Evaluation of Chromogenic Enzyme Substrate Mediums, Chromocult Coliform Agar(CCA) and XM-G, by Detection of Freeze-, Heat-, High-Pressure-Injured Coliforms, and Coliforms in Food Samples

Abstract

The two chromogenic enzyme substrate mediums of chromocult coliform agar (CCA) and XM-G were applied to detect freeze-, heat-, and high-pressure-injured coliforms (Enterobacter aerogenes, Enterobacter cloacae, Escherichia coli, Klebsiella ozaenae, and Klebsiella pneumoniae) as well as coliforms in various food items. Their detection abilities were then compared with the following three conventional media: tryptic soy agar (TSA), violet red bile agar (VRBA), and VRBA/TSA. The enumerated results of injured coliforms showed that the ability to detect injured cells was in the following descending order: non-selective medium TSA>VRBA/TSA>XMG≥CCA≫VRBA. The recovery rate of injured coliforms, when compared with selective agars, was higher in CCA and XM-G than in VRBA. Investigation of the total coliform counts from 100 food samples showed that the enumeration results of the three selected media (CCA, XM-G, and VRBA) were quite similar. The correlation coefficients were 0.89 for CCA vs. VRBA, 0.91 for XM-G vs. VRBA, and 0.91 for CCA vs. XM-G; indicating that CCA and XM-G are recommendable and can substitute for the conventional selective medium VRBA. In addition to the advantage of simultaneous detection of coliforms and Escherichia coli by CCA and XM-G, their superiority in detecting injured coliforms reveals that these two methods were highly effective and suitable to monitor total coliforms and E. coli including injured cells in food samples.