TCBS AGAR (VIBRIO SELECTIVE AGAR)

Description

TCBS Agar was developed by Kobayashi et al, who modified the selective medium of Nakanishi. Although this medium was originally designed for the isolation of V. cholerae and V. parahaemolyticus, most Vibrios grow to healthy large colonies with many different colonial morphologies. It is also recommended by APHA for the selective isolation of V.cholerae and V.parahaemolyticus. Enrichment in Alkaline Peptone Water (M618), followed by isolation on it is routinely used for isolation of V.cholerae. TCBS Agar, Selective has an additional selective ingredient i.e. sodium cholate for improved selectivity. Strains of V. cholerae produce yellow colonies on TCBS Agar because of fermentation of sucrose. V. alginolyticus also produce yellow colonies. V. parahaemolyticus is a sucrose non-fermenting organism and therefore produces blue-green colonies, as does V. vulnificus. Proteus species that are sucrose-fermenters may form yellow colonies. TCBS Agar is not a suitable medium for oxidase testing of Vibrio species. A few strains of V. cholerae may appear green or colourless on it due to delayed sucrose fermentation. It is highly selective for Vibrio species. However, occasional isolates of Pseudomonas and Aeromonas may also form blue green colonies on it. Any H2S negative colony of TCBS Agar can be considered presumptive positive for Vibrio. The medium should be inoculated heavily with faecal specimens because growth of few species may be inhibited on the medium due to fermentation of sucrose and accumulation of acids.