment in a cell converts the bacterium into small compass needle so that it must align to thgeomagnetic field. For those bacteria residing in thtemperate zones (either north or south) of the Earth, thgeomagnetic alignment points the cells downwards so thathey can migrate towards the microaerophilic (low oxygetension) conditions they prefer.

Other bacteria produce intracellular granules to assis

their nutritional needs. For example, glycogen granules ardeveloped by some bacteria as quick-energy reservoirGlycogen is a polymeric carbohydrate that can be easildrawn-on by the cell when it is under carbon limitationPolyhydroxyalkanoate granules (Figure 2; usually consising of b-hydroxybutyrate, b-hydroxyvalerate or copolymers of the two compounds) can also be found in sombacteria and these are a source of energy which can bdrawn-on under longer periods of nutrient limitationSulfur granules are found within bacteria (e.g. the purpl

Figure2 Electron micrograph of a thin section ofLeptothrix discopherawhich shows many structural attributes of a bacterialcell.This is a Gram-negativ

cell and the inner bilayer of the cell envelope is called the plasma membrane which encloses the cytoplasm. Reprinted from Beveridge (1988) withpermission of the Canadian Journal of Microbiology.

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Figure 3 Thin section of a magnetotactic spirillum (a Gram-negative bacterium) which contains a chain of magnetosomes containing small particlesmagnetite (Fe3O4). This sample was supplied by D. Bazylinski, University of Iowa.

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sulfur bacteria) that oxidize sulfur compounds. Thesegranules are spread throughout the cytoplasm and areshown to be highly refractile by light microscopy (bright-field or phase) and transmission electron microscopy. Aslong as these bacteria have a supply of reduced sulfur intheir environment, the granules continue to grow as moreand more elemental sulfur is laid down. But, as this supply

dwindles, the granules become oxidized (usually to sulfate)and are reduced in size. Many bacteria accumulate reservesof phosphate during growth since they require it for high-energy compounds to drive metabolic processes and forstructural constituents (e.g. for membrane phospholipidsand chromosomal DNA). In this case, the storedphosphate is partitioned into small cytoplasmic polypho-sphate granules, which are readily stained by basic dyesused for light microscopy. This metachromatic (or colour)change makes the particles visible and accounts for theirother name, metachromatic granules.

Bacteria living in aqueous environmentssometimes needto alter their ability to float. These cells produce a most

interesting internal flotation device called a gas vesicle. Thevesicles are constructed of proteinaceous subunits withhigh b structure that assemble with one another to form ahollow elliptical structure (it resembles a rugby football)about 300600 nm 100 nm. The fit of the subunits is sotight that the cytosol cannot pass through; only gases(dissolved in the cytosol) can pass into the lumen of thevesicle, where they collect and concentrate to produce $ 1atmosphere of pressure. Gas vesicles can regulate the levelat which a bacterium floats within a natural water columnsince the gases within a vesicle effect the cells buoyancy.This can be important for phototrophs (such as cyano-bacteria) which depend on discrete wavelengths of light to

grow.The light-harvesting apparatus of phototrophs can be

shaped into a range of different structures but there isalways a common characteristic: the photosyntheticpigments are embedded in a lipid bilayer or nonunitmembrane which has been derived from the plasmamembrane. Cyanobacteria, such as Synechococcus spp.,arrange their photosynthetic bilayers as concentric lamel-lae underneath the plasma membrane (Figure 4a), whereasChlorobium spp. (green sulfur bacteria) partition theirbacteriochlorophylls within cigar-shaped chlorosomes(which are bounded by nonunit membranes) arranged atthe cytoplasmic periphery (Figure 4b). Those phototrophs

that fix carbon dioxide via the Calvin cycle require theenzyme ribulose bisphosphate carboxylase (Rubisco)which is frequently found in carboxysomes (Figure 4a).

Bacteria that require other dedicated enzyme systems fortheir growth in particular ecological niches also frequentlypartition these enzymes into intracellular membranes. Forthis reason, nitrifiers (e.g. Nitrosococcus) and methano-trophs (e.g. Methylococcus) typically use such membra-nous structures.

