Use of the Chromogenic Substrate 5-Bromo-4-Chloro-3-Indolyl-B-D-Glucuronide (X-GLUC) for Enumerating Escherichia coli in 24 H from Ground Beef

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Abstract
A 24-h direct plating method for Escherichia coli using the chromogenic substrate 5-bromo-4-chloro-3-indolyl-B-D-glucuronide (X-GLUC) incorporated into a Peptone-tergitol agar base (PTX) was compared with the standard 3-tube Most Probable Number (MPN) method on 50 naturally contaminated ground beef samples. A paired-comparisons t-test showed no significant difference between the two methods. A positive linear correlation between the two methods was observed over the entire range of values. Ninety-seven percent of the positive colonies (blue colonies) on PTX agar were indentified as E. coli, whereas no atypical colonies (nonblue) were characterized as such. Thus, a simple and reliable enumeration of E. coli can be made within 24 h using the X-GLUC substrate in a selective agar as an indicator of B-glucuronidase activity.