MICR 201 Microbiology for Health Related Sciences

MICR 201 Microbiology for Health Related Sciences

Microbiology- a clinical approach by Anthony Strelkauskas et al. 2010

Chapter 19: Antibiotics

Antibiotics have changed the face of health care.

They have drastically reduced the number of deaths due to infection.

Infectious disease are among the few diseases we can actually cure with drugs.

Why is this chapter important?

Map for chapter 19

Original term was used for antibacterial compounds produced by living microorganisms.

These were products of secondary metabolic pathways and served to defend the producer against other microbes in the environment.

Some of them were chemically modified. Chemotherapeutics were entirely chemically

synthesized compounds. Today, the term “antibiotics” many times

used for any drug inhibiting bacteria.

What is an antibiotic?

Bacterial and fungal natural antibiotic producers

Historical perspectives No major discoveries of

natural antibiotic substances have occurred for several years.

Efforts have now shifted to modifying existing antibiotics.

Searching in new places for potential antibiotics has also gained in prominence.

The original natural molecules used by humans as antibiotics have a very narrow spectrum.◦ Penicillin activity is typically against Gram-

positive bacteria. Natural molecules can be chemically

modified making it possible to broaden their spectrum.

Antibiotics are classified as either broad-spectrum or narrow-spectrum.

Broad spectrum antibiotics are active against a wide range of bacteria including Gram-positive and Gram-negative bacteria.◦ Ciprofloxacin is a broad spectrum antibiotic.

Antibiotic spectra

The ideal antibiotic should target structures and processes only found in the pathogen.

A fundamental criterion of antibiotics for medical use is selective toxicity.◦ The antibiotic should be destructive to the disease-

causing organism but have no effect on the human host.

The first antibiotic discovered, penicilin, showed high selective toxicity.

Many antibiotic molecules are toxic if administered at high concentrations.

Toxicity necessitates extensive testing. It can take years and cost millions of dollars.

Antibiotic targets

Antibiotic targets

, rifampin

The bacterial cell wall The bacterial plasma

membrane Synthesis of bacterial

proteins Bacterial nucleic acids Bacterial metabolism

The bacterial cell wall is built by many enzymatic reactions.◦ These enzymes can be used as targets of antibiotic

molecules. The cell wall is made up of the peptidoglycan

molecules NAG and NAM.◦ They are cross-linked through activity of

transglycosylase and transpeptidase enzymes.◦ Many antibiotics inhibit the activity of these two

enzymes. Results in improper cell wall cross-linking Organism not able to withstand cell osmotic pressure.

◦ Some antibiotics inhibit the production of peptidoglycan precursors molecules and their transport across the cell membrane.

Bacterial cell wall

Penicillin-binding proteins (PBPs) are involved in the construction of the cell wall.

Penicillins contain a β-lactam ring which binds to these proteins.

New cell wall continuously built during active growth◦ Penicillin prevents the

formation of an intact cell wall.

◦ Penicillin is most effective during this phase.

Bacterial cell wall: Penicillins

http://en.wikipedia.org/wiki/Beta-lactam_antibiotic

Bacterial cell wall: Penicillins

All forms of penicillin contain the b-lactam ring.

Chemically changing the side chain can change:

Antimicrobial activityResistance to stomach acidOverall half-life in bodyMore than 50 derivatives

Cephalosporins have similar activity to penicillins.◦ They prevent the construction of a stable cell wall.

Cephalosporins have a much greater affect on Gram-negative bacteria than penicillins because they can enter the porines of Gram-negative bacteria.◦ They are naturally broad spectrum antibiotics.◦ They are not susceptible to some of the β-lactamase

enzymes. Cephalosporins are frequently used both

preoperatively and postoperatively. Frequent use has increased resistance.

Bacterial cell wall: Cephalosporins

Glycopeptide antibiotics are derived from Streptomyces organisms.◦ Vancomycin is a glycopeptide antibiotic.

Glycopeptide antibiotics have serious side effects.◦ Toxicity level reduced in recent years by improving

purification. They inhibit cell wall synthesis by forming a

complex with the substrates that make up peptidoglycan.

They cannot penetrate the porins of Gram-negative cells.◦ Narrow spectrum antibiotics restricted to Gram-positive

bacteria. Vancomycin used to be the drug of choice for

MRSA. However, there are now vancomycin resistant S. aureus strains (VRSA) described.

Bacterial cell wall:Glycopeptide antibiotics

These antibiotics target mycolic acids and are used against Mycobacterium tuberculosis.◦ Their cell walls are modified

by incorporation of mycolic acids.

Isoniazid inhibits the synthesis of mycolic acid.

Ethambutol inhibits the incorporation of mycolic acids into the cell wall.

Used in combination (plus rifampin) for Tb therapy.

Bacterial cell wall:Isoniazid and Ethambutol for Tb

The plasma membrane is involved with important physiological functions.◦ It is a prime target for antibiotics.◦ Any disruption of the membrane destroys the bacteria.

