Understanding Antibiotic Susceptibility Testing in Medical Microbiology Lab

Antibiotics are chemical substances produced by microorganisms to inhibit or kill other microorganisms. In the lab, antibiotics susceptibility testing is conducted using the Disc Diffusion method (Kirby-Bauer method) on agar plates. The sensitivity of bacteria to antibiotics is determined by measuring the zones of inhibition around the discs. Factors affecting the testing results include the nature of the medium, depth of the medium, density of inoculum, type of microorganisms, and incubation period.

• Antibiotics
• Susceptibility Testing
• Medical Microbiology
• Disc Diffusion Method
• Bacteria

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1. Al-Mustaqbal University college 2 st Class, 1 st Semester Department of pharmacy Medical microbiology Lab 5: Antibiotics susceptibility testing. Asst. Lec. Mariam A. Ali

2. Antibiotics a chemical substance consider as secondary metabolism product produced by microorganism at stationary phase and able to inhibit or kill another microorganism

3. Mode of action of antibiotic on bacteria

4. Antibiotics susceptibility Antibiotics susceptibility testing testing

5. Diffusion methods of sensitivity testing Disc diffusion methods (Kirby-Bauer method) The test depends on the inoculation of the bacteria under test on a solid medium in the petri dish and use a blotting paper disc containing antimicrobials with different concentrations and placed on the surface of the fertilized medium

6. During the incubation period, the antibody spreads from the disc to the center. If it is sensitive to the antibody, areas of non-growth around the disc appear. The greater the sensitivity, the greater the diameter of the inhibition.

7. Disc diffusion methods (Kirby-Bauer method) procedure 1- attended Muller-Hinton agar media. 2-Wipe the medium heavily with bacteria you want to test under sterile conditions (streaking, swabbing). 3-Using sterile forceps, antimicrobial tablets are placed on the surface with equal distances (6 tablets per medium) 4- incubation of dishes for the period 18-24 hr at 370C 5-Read the inhibition zones around each disc in millimeters and record the results

9. Factors Affecting the Reading of Antibiotics susceptibility Test Results The nature of medium Depth of medium Density of inoculum The disc Incubation period Type pf microorganisms

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