Antimicrobial Antimicrobial AgentsAgents

Use when balance tips Use when balance tips in favor of invading MOin favor of invading MO

Antimicrobial Therapy When balance between MO and

host tilts in direction of MO, body’s normal defense cannot prevent or overcome disease

Turn to Chemotherapy - treatment of disease with chemical drugs into body

Chemotherapeutic Agents

Antimicrobial - to treat infectious disease, act within host

Antibiotic - produced naturally by MO (bacteria, fungi)

Synthetic drug – synthesized, made in laboratory

Successful Antimicrobial

Selective toxicity - harm MO not host (all drugs have some side-effects)

No hypersensitivity reaction – does not elicit harmful host immune reaction

Penetrate - gets to site of tissue infection rapidly, retain for adequate time

No resistance - MO not readily able to counteract it

Activity of Antimicrobial

Easier to find against prokaryote as different from eukaryotic cell

Fungi, protozoan, helminth are eukaryotes; make finding drug with selective toxicity more difficult

Especially difficult to find drug against virus, require host cell to replicate

Spectrum of Antibiotics Narrow spectrum - affects relatively

few kinds of bacteria Broad spectrum - effective against

large number Gram(+) & Gram(-) bacteria

Problem of broad spectrum antibiotic use is NF destroyed, allow certain NF to flourish and cause opportunistic infection

Superinfection - overgrowth of NF due to antibiotic treatment for an initial infection

Action of Antimicrobial Bacteriocidal - kill bacteria Bacteriostatic - prevent growth of

bacteria; host’s defense of phagocytosis and antibody eliminate bacteria

Different areas in bacteria serve as target for action of antimicrobial: Cell wall Ribosome Plasma membrane DNA, RNA Metabolite

Bacterial Cell Wall Cross Linking Interference with synthesis of bacterial cell

wall should not harm host Bacterial cell wall contain peptidoglycan

not found in eukaryotic cell Many antibiotics prevent synthesis of

peptidoglycan by interfering with linkage by peptide cross-bridge

Inhibition of Bacteria Cell Wall Biosynthesis: Lactam

Ring These antibiotics contain beta lactam

ring that bind to group of bacterial enzymes called penicillin binding proteins (PBP)

PBP involved in peptidoglycan cell wall synthesis

Binding of PBP prevents peptide cross linking, cell wall weakened, bacteria undergoes lysis

Beta Lactam Ring Antibiotics

Affect cell wall synthesis, only effective on actively growing MO

These antibiotics include: Penicillin and derivatives (ampicillin,

methacillin, oxacillin, amoxacillin, augmentin)

Cephalosporin (cephalothin, cefuroxime, ceftazidime, cefoxitin)

Carbapenem (imipenem) Monobactam (aztrenam)

Inhibition of Bacterial Cell Wall Biosynthesis: Others

Bacitracin - interferes with synthesis of peptidoglycan by inhibiting recycling of metabolites

Vancomycin - binds to precursors used in cell wall synthesis; interfere with enzymes that incorporate these precursors into growing cell wall

Inhibition of mRNA Translation

Protein synthesis common feature of all cells

Ribosome structure of eukaryote and prokaryote cell differ (80S vs 70S)

Many antimicrobials specifically interfere with mRNA protein synthesis on prokaryotic 70S ribosomes

Some antimicrobials act on 50S subunit of the ribosome, while others act on 30S subunit of ribosome

Inhibition of Bacteria Translation Chloramphenicol - acts

at 50S, inhibit formation of peptide bond

Erythromycin - acts at 50S, prevent translocation movement of ribosome

Tetracycline - acts at 30S, interfere with tRNA attachment

Aminoglycosides (gentamycin, streptomycin) - act at 30S, cause misreading of mRNA

Injury to Bacteria Plasma Membrane

Polypeptide antimicrobials Polymyxin B Colistin Affect permeability of cells Result in leakage of macromolecules

and ions essential for cell survival

Inhibition of Bacteria DNA/RNA Synthesis

Ciprofloxacin (fluoroquinolone) - bind and interfere with DNA gyrase involved in DNA supercoiling

Metronidazole - breaks DNA strand Rifampin - binds to DNA dependent-

RNA polymerase to inhibit mRNA synthesis

Inhibition Bacteria Folate Synthesis

Antimetabolite - closely resemble normal substrate (analogue), competes for enzyme

