Diverse Geno- and Phenotypes of Persistent Listeria monocytogenes Isolates from Fermented Meat Sausage Production Facilities in Portugal

Abstract

ABSTRACTThe persistence ofListeria monocytogenesin food-associated environments represents a key factor in transmission of this pathogen. To identify persistent and transient strains associated with production of fermented meat sausages in northern Portugal, 1,723L. monocytogenesisolates from raw material and finished products from 11 processors were initially characterized by random amplification of polymorphic DNA (RAPD), PCR-based molecular serotyping, and epidemic clone characterization, as well as cadmium, arsenic, and tetracycline resistance typing. Pulsed-field gel electrophoresis (PFGE) typing of 240 representative isolates provided evidence for persistence ofL. monocytogenesfor periods of time ranging from 10 to 32 months for all seven processors for which isolates from different production dates were available. Among 50L. monocytogenesisolates that included one representative for each PFGE pattern obtained from a given sample, 12 isolates showed reduced invasion efficiency in Caco-2 cells, including 8 isolates with premature stop codons ininlA. Among 41 isolates representing sporadic and persistent PFGE types, 22 isolates represented lysogens. Neither strains with reduced invasion nor lysogens were overrepresented among persistent isolates. While the susceptibility of isolates to lysogenic phages also did not correlate with persistence, it appeared to be associated with molecular serotype. Our data show the following. (i) RAPD may not be suitable for analysis of large sets ofL. monocytogenesisolates. (ii) While a large diversity ofL. monocytogenessubtypes is found in Portuguese fermented meat sausages, persistence ofL. monocytogenesin this food chain is common. (iii) PersistentL. monocytogenesstrains are diverse and do not appear to be characterized by unique genetic or phenotypic characteristics.