Shape and Form

The characteristics of shape and form are of extremimportance to bacteria. Bacteria cannot reach out and grahold of food nor can they engulf it. Instead, they must relon diffusion in the outside environment to bring food tthem and to take waste materials away. Therefor

anything they can do to encourage diffusive processes wibe helpful. Simple design modifications that affect shapcan be quite advantageous. For example, a coccus (such aStaphylococcus) has a low surface area-to-volume ratiothere is not much surface area for adsorption of nutrientcompared to the relatively large volume of cytoplasm to bnourished. Yet, if a cell of similar volume is converted to rod (e.g. Bacillus), the surface area (and adsorption) idrastically increased. Supposedly, this is one of the reasonwhy B. subtilis can outgrow S. aureus under similar growtconditions. In the microbial world, a number of differenbacterial shapes are encountered (e.g. spheres, rodcommas, spirals, prosthecate (Figure 5), etc.) and the tren

towards a greater surface area-to-volume ratio for betteexchange of nutrients and wastes must be a potent selectivdriving force.

Biofilms

In natural settings, it is not uncommon to find mosbacteria attached to interfaces where they form so-callebiofilms. In aqueous habitats, the airwater interface ansolid surfaces are preferred because they tend to accumulate nutrients as a result of adsorptive and interfaciaeffects. Bacteria take advantage of these collected nutr

ents, adhere to the surface, grow and divide. Soon, theoutgrow one another, microbial consortia become interdependent (some cells require cofactors or metabolsubstrates produced by others) and stratification occur(nutrient, pH and redox gradients develop). Here, ceshape is still important. Caulobacter, which is a prosthecatbacterium, attaches its stalk to the substratum by meanof a proteinaceous glue onthe holdfast of its stalk (Figure5Once fastened, the stalk gradually lengthens as the biofilm(composed of other bacteria) thickens. This keeps thCaulobacters cell body in thebiofilms outer reacheswheroxygen and nutrients are plentiful. Here too, chains ofilamentous cyanobacteria grow by orienting their fila

ments (chains of cells) to the surface of the biofilm so thathey contact sunlight. As these natural aquatic biofilmflourish they become thicker, the innermost regionbecoming anoxic, encouraging the growth of anaerobes.

Enveloping Structures

In the Bacteria, all cells except the Mollicutes (the so-callewall-less bacteria) have additional structural layers abov

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the plasma membrane. The number and type of extralayering depends on the variety of bacterium,but almost allpossess a cell wall immediately above the membrane.

Gram-positive bacteria possess a thick wall (usually$ 2030nm thick) composed of peptidoglycan andsecondary polymers, usually consisting of teichoic acid,teichuronic acid, or both polymers. Peptidoglycan is theprimary structural component because its individualglycan strands are covalently linked to immediately

adjacent strands, thereby forming a bonded networkcompletely surrounding the cell. This network can be 20or more molecular layers thick. It is a strong network and isnecessary to resist the cytoplasmic turgor pressurediscussed in a previous section. The secondary polymersare attached to and intercalated with the peptidoglycannetwork so that an amorphous matrix (i.e. the Gram-positive cell wall) results.

Gram-negative walls are more complex (Figure 2). Theytoo have peptidoglycan, but in much smaller amounts.

This network is only 13 molecular layers thick and resideabove the plasma membrane in the periplasmic spacImmediately above is the outer membrane, consisting of lipid/protein bilayer of different composition to that of thplasma membrane. Several of the major proteins (outemembrane proteins or OMPs) assemble together intmultimers which span the bilayer, forming aqueouchannels connecting the outside to the underlying perplasmic space. The outer membrane is one of the few

biological membranes to have an asymmetric distributioof lipid. The inner face contains phospholipid, whereas thouter face contains lipopolysaccharide (LPS). The regiobetween the outer and plasma membranes, the periplasmispace, is filled with a concentrated brine of organmolecules. Many of these are enzymes responsible for full range of activities including the breaking down of largnutrients to smaller ones essential for metabolismconveying essential vitamins and minerals to the cell, andetoxifying harmful agents.

