Transcript MIC

PHT 416
Lab no 10
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.
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.
Susceptible organisms :
are those with MICs at/below the breakpoint.
Resistant organisms:
are those with MICs above the breakpoint.
INDICATIONS FOR DETERMINING
THE MIC
1- Serious infections e.g., infective endocarditis.
2- Slow-growing organisms e.g., Mycobacterium
tuberculosis.
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: 






Staph. aureus
Cefoxitin 80µg ⁄ml
Double strength nutrient broth (DSNB)
7 sterile test tubes
1 sterile pipette 5 ml
2 sterile pipette 1 ml
Procedure: 
1ml reject
1ml
1ml
1ml
1ml
1ml
1ml
AB
MO
1ml
1
DSNB
2
3
4
5
6
7
AB
-ve Control
S
Incubate all tubes at 37°c for 24h
+ve Control
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
40
20
10
5
2.5
1.25
-
Turbidity
-ve
-ve
-ve
+ve
+ve
+ve
+ve
No
bacteria
No AB
conc of the last tube clear + conc of first tube turbid
MIC =
2
SENSITIVITY TESTING
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].

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.


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
The
recommended medium →
Mueller Hinton agar
** The pH of the medium ( 7.2-7.4)
**5% defibrinated sheep blood is added to the
medium for certain fastidious organisms.
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).
SENSITIVITY TESTING

Materials:






Cultures of E. coli.
Filter paper disc impregnated in antibiotic
solutions.
Different antibiotic solutions.
Muller Hinton agar plate.
Loop.
Forceps.
Procedure:
m.o
Az
45°c
24 h
at 37°c
Don’t invert
Aug
Cef
Az
Van
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
SENSITIVITY TESTING
 Results:
( in mm)
Mo
AB
1
2
3
4
E
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