Comparative study of antibiotic-impregnated discs and tablets

In the majority of clinical laboratories, agar diffusion is the method used routinely for AST of a wide range of microorganisms. It is generally accepted that reliable results can be obtained by agar diffusion, when a standardised methodology is used and the diameter of the inhibition zones has been correlated with minimal inhibitory concentration (MIC).

Rosco Diagnostica (Copenhagen, Denmark) has produced a new range of antibiotic discs called Neo-Sensitabs. These 9-mm uniform tablets are not paper, as used by many other manufacturers, but dry, crystalline antimicrobial agent encapsulated in tablet form. Neo-Sensitabs are a very stable product with up to four years’ shelf life and can be stored at room temperature. They permit the uniform release of antimicrobial agent into the agar, which gives regular and reliable zones sizes.(1)

Testing times
Consecutive urinary Enterobacteriaceae isolates (n=535) from four laboratories across the UK were tested. Each isolate was tested against coamoxyclav, gentamicin and nalidixic acid obtained from three different manufacturers. Neo-Sensitabs were provided by Rosco Diagnostica, and antibiotic-mpregnated filter paper discs by Mast (Bootle, UK) and Oxoid (Basingstoke, UK). IsoSensitest agar (Oxoid) was inoculated using the method recommended by the British Society for Antimicrobial Chemotherapy (BSAC),(2) and the discs were placed on the plate using disc dispensers. The zone diameters were read using the Mastascan elite (Mast) to eliminate any reading bias.