Page 168 - MNU-PM502- Pharmaeutical Microbiology Theoritical Book

Pharm D- Clinical Pharmacy Program Third Level Pharmaceutical Microbiology& Antimicrobials (PM 502)

Factors Affecting Size of Zone of Inhibition

1- Size of the plate Smaller plates accommodate less number of discs

2- Depth of the agar medium (4 Thin media yield excessively large inhibition
mm) zones and vice versa
3- Proper spacing of the discs (2.5 Avoids overlapping of zones
cm)

4- Inoculum density Larger zones with light inoculum and vice versa

5- Timing of disc application If after application of disc, the plate is kept for
longer time at room temperature, small zones may

form
o
6- Temperature of incubation Larger zones are seen with temperatures < 35 C

7- Incubation time Ideal 18-20 hours; less time does not give reliable
results

8- Potency of antibiotic discs Deterioration in contents leads to reduced size

9- Composition of medium Affects rate of growth, diffusion of antibiotics and
activity of antibiotics
10- Acidic pH of medium Tetracycline, novobiocin, methicillin zones are
larger

11- Alkaline pH of medium Aminoglycosides, erythromycin zones are larger

12- Reading of zones Subjective errors in determining the clear edge

Agar well diffusion

• The agar plate surface is inoculated by spreading a volume of the microbial
inoculum over the entire agar surface.

• a hole with a diameter of 6 to 8 mm is punched aseptically.

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