Micro-Chpt 12 and 13 Susceptibility
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What are the factors to consider in susceptibility testing?
Body site from which organism was isolated
What is considered to be contamination in a specimen?
More than 2 organisms greater than 105 CFU/ml. (espcially in urines).
What are the three groups for susceptibility reporiting?
- Group A - Primary agents
- Group B-secondary agents
- Group C - Very broad spectrum
When would you report group B agents?
- When isolate is resistant to primary agents (group A).
- Patient can't tolerate primary agents.
What is MIC?
Minimal inhibitory concentration. Lowest concentration of antimicrobial agent required to inhibit the growth of a bacterial isoate.
What is MBC?
Minimum bactericidal concentration. Antimicrobial agent required to kill 99.9% of bacterial isolate.
How should antimicrobials stock solutions be stored?
Only stored in nonfrost free freezers. Never thaw and refreeze.
What is Broth macro diltion test good for?
Good for fastidious organisms with high growth requirements.
What is Broth microdiltution good for?
Good for large batches of susceptibilty.
What are Breakpoint MIC panels?
It is a variation of microdilution panel where only one or a few concentrations of each antimicrobial agent is tested on a panel.
What are two antibiotic targets?
- Cell wall-interfere or disrupt cell integrity
- metabolic functions -interrupt basic metabolic functions such as protein synth. nucleic acid metabolism, and inhibition of essential metabolites.
Give example of antimicrobials that inhibit cell wall synthesis?
Beta lactam drugs-Penicillins, cephalosporins, cabapenems, monobactams
What is vancomycin?
- It is a antibacteril drug that inhibits cell wall biosynthesis , a narrow spectrum antibiotic
- inhibits formation of glycan chains
What is vancomycin useful against?
Staphylococcus, streptococcus, and enetrococci.
What antimicrobials inhibit folate synthesis?
sulfonamides and trimethoprim
What do sulfonamides inhibit and how do they work?
Inhibit gram positive and gram negative through competitive inhibition of enzyme that aids in production of folic acid.
Resistance to Sulfonamides is commonly due to?
What is synergy?
Combined effect is greather than the additive effect. Enhanced acitivity.
What kind of drugs interfere with DNA replication (synthesis)?
Nalidixic acid and flurinated quinolones.
What does rifampin interfere with?
- Interferes with the production of mRNA.
- Blocks RNA polymerase.
What antimicrobials interfere with DNA Translation?
- Aminoglycosides-bind to 30S subunit
- Tetracyclines,doxycycline, minocycline
- Etrythromicin, azithromycin-bind to 50Sunit
- lincosamide, clindamycin
What kind of drugs target protein synthesis?
- Oxazolidinone-bind 50S,
- Streptogramin-bind 50S,
- Both effective against gram + that are resistant to B lactams and vancomycin.
- Interfere with initiating protein synth.
Glycylcycline-(tigecycline)bind 30s prevent elongation of peptide chains.
What is efflux?
Increase secretion of drug.
What are types of intrinsic mechanisms of resistance?
- Enzymatic inactivation
What are origins of antibiotic resistance?
- Intrinsic-found naturally
- Acquired- through exogenous DNA as in plasmids, conjugation, transposons, bacteriophage etc.
What do tetracyclines inhibit?
Inhibit rotation of bound tRNA into the A site and interfere with DNA translation.
What are types of acquired mechanisms of resistance?
- Efflux pumps
- Target site of modification
- Enzymatic inactivation
- Horizontal transfer-plasmids
- Transposons- Insertion of sequences to confer resistance.
Name antimicrobials that inhibit at 30S?
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