ANTIMICROBIAL/ANTIBIOTIC SUSCEPTIBILITY/SENSITIVITY TESTING
Biology Exam 2025/2026
Antibiotic sensitivity tests are performed in vitro to determine;
 The susceptibility of a given micro-organism to known concentration of the drug
 The potency of an antimicrobial agent
Two types of antimicrobial susceptibility tests
1 DIFFUSION TESTS
2 DILUTION TESTS
DIFFUSION TESTS
The most commonly used diffusion method is Disc diffusion test.
Principle-Filter paper discs are impregnated with known concentration of antimicrobials and
are placed on agar plate that has been inoculated with a culture of test bacterium to be
tested.
Diffusion test methods include;
1 Kirby Bauer method
2 Stokes method
KIRBY BAUER METHOD
Simple and reliable method, routinely employed in laboratory.
Procedure
1 Inoculate the Muller Hinton agar plate with standardized inoculum by streaking the
swab over the entire agar surface.
2 Antibiotic discs are applied firmly to the surface of the agar plate using sterile
forceps.
3 On a plate of 100 mm, seven discs are placed.
4 Control strains of S.aureus (ATCC), E.coli, and P.aeruginosa should also be tested.
Results