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Applied and Environmental Microbiology, June 2007, p. 3887-3895, Vol. 73, No. 12
0099-2240/07/$08.00+0     doi:10.1128/AEM.02912-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Recurrent and Sporadic Listeria monocytogenes Contamination in Alheiras Represents Considerable Diversity, Including Virulence-Attenuated Isolates ^,

Escola Superior de Biotecnologia, Universidade Católica Portuguesa, Porto, Portugal,¹ Escola Superior Agrária de Castelo Branco, Castelo Branco, Portugal,² Department of Food Science, Cornell University, Ithaca, New York³

Received 17 December 2006/ Accepted 30 January 2007

Microbiological characterization of alheiras, traditional smoked meat sausages produced in northern Portugal, had previously shown that more than 60% of the lots analyzed were contaminated with Listeria monocytogenes at levels higher than 100 CFU/g. In order to better understand L. monocytogenes contamination patterns in alheiras, we characterized 128 L. monocytogenes isolates from alheiras using a variety of subtyping techniques (i.e., molecular serotyping; arsenic, cadmium, and tetracycline resistance typing; and pulsed-field gel electrophoresis [PFGE]). Subtyping of isolates from products collected on two separate dates provided evidence for the persistence of specific L. monocytogenes PFGE types in the production and distribution chains of alheiras from four different processors. A subset of 21 isolates was further characterized using ribotyping and Caco-2 cell invasion assays to evaluate the pathogenic potential of L. monocytogenes present in alheiras. Caco-2 invasion assays revealed seven isolates with invasion efficiencies that were less than 20% of that of the control strain 10403S. All seven isolates had premature stop codons in inlA that represented three distinct mutations, which had previously been observed in isolates from the United States or France. Our findings indicate the need for a comprehensive approach to control L. monocytogenes in alheiras, including strategies to reduce persistence. The presence of considerable diversity in invasion phenotypes among L. monocytogenes strains present in alheiras, including the presence of subtypes likely to be virulence attenuated, may provide an opportunity to initially focus control strategies on the subtypes most likely to cause human disease.

Published ahead of print on 20 April 2007.

Supplemental material for this article may be found at http://aem.asm.org/.

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