Figure 4 (a) Thin section of the cyanobacterium SynechococcusGL-24 showing the concentric arrangement of the photosynthetic membranes and th

carboxysomes. (b) Thin section of a Chlorobium sp. which is a green sulfur bacterium showing the photosynthetic chlorosomes. Both (a) and (b) wereprovided by S. Douglas, University of Guelph and (a) is with permission of the Journal of Bacteriology.

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Additional layers above the cell wall can often be found.Sometimes a gel-like matrix can be produced; if it isattached to the cell wall it is called a capsule (Figure 2), ifnot, it is a slime. Both extracellular polymeric substancesare frequently found separating individual cells in biofilms.Another capsule-like structure in which chains of cells canreside is a sheath. This is a cylindrical structure thatsometimes is the outermost boundary layer ofLeptothrix,Sphaerotilus, Beggiatoa and cyanobacteria (Figure 2). S

layers are more ordered structures that can be encounteredattached to cell walls. These consist of proteins (orglycoproteins) which interact with each other once theyreach the wall surface so that they self-assemble inregularly ordered, planar arrays (Figure 4). These para-crystalline arrays, or S layers, can be arranged in oblique(p2), square (p4) or hexagonal (p3, p6) lattices.

Gram-staining Properties

The structural format and chemical composition obacterial cell walls dictate the staining response of cells tthe Gram stain. This stain for bright-field light microscopwas formulated by Christian Gram in 1884 and is still prime method for distinguishing different types of bacteriduring their taxonomical identification. Bacteria are heatfixed to a glass slide and stained with crystal violet (purple

for 60 s. Grams iodine (containing potassium iodide) inext added as a chemical mordant for 180 s. The slide irinsed with a steady flow of ethanol for 20 s and, lastly, thcells on the slide are stained with safranin (red) as counterstain for 60 s.

Crystal violet enters all bacteria and stains them purpleGrams iodine clusters the stain into large precipitates thacannot be removed from the cell unless the cell wall ibroken down. When the ethanol rinse is applied to thslide, the cell wall of Gram-positive bacteria is not affected

Figure 5 Electron micrograph of a negatively stained Caulobactersp. showing its unusual shape complete with stalk and holdfast. Reprinted fromBeveridge (1988) with permission of the Canadian Journal of Microbiology.

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the crystal violet cannot be removed from its intracellularlocation and the cells remain purple. Gram-negativebacteria, however, are decolorized by the ethanol rinse.Since the outer and plasma membranes dissolve in alcohol,the crystal violetiodine complex is washed away and thesafranin counterstain dyes the bacteria red. Safranin doesnot affect the purple colour of Gram-positive cells when it

is applied.

Motility Structures

The most commontype of motility in bacteria is swimming.Here, one or more flagella rotate to push (or pull) the cellthrough an aqueous milieu. The bacterial flagellumconsists of a long spiral filament (Figure 6) made up of ahelical arrangement of proteins referred to as flagellins.The filament is $ 20 nm thick, can be one or more celllengths long and is hollow (so that newly synthesized

monomeric flagellin can migrate through the hollow cavityfrom the cell body to the distal tip of the flagellum where itself-assembles). Most filaments consist of only one varietyof flagellin (e.g. Escherichia coli), but some have two ormore flagellins (e.g. Campylobacter and Vibrio).

The filament is attached to a hook which is connected toa basal body (Figure 6). The basal body is a complexorganelle that attaches the flagellum to the cell body and isconstructed of a number of structural parts. In Gram-negative bacteria, it consists of a rod with attached rings;the L ring is inserted into the outer membrane, the P ring isaligned with the peptidoglycan layer, and the S-M ring isembedded into the plasma membrane. A proton gradient

across the plasma membrane is the driving force whichrotates the S-M ring and, thus, rotates the entire flagellum,acting as a propeller and driving the bacterium forward.

