1. PHT 226 Lab no 9

2. MIC: 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 the test organism.

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

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

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

6. 1- Serious infections e.g., infective endocarditis. 2- Slow-growing organisms e.g., Mycobacterium tuberculosis.

7. 1. Serial dilution method 2. Agar diffusion method 3. E- test

8.  Materials:  Staph. aureus  Cefoxitin 80 µ g ⁄ml  Double strength nutrient broth (DSNB)  7 sterile test tubes  1 sterile pipette 5 ml  2 sterile pipette 1 ml

9.  Procedure: DSNB 1ml 12 3 4567 AB 1ml 1ml reject S o.1 ml AB MO -ve Control +ve Control Incubate all tubes at 37°c for 24h

10.  Result: Tube#1234567 AB conc Turbidity 40 20 10 5 2.5 1.25 - 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 =