9/18/2016 Thiosulfate-Citrate-Bile salts-Sucrose Xylose Desoxycholate(XLD)Agar ·Vibrio ·Enteric species like Selective agents-sodium Salmonella and Shigella citrate,sodium thiosulfate, Selective agents-sodium oxgall wh desoxycholate inhibits gram- alka positive bacteria ￥ee nella from other enteric bacteria vibrios Xylose fermentation V.cholerae ferments sucrose turning pH stimulates Salmonella-black centers icator brom thymol blue Lysine decar ces alkaine ylation are due to hydrogen yellow pro sulfide production Other enteric bacterio Hydrogen sulfide and appears blue-green production ow or red 1.0ml 1.0ml Most Probable Number Methods rcem Selective agents are added to a broth medium ” mite of smple is rnferred to the 签P11111 1111计iiii训 tctate2re1o-fodseraldltiorsare 、 Turbid growth is recorded MPN is derived from a statistical chart 血i诚诚 After incub MPN/100 ml of water on a statistical table.9/18/2016 5 Thiosulfate-Citrate-Bile salts-Sucrose • Vibrio • Selective agents – sodium citrate, sodium thiosulfate, oxg p all which provide an alkaline pH of 8.6 and slows growth of other microbes • Sucrose fermentation differentiates between vibrios - V. cholerae ferments sucrose turning pH indicator brom thymol blue yellow - V. parahaemolyticus cannot ferment sucrose and appears blue-green 17 Xylose Desoxycholate (XLD) Agar • Enteric species like Salmonella and Shigella • Selective agents – sodium desoxy g cholate inhibits gram￾positive bacteria • Three reactions differentiate Salmonella from other enteric bacteria - Xylose fermentation stimulates Salmonella black centers - Lysine decarboxylation produces alkaline conditions which stimulate - Hydrogen sulfide production Salmonella - black centers are due to hydrogen sulfide production Other enteric bacteria remain acid yellow or red without a black center 18 Most Probable Number Methods • Used to estimate MPN of organisms in a sample when low numbers of bacteria are exp( ) ected (<30 CFU / ml) - Selective agents are added to a broth medium - One milliliter of sample is transferred to the broth - At least three 10-fold serial dilutions are inoculated - Turbid growth is recorded - MPN is derived from a statistical chart 19 After incubation, a 3-digit number is produced based on the number of positive tubes per set. The number corresponds to a MPN/100 ml of water on a statistical table. 20