As explained in a previous section, Gram-positivebacteria do not have an outer membrane as a structure in

their cell walls, but only a relatively thick matrix opeptidoglycan and secondary polymers. The flagella othese bacteria do not, then, have L and P rings in the basabody, but only the S-M ring. Rotation of Gram-positivflagella is by the same mechanism as explained for Gramnegative cells.

There aresome variations fromthis general theme. Som

bacteria possess only one flagellum and often this is at single polar location on the cell (e.g. Pseudomonaaeruginosa). This arrangement is called monotrichou(i.e. single hair). Others can have tufts of flagella at onor both poles (e.g. Aquaspirillum serpens; such an arrangement is called lophotrichous (i.e. tuft hair)). Thosbacteria with flagella placed continuously around the cebody (e.g. E. coli) have a peritrichous arrangement (i.e[all] around hairs). Vibrio and Bdellovibrio have sheatheflagella in which the lipidprotein bilayer of the outemembrane extends over the length of the flagellumSpirochaetes, such as Treponema pallidum, have adoptean entirely different strategy by burying their flagella in th

periplasm, aligning them along the cell axis so that thesendoflagella are never exposed to the outside.

Two other types of motility have been described fobacteria: twitching and gliding. Twitching is the result ocontractile pili (also called fimbriae) and this motilitsystem seems to be nondirectional and has not beeextensively studied. Gliding has been better studied anrequires an interface (usually a solid surface) for thbacteria to move on. By inference, this motility systemmust (somehow) push or pull on the substratum surfacand a number of propulsion devices have been propose(i.e. directed extrusion of exopolymers, surface activagents, small rotating discs, moving fibrils, etc.). So far, th

exact nature of gliding motility is unclear.

Figure6 Negativestainof theproximalend of a flagellumfrom Campylobacterfetussubsp. fetuswhichshowsthe filament,hookand basalbody (L,P an

S-M rings).

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Specialized Structures for Survival

The best-studied survival structure for bacteria is a restingcell called an endospore. This is a highly resilient cell that isformed within a mother cell. It is encased in a number of

robust coatings and has almost no detectable metabolism(Figure7). As the endospore forms, much of the cytoplasmicwater is removed, a specialized substance (dipicolinic acid)is synthesized to complex Ca21 , and special coatings arelaid down. For the latter,a thick peptidoglycan-containingcortex is formed first above the plasma membrane of thespore. Next, the spore coat (consisting of a regulararrangement of high cysteine-containing proteins) ap-pears, followed by a more loosely arranged exosporium. Atthis point, the cytoplasm of the spore has lost so much

water it has become very dense and its metabolic processebarely detectable; this region is now called the core. Usinlight microscopy, endospores are highly refractile.

Endospores are highly resistant to desiccation, poonutrition, extremes in temperature, harsh chemical

extreme pH and radiation, and can survive under sucstressful conditions for long periods of time. Once theencounter better environments, they germinate; therobust encompassing garments break open and a vegetative cell emerges; this is called outgrowth. Endospores armost frequently encountered in Bacillus and Clostridiumspp.

Azotobacter produces another type of restingcell calledcyst. These are also produced during harsh conditions buin this case (unlike an endospore), a cyst is not produce

Figure 7 Thin section of a Bacillus megaterium endospore.

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within a mother cell. The mother cell itself begins todehydrate and fibrous material is laid down outside of theouter membrane of this Gram-negative bacterium. Even-tually, this fibrous material arranges itself as concentriclayers around the cell body and can be $ 0.5mm thick. Asthis is happening, large polyhydroxyalkanoate granules (ascarbon reserves) begin to take up much of the cytoplasmic

volume.More recently, there has been a general recognition that

many Gram-negative bacteria under nutrient limitation innatural environments have another strategy for survival.They gradually cannibalize themselves until only the bareessential structures are left. By doing this they becomedrastically reduced in size, contain few ribosomes, possessa condensed chromosome and are bounded by a typicalGram-negative envelope. These small cells can remain inthis state of stasis until conditions become more favour-able. They are referred to as ultramicrobacteria ornanobacteria.

