Comparison of the plate count and the MPN-method for detection of Listeriain experiment and routine testing

In order to compare the plate count method for quantitating Listeria, as pu blished in the "Official Collection of Testing Methods" in 35 LMBG (L. 00.0 0-22), to an MPN-method for Listeria based on the same mediums, these two d etection methods for Listeria were tested in three sets of experiments and a routine sample status evaluation, A pure broth culture of L. monocytogene s, artifically with L. moncytogenes contaminated ground meat, artificially contaminated and cold stored ground meat as well as 77 ground beef samples from Berlin retail food stores were used in the four trials. The detection limit of the MPN-method is about, 66% lower than the plate count method all owing detection of a clearly greater number of Listeria-positive samples fr om naturally contaminated ground meat. The MPN-method yielded more Listeria spp-positive samples (rel. 43%) and more L. monocytogenes-positive samples (rel. 21%) versus the colony count method based on the results from the fi eld trial using ground beef samples from retail food stores in Berlin. Neve rtheless the standardized colony count method is preferred over the MPN-met hod for routine use because of its slightly higher productivity and much sm aller variation in the results. However, the MPN-method is preferable for e pidemiological studies because of the significance of the lower detection l evel. The random sampling evaluation of ground beef from retail stores indi cated that 39% of the samples were Listeria spp.-positive and 31% were L. m onocytogenes-positive when using the colony count method. A total of 56% of the meat samples were found to be Listeria spp-positive and 38% L. monocyt ogenes-positive when the MPN-method was used. Population levels ranged from 10 to 580 cfu/g (Listeria spp.-positive samples) and from 10 to 270 cfu/g (L monocytogenes-positive samples) for the colony count method. The MPN-met hod yielded population levels of 3,6 to 930 MPN/g for Listeria spp.-positiv e samples and 3,6 to 150 MPN/g for L monocytogenes-positive samples. L monocytogenes strains isolated using the colony count method belonged to the following serovars: 1/2 a (46%), 1/2 b (13%), 1/2 c (33%), 3 b (4%) and 4 c (4%). A similar serovar isolation pattern was found for L. monocytogen es-positive MPN-tubes. The most common serotype was 1/2 a (43%), followed b y 1/2 c (32%) and 1/2 b (14%). The serotypes 3 c, 4 b and 4 c were all isol ated 4% of the time.