What are the factors that influence the size of the zone of inhibition of an antibiotic when employing the Kirby-Bauer method? How can this method determine if an antimicrobial agent is bactericidal or bacteriostatic?
The Kirby-Bauer method itself cannot state if the antimicrobial agent has bacteriostatic or bactericidal effect. After performing the Kirby-Bauer test, the bacteriostatic or bactericidal effect is decided by prelevating a sample from the inhibition zone and putting it in nutrient broth. If the growth of bacteria from the sample continues still, hence, then the antibiotic has a bactriostatic effect.
The results of Kirby-Bauer method regarding the actions different antibiotics can be considered only if the performing conditions of the test are standardized. Hence, the following conditions must be standardized, such that: the concentration of the nutrient broth, the time of incubation, the temperature of incubation, the size of discs and the concentration of antibiotics on the discs.
The zones of inhibition of different antibiotics cannot be directly compared. For example, two antibiotics can reach the same diameter of inhibition zone but this achievement being realized in two different conditions: one antibiotic diffusing faster because of the smaller size of it's molecules and other antibiotic diffusing slower because of the larger size of it's molecules.