Unfortunately the structure of the bacterial plasma membrane is similar to the eukaryotic plasma membrane.

This does not allow for selective toxicity. One of the few antibiotics that target the

bacterial cell membrane is polymyxin B. Polymyxin B acts like a detergent.

Bacterial plasma membrane

Ribosomes site of protein synthesis

Binding of mRNA, tRNA, decoding, peptide elongation, detachment

Disruption in production of protein devastating to cell.

Ribosomes of prokaryotes are not the same as eukaryotes.

70S with 30S+50S This allows for selective

toxicity. Eukaryotic cells have

mitochondria ribosomes similar to prokaryotes.

There is antibiotic interference in eukaryotic cell function if antibiotics given in excessive amounts.

Synthesis of bacterial proteins

Aminoglycosides◦ Gentamicin,

tobramycin, spectinomycin, kanamycin, streptomycin

Tetracyclines◦ Tetracycline,

doxycycline

Macrolide ◦ Erythromycin,

clarithromycin, azithromycin

Lincosamide◦ Clindamycin

Amphenicols◦ Chloramphenicol

Oxazolidinone ◦ Linezolid

Streptogramins◦ Pristanamycin,

streptogramin

Antibiotics that interfere with protein synthesis

VRE

Streptomyces species make pristinamycin and streptogramin.◦ They work synergistically to inhibit translation at

the 50S subunit.◦ Both have been synthetically modified.◦ They make up the antibiotic Synercid®.◦ Approved for the treatment of vancomycin-

resistant enterococci (VRE).

Antibiotics that interfere with protein synthesis: Synercid®

DNA and RNA are universal components.◦ Their structure in bacteria is no different from their structure

in humans.◦ This does not allow for selective toxicity.

Two families of synthetic compounds can target bacterial nucleic acids.◦ Rifamycins including rifampin inhibit transcription.

Used for Tb treatment◦ Quinolones including ciprofloxacin target bacterial

topoisomerase gyrase and block DNA replication and DNA repair. Used for urinary tract infections, osteomyelitis, community-

acquired pneumonia and gastroenteritis, and anthrax

Bacterial nucleic acids

Two targets for inhibiting bacterial growth:◦ Metabolic pathways at

the plasma membrane.◦ Production of nucleic

acid precursors; folic acid from para-aminobenzoic acid (PABA)

◦ Humans consume folic acid with diet

Sulfa drugs block folic acid synthesis

Sulfa drugs use longer any antibacterial agent.

Sulfamethoxazole used in combination with trimethoprim; treat urinary tract infections.

Bacterial metabolism

Viruses pose a different set of problems for antibiotic therapy.◦ They are obligate intracellular parasites and use

cell machinery for replication.◦ Drugs that can eliminate the virus are dangerous

to non-infected cells.◦ This makes selective toxicity difficult.

Many viruses difficult to grow.◦ It is difficult to test potential antiviral drugs.

Antiviral drugs

Many acute viral infections have a short duration.

The lack of rapid tests means it is difficult to differentiate between various viral infections.

Successful antiviral drugs must eliminate all virions.

The escape of even one virion could restart the infectious cycle.

Antiviral drugs

Inhibition of uncoating◦ Amantadine for Influenza A virus infections

Nucleoside analogues block and terminate viral nucleic acid replication ◦ Acyclovir (and Valtrex® and Famvir® ): Herpes

simplex virus (HSV) infections◦ Gancyclovir: Cytomegalovirus (CMV) infections ◦ Ribavirin: Lassa fever, Hantavirus infections, also

Hepatitis C (in combination with INFa) Nonnucleoside polymerase inhibitors

◦ Foscarnet: HSV and CMV resistant to nucleoside analogues

Neuraminidase inhibitor◦ Zanamivir: Influenza virus

Antiviral drugs

All anti-HIV drugs have numerous side effects Common side effects include nausea, vomiting, headache, fatigue,

weakness, and/or muscle pain. Other side effects included changes in body fat distribution

(lipodystrophy), inflammation, insomnia, and kidney disorders.

Anti-HIV drugs

Examples for lipodystrophy in AIDS patients

http://drugster.info/medic/term/lipodystrophy-syndrome/

Reverse transcriptase inhibitors◦ Nucleoside analogues such as AZT (azidothymidine)

Prevent successful synthesis of cDNA by RT◦ Non-nucleoside analogue RT inhibitors

Bind non-competitively to RT to block its polymerization function

◦ Nucleotide analogues Work similar to the nucleoside analogues

Protease inhibitors◦ Prevent cleavage of polyproteins (by viral protease)

required to make mature virus Integrase inhibitors Fusion inhibitors

Anti-HIV drugs

Combination therapy – helps prevent the development of resistant strains

HAART - highly active anti-retroviral therapy (3 or more different classes of drugs in combination)◦ Serious long-term side effects

accumulation of lactic acid in the bloodstream physical and metabolic changes that cause changes in

fat distribution cholesterol and glucose abnormalities that can lead to a

risk of heart disease.◦ Long-term use of the drugs can also promote

the development of drug-resistant strains of HIV.

