Comparison of chromogenic and fluorogenic substances for differentiation of coliforms and Escherichia coli in soft cheese

Abstract

In food hygiene the differentiation of Escherichia coli and coliforms as index resp. indicator organisms is very important as a basis for the assessment of good manufacturing practice (gmp). Using a fluorogenic (MUG) and chromogenic (X-gal) substrate the laborious methods have become more simple and reliable. A new German DIN standard method (10.183, part 3) was used as a base to examine 200 soft cheese samples for coliforms and Escherichia coli comparing different media, parameters and incubation times. It could be shown that an incubation of 48 h is absolutely necessary for E. coli and coliforms in both media, that X-gal is a quicker and more sensitive parameter for total coliforms than gas production and that the combination of fluorescence and indole is slightly superior to fluorescence and gas for the identification of E. coli. To sum up, the new fluorescence principle, integrated in new standard methods will be an excellent tool to simplify the differentiation of E. coli and coliforms in food hygiene. Additionally, X-gal as a chromogenic substrate for all coliforms, including E. coli, may be integrated in further standard methods.