Development of a membrane filtration method for enumeration of Listeria monocytogenes from soft cheese

A new and simple analytical method has been developed to quantify low levels (≤50 cfu g−1) of Listeria monocytogenes in soft cheese. The technique allows the analysis of 1 g of cheese instead of 0·1 g or 0·01 g using the ISO 11290-2 standard method. The analysis protocol combines filtration of the decimally diluted cheese suspension through a 0·45-μm pore size cellulose ester membrane, and a culture of the filter on a Palcam agar. A tween 80–trypsin treatment used to increase filterability of cheese did not reduce L. monocytogenes counts. The tested method provided more precise results (nearer to the true value) compared to the ISO 11290-2 standard method in the enumeration of L. monocytogenes from artificially contaminated cheese. However, it improves neither repeatability nor reproducibility since the selected medium, Palcam, does not allow distinction between the different Listeria species.