HiMedia – M1991I_HiCrome™ Chromogenic Coliform Agar (CCA)

Principle and Interpretation
HiCrome™ Chromogenic Coliform Agar is a selective medium recommended for the simultaneous detection of Escherichia coli and total coliforms in water samples (1). The medium contains three chromogenic substrates. The enzyme b-D-galactosidase produced by coliforms cleaves 6-chloro-3-indoxyl-b-D-galactopyranoside to form pink to red coloured colonies (3). The enzyme b-D-glucuronidase produced by E.coli, cleaves 5-bromo-4chloro-3-indoxyl-b-D-glucuronic acid (2). Colonies of E.coli give dark blue to violet coloured colonies due to cleavage of both the chromogens. The presence of the third chromogen IPTG enhances the colour reaction. Addition of L-Tryptophan improves the indole reaction thereby increasing the detection reliability. Tryptone, yeast extract, sodium pyruvate and sorbitol provide nitrogenous substances, fermentable carbohydrate and other essential growth nutrients for the organisms. Phosphates buffer the medium. The media formulation helps even sublethally injured coliforms to recover and grow rapidly. Tergitol-7 inhibits gram-positive as well as some gram-negative bacteria other than coliforms (3). The medium is inoculated either by pour plate technique or by spreading the sample on the surface of plated medium. Membrane filter technique can also be used. To confirm E.coli, add a drop of Kovacs reagent on the dark blue to violet colony. Formation of cherry red colour indicates a positive reaction.

原理與解釋
HiCrome™顯色大腸桿菌瓊脂是一種選擇性培養基，推薦用於同時檢測水樣中的大腸桿菌和總大腸菌群（1）。該培養基含有三種生色底物。由大腸桿菌產生的酶b-D-半乳糖苷酶裂解6-氯-3-吲哚氧基-b-D-吡喃半乳糖苷形成 粉紅色到紅色的菌落（3）。由大腸桿菌產生的酶b-D-葡醣醛酸糖苷酶切割5-溴-4-氯-3-吲哚基-b-D-葡醣醛酸（2）。由於兩種色原的裂解，大腸桿菌的菌落產生深藍色至紫色的菌落。第三色原體IPTG的存在增強了顏色反應。 L-色氨酸的添加改善了吲哚反應，從而提高了檢測可靠性。 胰蛋白腖，酵母提取物，丙酮酸鈉和山梨糖醇為生物提供含氮物質，可發酵的碳水化合物和其他必需的生長營養素。磷酸鹽緩衝培養基。媒體配方甚至可以幫助亞致死的大腸菌群恢復並迅速生長。 Tergitol-7抑制革蘭氏陽性以及除大腸桿菌外的一些革蘭氏陰性細菌（3）。 通過傾注平板技術或通過將樣品鋪展在電鍍培養基的表面上來接種培養基。也可以使用膜過濾技術。為確認大腸桿菌，在深藍色至紫色菌落上加入一滴Kovacs試劑。櫻桃紅色的形成表示陽性反應。