Comparison of the ISO method and three modifications of it for the enumeration of low concentrations of Listeria monocytogenes in naturally contaminated foods

Sensitive methods for enumeration of Listeria monocytogenes (L. monocytogenes) are needed to verify compliance with microbiological criteria in ready-to-eat foods. Here, we assessed the reference EN ISO 11290-2 method and three modifications of it with lower threshold levels for enumeration in terms of specificity, false results and practical limitations for use. Two of the methods, called the EURL and the Cyprus protocols, use membrane filtration to obtain a more concentrated test suspension, and the third, called the Norway protocol, uses less diluent. This study included 18 samples of foods naturally contaminated with L. monocytogenes at concentrations of 0.2-80 CFU/g. All four tested methods yielded valid results with good repeatability (Fisher’s test, p<0.01). The Norway protocol was the least laborious method and gave good results even for samples that could not be filtered.