PHT 381 Lab # 8

MIC: MIC: It is the lowest concentration of the antimicrobial agent that inhibits the growth of the test organism but not necessarily kills it. It is the lowest concentration of the antimicrobial agent that inhibits the growth of the test organism but not necessarily kills it. MBC (minimum bactericidal conc.): It is the lowest concentration of the antimicrobial agent that kills It is the lowest concentration of the antimicrobial agent that kills the test organism. the test organism.

Bactericidal drugs have MBC 2- 4 times MIC. Bacteriostatic drugs have MBC much higher than MIC.

The breakpoint of an antimicrobial agent: It is the concentration of the drug which can be achieved in the serum with optimal dose. It is the concentration of the drug which can be achieved in the serum with optimal dose.

Susceptible organisms : are those with MICs at/below the breakpoint. are those with MICs at/below the breakpoint. Resistant organisms: are those with MICs above the breakpoint. are those with MICs above the breakpoint.

1- Serious infections e.g., infective endocarditis. 2- Slow-growing organisms e.g., Mycobacterium tuberculosis. Indications for determining the MIC

Methods for determination of MIC 1. Serial dilution method 2. Agar diffusion method 3. E- test

Determination of MIC by the serial dilution method Materials: Materials: Staph. aureusStaph. aureus Cefoxitin 40 µ g ⁄mlCefoxitin 40 µ g ⁄ml Double strength nutrient broth (DSNB)Double strength nutrient broth (DSNB) 7 sterile test tubes7 sterile test tubes 1 sterile pipette 5 ml1 sterile pipette 5 ml 2 sterile pipette 1 ml2 sterile pipette 1 ml

Procedure: Procedure: DSNB 1ml AB 1ml 1ml reject S o.1 ml AB MO -ve Control +ve Control Incubate all tubes at 37°c for 24h

Determination of MIC by the serial dilution method Result: Result: Tube# AB conc Turbidity No AB -ve -ve -ve +ve +ve +ve +ve No bacteria If Original Antibiotic Conc. = 80 µg/ml conc of the last tube clear + conc of first tube turbid conc of the last tube clear + conc of first tube turbid 2 MIC =