Enumeration of E. coli in environmental waters and wastewater using a chromogenic medium

CHROMagar Liquid ECC (CLECC) was compared with membrane lauryl sulphate broth plus urea (mLS-UA) used routinely in Hong Kong for E. coli enumeration. E. coli appear as distinctive greenish-blue colonies on CLECC while other faecal coliforms are red in colour. CLECC performance was comparable to mLS with mixtures of faecal coliforms, E. coli and non-faecal coliforms. Beach, river, ground and waste water samples were used for further comparison. Results given by the two media were significantly correlated (P = <0.001). CLECC was superior to mLS-UA in sensitivity (99.1% vs 87.2%) and specificity (96.9% vs 59.8%) for detecting E. coli. On CLECC 95% of E. coli colonies were correctly identified but only 66.9% on mLS-UA. On average, time required for performing one E. coli enumeration test with CLECC was 1.2min/sample less than with mLS-UA. CLECC may enable the development of complete automation for enumerating E. coli.