23. Laboratory Aspects of Antimicrobial Agents

23. Laboratory Aspects of Antimicrobial Agents -...

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Unformatted text preview: Antimicrobial Susceptibility Testing Terminology / Definitions: Bacteriostatic - agent (drug) that inhibits but does not kill an organism Bacteriocidal - agent (drug) that kills an organism Minimum inhibitory concentration (MIC) - lowest concentration of antibiotic that inhibits visible growth Minimum bacteriocidal concentration (MBC) lowest concentration of antibiotic that kills 99.9% of the inoculum Laboratory Aspects of Antimicrobial Agents Antimicrobial Susceptibility Testing Terminology / Definitions: Serum inhibitory titre - dilution of serum that inhibits visible growth Serum bacteriocidal titre - dilution of serum that kills 99.9% of the inoculum Antimicrobial levels - concentration (g ml) ( of antibiotic in serum Criteria for Effective Antibiotic 1. Microorganism - unique target 2. Antimicrobial Agent - penetrate bacterial surface - reach infected tissue - penetrate mammalian cells 3. Host - intact immune system - blood supply and drainage Criteria for Effective Antibiotic Antibiotics rarely kill all the bacteria present, they just alter the balance of the host/pathogen interaction in favor of the host. In the absence of an adequate immune response antibiotics are of little use AIDS Very ill ("complications") Transplants Etc. General Principles of Effective Antibacterial Therapy - based on susceptibility tests, epidemiologic data - empiric treatment for lifelifethreatening infections; usually broad spectrum - collect samples before therapy 2. Prompt therapy - low bacterial count - prevents super-infection super1. Selective use General Principles of Effective Antibacterial Therapy 3. Special situations - bactericidal agents - drain or remove obstruction 4. Proper use - dose - duration - route Antimicrobial Susceptibility Testing Guide for the clinician; not a guarantee that an antimicrobial agent will be effective in therapy Factors which may affect correlation between laboratory results and clinical outcome include: pH - test at physiologic pH (7.2 to 7.4) cations - Mg2+, etc. inoculum - amount of bacteria tested clinical pharmacology - intracellular concentration, CSF, half life, etc. Antimicrobial Susceptibility Testing If antimicrobial testing is not done according to an accepted protocol (e.g. NCCLS) then the results are meaningless! Even when performed correctly many other factors (partial immunity, general health, etc) may determine the outcome. Antimicrobial Susceptibility Testing Indications: rapidly growing bacteria with unpredictable susceptibility clinically significant bacteria epidemiologic purposes only for bacteria where standardized methods have been established Antimicrobial Susceptibility Testing Methods: Disk diffusion (Kirby-Bauer test) (Kirby Broth dilution test Macrobroth Microbroth Commercial systems Agar dilution Beta-lactamase test Beta- Antimicrobial Susceptibility Testing Results Reported as: Susceptible (S) Intermediate (I) Resistant (R) Selective antibiotics reported only Reflex rules assume that the physician will rules only read the report until they find an antibiotic that will work Antimicrobial Resistance Antimicrobial Resistance 1. Intrinsic - usually related to structural features (e.g. cell wall, lipids) 2. Acquired - mutations that alter binding site of drug transfer of genetic material (eg. Plasmids with enzymes) eg. 4 mechanisms: 1) Production of inactivating enzyme(s) eg. -lactamases, aminoglycoside eg. lactamases, inactivating enzymes 2) Altered target / receptor eg. Penicillin binding proteins, eg. Ribosomal subunits 3) Decreased permeability eg. aminoglycosides eg. 4) Active export from bacterial cell eg. Tetracycline eg. Combination Antimicrobial Testing Reasons why 2 or more antibiotics may be used to treat a patient: i) Mixed infections ii) Prevention / Delay of bacterial resistance iii) Ability to use nontoxic amounts of 2 or more antibiotics iv) Combination is more effective against an organism or disease syndrome Combination Antimicrobial Testing Interaction between antimicrobial agents include: i) Autonomous / indifferent effect ii) Anatgonistic effect iii) Additive effect iv) Synergistic effect Refers strictly to in vitro effects Not routinely tested (CF patients?) Combination Antimicrobial Testing Synergy may occur when: antibiotics work sequentially or at different sites to inhibit synthesis of bacterial cell components eg. Trimethoprim - sulfamethoxazole eg. uptake of an antibiotic is enhanced by another eg. penicillin plus aminoglycoside eg. one member of the combination inhibits bacterial enzyme activity eg. -lactamase inhibitors eg. (amoxicllin plus clavulanic acid) ...
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