1. PHT 416
Lab no 10
Minimum
Inhibitory
Concentration
[MIC]

2. MBC(minimum bactericidal conc.):
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
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. Indications for determining the MIC
1- Serious infections e.g., infective endocarditis. 2- Slow-growing organisms e.g., Mycobacterium tuberculosis.

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

8. Determination of MIC by the serial dilution method
Materials:
Staph. aureus
Cefoxitin 80µg ⁄ml
Double strength nutrient broth (DSNB)
7 sterile test tubes
1 sterile pipette 5 ml
2 sterile pipette 1 ml

9. Procedure: o.1 ml S Incubate all tubes at 37°c for 24h 1ml reject 1mlAB MO
1ml 3 1 2 4 5 6 7 AB
DSNB
-ve Control
+ve Control
o.1 ml S
Incubate all tubes at 37°c for 24h

10. Determination of MIC by the serial dilution method
If Original Antibiotic Conc. = 80 µg/ml
Result:
Tube# 1 2 3 4 5 6 7
AB conc -
Turbidity
-ve ve ve ve ve ve ve No
bacteria
No AB
conc of the last tube clear + conc of first tube turbid 2
MIC =

11. Sensitivity Testing

12. Sensitivity Testing
Sensitivity testing is used to determine the susceptibility of the microorganism to various antimicrobial agents.
It can be done by :
Plate diffusion method using disc.
Minimum inhibitory concentration [MIC].

13. Sensitivity Testing
Antibiotic sensitivity testing by the disc diffusion method:
Principle;
Rapid
Accurate
Inexpensive
The diameter of the resulting zones of inhibition that surrounds a disc that has been impregnated with a specific concentration of the agent will be a measure of the effectiveness of the antibiotics on the microorganism.

14. Sensitivity Testing There are two types of antibiotic:-
Narrow spectrum
active against Gram +ve only or Gram –ve only
Broad spectrum antibiotic
active against both types of bacteria

15. The recommended medium →
Mueller Hinton agar
** The pH of the medium ( )
**5% defibrinated sheep blood is added to the medium for certain fastidious organisms.

16. “0.5 McFarland” (a barium sulphate standard)
The inoculum:
The turbidity of a broth culture / saline suspension of the test organism has to match a defined standard
“0.5 McFarland” (a barium sulphate standard)
( the matched inoculum should give confluent growth).

17. Sensitivity Testing Materials: Cultures of E. coli.
Filter paper disc impregnated in antibiotic solutions.
Different antibiotic solutions.
Muller Hinton agar plate.
Loop.
Forceps.

18. Procedure: incubate Don’t invert 0.2ml inoculate 45°c Aug Cef 24 h
m.o
0.2ml Az
inoculate
45°c
Aug
Cef
incubate
24 h
at 37°c Az
Van
Don’t invert

19. Results: Measure the diameter of each inhibition zone
* The diameter of the inhibition zones are directly proportional to the susceptibility of the microorganism to the antibiotics.
Van
Aug Az
Cef

20. Sensitivity Testing
Results: ( in mm) Mo AB E 1 2 3 4

21. Thank you