AN EVALUATION OF FLUOROGENIC AND CHROMOGENIC ASSAYS FOR THE DIRECT ENUMERATION OF ESCHERICHIA-COLI

Three direct plating methods to enumerate Escherichia coli from food in 24 h are described. Unlike the majority of enterics, 96% of E. coli are able to cleave beta-glucuronic acid. This reaction can be observed by incorporating (1) 4-methylumbelliferyl-beta-D-glucuronate, (2) para-nitrophenyl-beta-D-glucuronate or (3) 5-bromo-4-chloro-3-indolyl-beta-D-glucuronate (BCIG) into a peptone tergitol agar base. Escherichia coli produce fluorescent, yellow or deep blue colonies from these three compounds respectively. BCIG agar proved the easiest to read and produced the least number of false positives and false negatives. An attempt is made to explain the poor correlation achieved between this method and a conventional most probable number technique.