PHT 381PHT 381 Lab # 8Lab # 8

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 ==