Product Code TBD 073
Disc diffusion test is a qualitative test method. This method is based on the principle that antibiotic-impregnated disk,
placed on agar previously inoculated with the test bacterium, picks up moisture and the antibiotic diffuse radially outward
through the agar medium producing an antibiotic concentration gradient. The concentration of the antibiotic at the edge
of the disk is high and gradually diminishes as the distance from the disk increases to a point where it is no longer
inhibitory for the organism, which then grows freely. A clear zone or ring is formed around an antibiotic disk after
incubation if the agent inhibits bacterial growth. The disk diffusion method is performed using Mueller-Hinton Agar
(MHA), which is the best medium for routine susceptibility tests because it has good reproducibility, low in sulfonamide,
trimethoprim, and tetracycline inhibitors, and gives satisfactory growth of most bacterial pathogens.
The purpose of the Antimicrobial susceptibility testing (AST) by Kirby-Bauer disk diffusion susceptibility test is to
determine the sensitivity or resistance of pathogenic microbes to various antimicrobial compounds in order to assist the
clinical laboratories in selecting treatment options for patients. The findings of these tests are the basis of selection of
the most appropriate antimicrobial agent for treatment against the infection.
Till the 1950s, clinical laboratories were lacking in the methodologies for the accurate determination of in vitro responses
of organisms to antimicrobial agents. Kirby and co-workers, extensively reviewed the susceptibility testing literature.
They consolidated and updated all the previous descriptions of the disk diffusion method in 1960s. Based on the Bauer-
Kirby Method, standardized reference procedures for the disc systems were published by WHO and FDA and are
periodically updated by the CLSI (formerly NCCLS).
The Clinical Laboratory Standards Institute (CLSI) and The European Committee on Antimicrobial Susceptibility Testing
(EUCAST) have published comprehensive documents regarding the disc diffusion systems. Now-a-days, the agar disc
diffusion test is a convenient and widely used method for routine antimicrobial susceptibility testing.
For antimicrobial susceptibility testing of bacterial cultures by disk diffusion method of Bauer-Kirby.