Principle
- Chromogenic Coliform Agar is a selective medium recommended for the simultaneous detection and recovery of sub-lethally injured coliforms in water samples.
- The CCA media is composed of tryptone, yeast extract, sodium chloride, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyruvate, sorbitol, tryptophan, tergitol-7 and three chromogenic substrates.
- The three chromogenic substrates are 6-chloro-3-indoxyl β-D-galactopyranoside, 5-bromo-4-chloro-3-indoxyl- β-D-glucuronic acid cyclohexamine ammonium salt, monohydrate and IPTG (Isopropyl-β-D-thiogalactopyranoside).
- Tryptone, sodium pyruvate, and sorbitol provide nitrogenous substances, fermentable carbohydrates, and other essential growth nutrients for the organisms.
- The L-Tryptophan improves the indole reaction thus increasing the revealing reliability. Sodium dihydrogen phosphate and disodium hydrogen phosphate buffer the medium.
- Tergitol-7 inhibits gram-positive as well as some gram-negative bacteria other than coliforms. The enzyme ß-D-galactosidase produced by coliforms hydrolyzes 6-chloro-3-indoxyl-β-D-galactopyranoside to form pink to red colored colonies.
- The enzyme ß-D-glucuronidase produced by E.coli cleaves 5-bromo-4chloro-3-indoxyl-β-D-glucuronic acid. Colonies of E.coli give dark blue to violet colored colonies due to cleavage of both the chromogens. The presence of the third chromogenic IPTG enhances the color of the reaction.
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