Evaluation of a chromogenic medium for total coliforms and Escherichia coli determination in ready-to-eat foods

Abstract

Chromocult ® coliform agar (CCA) was compared with the standard medium violet red bile agar (VRBA) for total coliform counts (TC) in different ready-to-eat foods. In addition, the specificity of CCA to detect E. coli was evaluated. CCA is a medium designed to detect TCs and E. coli by the specific action of β- d-galactosidase and β- d-glucuronidase. A total of 94 ready-to-eat food samples were obtained from restaurants, fast food shops and bars in Córdoba city, Argentina. The foods included boiled vegetable salads, cooked vegetables, meat and chicken meals, salad and sandwiches. One hundred and one strains were isolated at random from food samples. In CCA, the value of confirmation for TC was 81.3% and for E. coli was 93.3%. All E. coli strains isolated were β- d-glucuronidase positive. The median TC count was 4.61 log cfu/g on CCA and 3.58 log cfu/g on VRBA. The Spearman's correlation coefficient between TC count on VRBA and CCA was 0.72 ( P=0.00001). The Wilcoxon signed rank test revealed statistically significant differences between both variables. Disagreement between the two methods for TC enumeration was due to false positive results in CCA. The results of this study showed that CCA is an efficient method for simultaneous detection of E. coli and coliforms from ready-to-eat foods.