HAART

Survival rates with HAART

http://emedicine.medscape.com/article/1533218-overview

The emergence of diseases that render a host immunocompromised has led to increased secondary fungal infections.

Drugs used for fungal infection have selective toxicity issues.◦ Fungi are eukaryotes.◦ Attacking common targets can

cause serious side effects. Major antifungal drugs

◦ Griseofulvin◦ Polyenes (Amphotericin B)◦ Azoles (clotrimazole, miconazole,

ketoconazole, fluconazole) Other antifungal drugs

◦ Flucytosine◦ Pentamidine

Antifungal drugs

Antifungal drugs:Griseofulvin

Griseofulvin is produced by a species of the fungus Penicillium.

Administered orally, effective superficial fungal infections.

React with keratin. Blocks formation of

microtubules. Inhibits mitosis in fungi.

http://www.myfootshop.com/detail.asp?condition=onychomycosis

Polyenes are produced by the soil bacterium Streptomyces.

They interact with sterols and increase the permeability of the plasma membrane.

They must be used with caution because of side effects.

Amphotericin B has high renal toxicity.◦ Serum levels are closely monitored

Antifungal drugs:Polyenes

Azoles inhibit the production of sterols.

Clotrimazole and miconazole◦ Sold without a prescription◦ Routinely used topically against

athlete’s foot and vaginal yeast infection

Ketoconazole ◦ Is a broad spectrum derivative for

systemic fungal infections◦ Can be taken orally◦ Less toxic than amphotericin B

Fluconazole and itraconazole ◦ Are the least toxic azoles◦ Widely used for systemic fungal

infections

Antifungal drugs: Azoles

Flucytosine interferes with DNA and RNA synthesis.◦ Taken up preferentially by fungi.◦ High level of toxicity in kidney and bone marrow

Pentamidine bind to fungal DNA.◦ Used in treatment of Pneumocystis pneumonia

Antifungal drugs:Other antifungal drugs

http://www.health-pic.com/pneumocystis-carinii-pneumonia-in-hiv/

The development of drugs for parasitic infections has lagged behind.◦ Parasitic infections do not occur often in

developed nations.◦ Low profit incentive for corporate drug company

development. Two widely used anti-parasitic drugs are:

◦ Quinine◦ Metronidazole

Drugs for protozoa

Quinine has been used as a treatment for malaria since the 1600s

It has been chemically modified into several synthetic forms.

Chloroquine has been widely used.

Drugs for protozoa:Quinine

Metronidazole is one of the most widely used anti-protozoan drugs.◦ Sold under the name

Flagyl® It is the drug of choice

for: ◦ Vaginitis resulting from

Trichomonas vaginalis◦ Giardiasis ◦ Amebic dysentery.

It interferes with anaerobic metabolism.

Drugs for protozoa:Metronidazole

Trichomonas vaginalis

Anti-helminthic drugs have also been largely ignored until recently.◦ Affected populations were not found in developed

countries.◦ The popularity of sushi has led to an increase in

tapeworm infestations.◦ Increased world travel has also increased helminth

infections. Important drugs are

◦ Niclosamide◦ Praziquantel◦ Mebendazole◦ Ivermectin

Drugs for helminths

Niclosamide is the choice of treatment for these infections.◦ Inhibits the production of ATP

Praziquantel is also the drug of choice for fluke diseases.◦ Increases the permeability of plasma membranes

for Ca+2.◦ It induces muscle spasms in the worm and

dislodges the worm.◦ This exposes antigenic sites for attack by the host

immune system.

Drugs for helminths:Tapeworm and flukes

Mebendazole is used against these infections.◦ It disrupts microtubule

formation.◦ This affects the motility

of the worm. Ivermectin is also

used.◦ It paralyzes the worm.◦ This induces the worm to

exit the body.

Drugs for helminths:Pinworm and Ascariasis

http://microbeworld.org/images/stories/twip/ascaris_from_child.jpg

There have been no new discoveries of natural antibiotics in decades.

Microorganisms produce toxic chemicals as part of their natural defense.

Antibiotics can be broad or narrow spectrum. Chemical modification of natural antibiotics can broaden their

spectrum. Some bacteria produce an enzyme called beta-lactamase that

inhibits the reactivity of penicillin. Antibiotics must be selectively toxic but most antibiotics will

have side effects.

Chapter 19 key concepts

The five prime targets for antibiotics are the cell wall, the plasma membrane, the ribosome, nucleic acids, and metabolic synthesis pathways.

Viruses present problems for antibiotic treatment because they are obligate intracellular parasites and use the host machinery for replication.

Antifungal drugs have serious side effects because fungi are eukaryotic cells.

The development of new drugs is an expensive and time-consuming process.

Drugs against parasitic protozoans and helminths have been slow to be developed because these infections occur mostly in developing countries.

Chapter 19 key concepts