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Applied and Environmental Microbiology, December 2005, p. 8115-8122, Vol. 71, No. 12
0099-2240/05/$08.00+0 doi:10.1128/AEM.71.12.8115-8122.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Microbial Food Safety Research Unit, Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Wyndmoor, Pennsylvania 19038,1 DuPont Qualicon, DuPont Experimental Station, Building 400, Wilmington, Delaware 19880,2 Food Products Association, 1350 I St., N.W., Suite 300, Washington, D.C. 200053
Received 24 March 2005/ Accepted 27 August 2005
Pulsed-field gel electrophoresis and serotyping were performed for 544 isolates of Listeria monocytogenes, including 502 isolates recovered from contaminated samples from 31,705 retail ready-to-eat (RTE) food products and 42 isolates recovered from human cases of listeriosis. The isolates were from Maryland (294 isolates) and California (250 isolates) and were collected in 2000 and 2001. The isolates were placed into 16 AscI pulsogroups (level of relatedness within each group,
66%), 139 AscI pulsotypes (levels of relatedness,
25% to 100%), and eight serotypes (serotypes 1/2a, 1/2b, 1/2c, 3a, 3b, 4b, 4c, and 4d). The most frequently found pulsotypes belonged to either pulsogroup A (150 food isolates plus 4 clinical isolates) or pulsogroup B (104 food isolates plus 5 clinical isolates). The majority of the 502 food isolates were either serotype 1/2a (298 isolates) or serotype 1/2b (133 isolates), whereas the majority of the 42 clinical isolates were either serotype 1/2a (19 isolates) or serotype 4b (15 isolates). Additionally, 13 clinical isolates displayed pulsotypes also found in food isolates, whereas the remaining 29 clinical isolates displayed 24 unique pulsotypes. These data indicate that most (86%) of the L. monocytogenes subtypes found in the RTE foods sampled belonged to only two serotypes and that 90% of the isolates displayed 73 pulsotypes, with 107 isolates displaying pulsotype 1. These data should help define the distribution and relatedness of isolates found in RTE foods in comparison with isolates that cause listeriosis.
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