Concluding Remarks

This article serves as a general introduction to the structureof bacteria. Because it strives to encompass many of thestructures found in this large prokaryotic domain it cannotpresent all the details. It should be remembered that not allbacteria possess all of the structures mentioned in thisarticle. Many are species- or genus-specific, whereas othersare induced by environmental factors.

Further Reading

Beveridge TJ (1981) Ultrastructure, chemistry and function of the

bacterial cell wall. International Review of Cytology 72: 229317.

Beveridge TJ (1988) The bacterial surface: general considerations

towards design and function. Canadian Journal of Microbiology 34:

363372.

Beveridge TJ (1989) The structure of bacteria. In: Poindexter JS and

Leadbetter ER (eds)Bacteria inNature:A Treatiseon theInteraction of

Bacteria and their Habitats, pp. 165. New York: Plenum Publishing.

Beveridge TJ (1990) Mechanism of Gram variability in select bacteria.

Journal of Bacteriology 172: 16091620.

Beveridge TJ (1994) Bacterial S-layers. Current Opinion in Structur

Biology 4: 204212.

Beveridge TJ and Davies JA (1983) Cellular response ofBacillus subtil

and Escherichia coli to the Gram stain. Journal of Bacteriology 15

846858.

Beveridge TJ and Graham LL (1991) Surface layers of bacteri

Microbiological Reviews 55: 684705.

Beveridge TJ and Schultze-Lam S (1997) The response of select

members of the Archaea to the Gram stain. Microbiology 142: 28872895.

Beveridge TJ, Makin SA, Kadurugamuwa JL and Li Z (199

Interactions between biofilms and the environment. FEMS Micro

biology Reviews 20: 291303.

Costerton JW, Cheng K-J, Geesey GG et al. (1987) Bacterial biofilms

nature and disease. Annual Reviews of Microbiology 41: 435464.

Davies JA, Anderson GK, Beveridge TJ and Clark HC (1983) Chemic

mechanisms of theGram stain andsynthesis of a newelectron-opaq

marker for electron microscopy which replaces the iodine mordant

the stain. Journal of Bacteriology 156: 837845.

Koch AL (1995) Bacterial Growth and Form. New York: Chapman an

Hall.

Ko nig H and Messner P (special eds) (1997) 4th International S-lay

Workshop (Rothenburg o.d. Tauber, Germany).FEMS Microbiolog

Reviews 20: 5175.

Macnab RM and DeRosier DJ (1988) Bacterial flagellar structure an

function. Canadian Journal of Microbiology 34: 442451.

Messner P and Sleytr UB (1992) Crystalline bacterial cell surface layer

Advances in Microbial Physiology 33: 213275.

Poindexter JS and Leadbetter ER (eds) (1989) Bacteria in Nature:

Treatise on the Interaction of Bacteria and their Habitats, vol. 3. Ne

York: Plenum Publishing.

Salton MRJ (1963) The relationship between the nature of the cell wa

and the Gram stain. Journal of General Microbiology 30: 223235.

Walter MR (1983) Archean stromatolites: evidence of the earths earlie

benthos. In: Schopf JW (ed.)Earths Earliest Biosphere: Its Origin an

Evolution, pp. 187213. Princeton: Princeton University Press.

Whitfield CW (1993) Biosynthesis and expression of cell-surfa

polysaccharides in Gram-negative bacteria. Advances in Microbi

Physiology 35: 135246.

Wilson DR and Beveridge TJ (1993) Bacterial flagellar filaments an

theircomponentflagellins.Canadian Journal of Microbiology 39: 415

427.

WoeseCR, Kandler O andWheelisML (1990)Towards a natural syste

of organisms: proposal for the domains Archaea, Bacteria, an

Eucarya. Proceedings of the National Academy of Sciences of the US

87: 45764579.

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