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.
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MIC:MIC: It is the It is the lowest concentration of the lowest concentration of the
antimicrobial agent that inhibits the antimicrobial agent that inhibits the growth of the test organismgrowth of the test organism but not but not necessarily kills it.necessarily kills it.
MBCMBC (minimum bactericidal (minimum bactericidal conc.):conc.):
It is the It is the lowest concentration of the lowest concentration of the antimicrobial agent that killsantimicrobial agent that kills
the test organism.the test organism.
Bactericidal drugsBactericidal drugs have MBC have MBC 2-4 times2-4 times MIC. MIC.
Bacteriostatic drugsBacteriostatic drugs have MBC have MBC much higher thanmuch higher than MIC. MIC.
The breakpoint The breakpoint of of an antimicrobial an antimicrobial agent:agent:
It is the It is the concentration of the concentration of the drugdrug which can be which can be achieved in achieved in the serumthe serum with with optimal dose.optimal dose.
Susceptible organisms :Susceptible organisms : are those with are those with MICsMICs at/below at/below the the
breakpointbreakpoint..
Resistant organisms:Resistant organisms: are those with are those with MICs MICs aboveabove the the
breakpoint.breakpoint.
1- Serious infections 1- Serious infections e.g., infective e.g., infective endocarditis.endocarditis.
2- Slow-growing organisms 2- Slow-growing organisms e.g., e.g., Mycobacterium tuberculosis.Mycobacterium tuberculosis.
Indications for determining Indications for determining the MICthe MIC
Methods for determination Methods for determination of MICof MIC
1. 1. Serial dilutionSerial dilution method method
2. 2. Agar diffusionAgar diffusion method method
3. 3. E- testE- test
DeterminationDetermination of MIC by the serial of MIC by the serial dilution methoddilution method
Materials:Materials:• Staph. aureusStaph. aureus• Cefoxitin 40µg ⁄mlCefoxitin 40µg ⁄ml• Double strength nutrient broth Double strength nutrient broth
(DSNB)(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
1ml1 2 3 4 5 6 7
AB
1ml
1ml 1ml 1ml 1ml 1ml
1ml reject
S
o.1
ml
ABMO
-ve Control +ve Control
Incubate all tubes at 37°c for 24h
Determination of MIC by the Determination of MIC by the serial dilution methodserial dilution method
Result:Result:
Tube# 1 2 3 4 5 6 7
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 turbidconc of the last tube clear + conc of first tube turbid
2 2 MIC MIC ==
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