Question

What two factor influence the size of a zone of inhibition around an antibiotic disk on Kirby Bauer disk diffusion plate used to determine antibiotic sensitivities?

Answer 1

There are many factors which influence the size of Zone of inhibition, among which two factors are most important ;

Composition of the medium : the medium affects the growth rate of organisms, the activity of the agent and rate of diffusion of the antibiotic.

Inoculum density : when the inoculum is light, the zone of inhibition is larger and resistant strains behave as susceptible ( relatively) whereas, if the inoculum is heavy , the inhibition zone is smaller and susceptible strains behave as resistant.

Other factors like Size of plate , depth of agar medium , spacing of antibiotic disc , incubation time and temperature also influence the Zone of inhibition.