Clindamycin

Clindamycin sensitivity discs are designed for antimicrobial susceptibility testing of bacterial cultures using the Bauer-Kirby disk diffusion method. This qualitative test enables the observation of the inhibitory effect of clindamycin on a bacterial culture under standardised conditions. The test is performed by inoculating the bacterial suspension on a suitable agar surface. When placed on the inoculated agar, the clindamycin disc absorbs fluid from the medium. The antimicrobial agent then diffuses radially through the agar creating a concentration gradient around the disc. The concentration is maximum in the vicinity of the disc and falls off with increasing distance.

Clindamycin stops the growth of bacteria and this is observed as a clear area or ring after incubation around the disc. The observed zone of inhibition allows evaluation of the bacterial response according to the applicable criteria of antimicrobial susceptibility testing. The procedure is performed on Mueller-Hinton Agar (MHA) which supports good growth of most bacterial pathogens and offers good reproducibility for routine susceptibility testing. Over time the Bauer-Kirby method has been developed and standardised to provide uniform approaches for the assessment of antimicrobial responses. Reference procedures for disc systems have been published by organisations such as WHO and FDA and CLSI and EUCAST give comprehensive guidance for antimicrobial susceptibility testing.

To obtain accurate results, it is necessary to have proper test conditions. The depth of agar should be 4.0 ± 0.5 mm. The inoculum suspension should be used preferably within 15 minutes of preparation and always within 60 minutes. Flooding the agar plate during inoculation should be avoided. Clindamycin discs should be placed within 15 min of inoculation and incubation started within a further 15 min. Incubation should not exceed 16-20 hours, since prolonged incubation can cause indistinct zone edges or colonies within the inhibition zones.

Proper plate setup accommodates a maximum of six discs on a 90 mm plate and 12 discs on a 150 mm plate. Sensitivity discs should be refrigerated immediately upon receipt, at the recommended temperature. Consistent observations in antimicrobial susceptibility testing are made possible by specified conditions for preparation, inoculation, disc placement, and incubation.