Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory, TCBS Agar
Purpose
Thiosulfate Citrate Bile Sucrose (TCBS) Agar is an undefined, selective, and differential medium used for the primary isolation of Vibrio species. Clinical and nonclinical specimens suspected of fecal contamination are streaked on TCBS in an effort to recover Vibrio cholerae, the most important pathogen of the genus.
Principle
The medium’s alkaline pH (8.6) promotes growth of Vibrio spp., especially that of V. cholerae. Oxgall and sodium cholate are included to inhibit the growth of Gram-positive bacteria. Sucrose is the fermentable carbohydrate and sodium thiosulfate is included as an electron acceptor for sulfur reducers. Bromthymol blue is the pH indicator and ferric ammonium citrate is included to indicate sulfur reduction. Sucrose fermenters producing acid end-products (such as Vibrio cholerae) form yellow colonies (Figure 2-29) while those of sucrose nonfermenters are blue. Some Enterococci ferment sucrose but are inhibited by the oxgall. These organisms, as shown in Figure 2-30, produce small yellow colonies. Species able to reduce thiosulfate to H2S produce black colonies due to the reaction of H2S with the ferric ion in the medium.
2-29 VIBRIO CHOLERAE STREAKED ONTCBS AGAR The large, yellow colonies are
indicative of Vibrio cholerae.
indicative of Vibrio cholerae.
2-30 ENTEROCOCCUS FAECALIS STREAKED
ON TCBS AGAR This Gram-positive coccus
may also be recovered from fecally contaminated samples;
ON TCBS AGAR This Gram-positive coccus
may also be recovered from fecally contaminated samples;
however, its yellow colonies
are much smaller than those of V. cholerae.
Compare the E. faecalis colonies with those in
Figure 2-29.
are much smaller than those of V. cholerae.
Compare the E. faecalis colonies with those in
Figure 2-29.
Suggested Reading
- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011
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