Enumeration of Listeria monocytogenes at Low Contamination Levels in Several Food Matrices Using a Membrane Filtration M
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Enumeration of Listeria monocytogenes at Low Contamination Levels in Several Food Matrices Using a Membrane Filtration Method Nicolas Baudouin & Bertrand Lombard & Nelly Audinet & Nathalie Gnanou Besse
Received: 6 November 2008 / Accepted: 16 January 2009 / Published online: 5 February 2009 # Springer Science + Business Media, LLC 2009
Abstract For the enumeration of Listeria monocytogenes in cold-smoked salmon, a sensitive enumeration method, based on membrane filtration followed by transfer of the filter on a selective medium, has been recently developed. An evaluation of this method was performed with several categories of foods likely to be contaminated with L. monocytogenes. The results obtained with the technique were compared with those from the reference EN ISO 11290-2 method and found to provide more precise results in the enumeration of L. monocytogenes from both artificially and naturally contaminated products. In most cases, the filtration method enabled a greater quantity of food to be examined (from around 0.5 to 14 g, instead of 0.01 to 0.1 g with the reference EN ISO 11290-2 method), thus greatly improving the sensitivity of the enumeration. Keywords Listeria monocytogenes . Enumeration . Filtration . Microbiological methods . Food
Introduction Despite its low incidence, food-borne listeriosis is characterized by the seriousness of symptoms and a high rate of lethality (Anonymous 2000, 2007). In addition, the detection of Listeria monocytogenes in food has important economic consequences, such as withdrawal of products. N. Baudouin : B. Lombard : N. Audinet : N. Gnanou Besse (*) Laboratoire d’Etudes et de Recherches sur la Qualité des Aliments et sur les Procédés agro-alimentaires, Agence française de sécurité sanitaire des aliments, Afssa, 23 avenue du Général de Gaulle, 94706 Maisons Alfort Cedex, France e-mail: [email protected]
Moreover, since 2000, an increase of listeriosis cases has been observed in several European countries, but reasons for this phenomenon still remain unknown (Anonymous 2007). The recent European Community (EC) Regulation 2073/ 2005 (Anonymous 2005) on microbiological criteria for foodstuffs defines a quantitative limit for L. monocytogenes of 100 cfu g−1, which is applicable to certain categories of ready-to-eat food products during their shelf-life. The manufacturer must be able to demonstrate that its product will not exceed the limit of 100 cfu g−1 throughout the shelflife. For that purpose, different types of data and studies can be used, such as challenge tests, durability studies, and predictive microbiology. The European and International Standard method for enumeration of L. monocytogenes in food EN ISO 11290-2 (Anonymous 1998c, 2004) is cited as reference method in the quantitative criteria of EC Regulation 2073/2005 for L. monocytogenes. This method is characterized by a theoretical limit of enumeration of 10– 100 cfu g−1 or ml−1. Meanwhile, it has been shown that the precision of this standard method is relatively poor especially at such low levels. Even if th
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