Evaluation of a 24-Hour Fluorogenic Assay for the Enumeration of Escherichia coli From Foods

Abstract

A 24 h assay for the detection and enumeration of Escherichia coli in foods was evaluated. The procedure used 4-methylumbellifery1-β-D-glucuronide (MUG) incorporated into trypticase soy agar (TSA) and violet red bile agar (VRBA). The procedure was evaluated using 106 different stock cultures, and 165 isolates from a variety of naturally contaminated foods. Samples were first surface plated on TSA-MUG, and allowed to incubate at room temperature for 1–2 h. Plates were then overlaid with VRBA-MUG and incubated at 35°C for 24 h. The TSA/VRBA-MUG assay was able to detect 94.1% of the E. coli cultures tested, and 95.8% of E. coli strains isolated from foods, giving false-negative rates of 5.9% and 4.2% respectively. False positives were found with 2 of 31 Salmonella spp. isolates and 4 of 4 Shigella spp. isolates from the culture collection. There were no false positives from food isolates. The TSA/VRBA-MUG assay was able to detect E. coli in six food samples that were not detected by our routine procedure. The visible fluorescent halo around each E. coli colony gave immediate confirmation, without the need for further tests. The TSA/VRBA-MUG assay provided an accurate, simple, inexpensive method for detection of E. coli, from a wide variety of food products.