Both sulfonamide and trimethoprim interfere with folic acid pathway

Often in single pill used in combination drug therapy: Trimethoprim-Sulfamethoxazole (TMP-SMX, Bactrim)

Broad spectrum antimicrobial

Inhibition of Bacteria Enzymatic Activity

Nitrofurantoin - targets synthesis of several bacterial enzymes and proteins; may also directly damage DNA

Isoniazid - structural analogue of vitamin B6; inhibits synthesis of mycolic acid of Mycobacteria cell wall

Ethambutol - inhibits incorporation of mycolic acid into Mycobacteria cell wall

Summary: Bacteria Antimicrobial

Antifungal Drugs Nystatin and amphotericin B combine with

sterols to disrupt fungal plasma membrane Effective because animal sterols are mostly

cholesterol while fungal membranes contain mainly ergosterol against which the drugs target

Ketoconazole (imadazole) - interfere with sterol synthesis

Griseofulvin - binds to keratin on skin, hair, and nails; interferes with mitosis and fungal reproduction

Antiviral Drugs: Nucleoside Analogue In viral nucleic

acid, analogue insert in place of normal nucleoside

Nucleic acid synthesis stops

Nucleoside analogue binds more strongly with viral enzyme than host cell enzyme

Example: acyclovir for herpes virus; also several nucleoside analogues for HIV infection

Other Antiviral Drugs Interferon – protein made by host cell for

first line of antiviral defense; cloned by recombinant DNA technology, treatment for severe and chronic virus infections

Tamiflu, Relenza – interfere with release of influenza virus from host cell

Protease inhibitors – interfere with proteolytic cleavage of HIV polyproteins into individual proteins, stops replication process

Anti-sense or siRNA (small, interfering RNA) – experimental antiviral drugs, inhibits mRNA translation

Antimicrobial Susceptibility Testing

Important as different MO species and strains have different degree of susceptibility to different antimicrobials

Susceptibility of MO to antimicrobial may change with time, even during course of antimicrobial therapy

Drug Sensitivity Test:Diffusion Test

• Kirby-Bauer Test – standardized lab test with antibiotic impregnated disk, diffuses out in a concentration gradient, measure zone of inhibited bacterial growth

• E Test – utilizes plastic coated strip containing gradient of antibiotic that diffuses out, allows estimate Minimal Inhibitory concentration (MIC) that prevents visible bacterial growth

Drug Sensitivity Test:Test Tube Dilution

• Broth Dilution Test – measures more accurately serial antibiotic dilutions in broth test tube for MIC, followed by plating for Minimal Bacteriocidal Concentration (MBC)

Antibiotic Resistance Presently a common occurrence Bacterial drug resistance requires

interruption or disturbance of the steps for antimicrobial action

Antibiotic Resistance Intrinsic resistance - normal genetic,

structural, or physiologic state of MO; considered natural and inherited characteristic associated with majority of strains of bacterial group

Acquired resistance - altered cellular physiology and structure caused by changes in a MO genetic makeup; may be a trait associated with only some strains of bacterial group

Acquired Antibiotic Resistance

Acquisition of genes from other MOs via gene transfer mechanisms (i.e., resistance plasmids)

A combination of mutational and gene transfer events

Pathways of Antibiotic Resistance

Enzymatic Degradation: Penicillinase

Resistance to penicillin and other beta-lactam antibiotics

Production of an enzyme that breaks beta-lactam ring

Gram(+) MO Resistance to Beta-lactam Antibiotics

Enzymatic degradation – MO produces beta-lactamase, cleaves ring structure of antibiotic

Altered antimicrobial target – MO mutation of penicllin binding proteins (PBP) so antibiotic no longer binds to it

Gram(-) Resistance to Beta-lactam Antibiotics

Decrease uptake - of antibiotic Enzymatic degradation – of antibiotic Altered antimicrobial target - PBP

Dissemination of Antimicrobial Resistance

Prevention of Bacterial Antimicrobial Resistance

Use antimicrobial drugs only when necessary

Finish prescribed course of antimicrobial Use drugs in combination; microbe less

likely to develop resistance to two drugs at the same time:

Consider synergistic effects Consider antagonistic effects