Genetic characterization of genes specific to Listeria monocytogenes epidemic-associated serotype 4b strains

by Cheng, Ying.

CHENG, YING. Genetic Characterization of Genes Specific to Listeria monocytogenes Epidemic-Associated Serotype 4b Strains (Under the direction of Dr. Kathariou Sophia.) Listeria monocytogenes, a food-borne pathogen of humans and animals, can cause severe listeriosis with relatively high mortality. A cluster of closely related strains of L. monocytogenes (designated Epidemic Clone I) have been implicated in numerous outbreaks in Europe and North America, including the California outbreak of 1985. L. monocytogenes strains implicated in the 1998-1999 and the 2002 multistate outbreaks in the USA represent a unique epidemic-associated clonal group, designated Epidemic Clone II (ECII). Comparative genomic analyses across five genomes from different L. monocytogenes isolates and Listeria species identified a genomic region (region-18) in serotype 4b strains that may have been acquired by horizontal gene transfer. Region-18 is either absent or markedly divergent in ECII strains but conserved among other serotype 4b strains. Region-18 is flanked by a large gene encoding a putative cell-wall associated protein (wap) on one side and a well-known virulence gene internalin A (inlA) on the other side in serotype 4b strains. PCR primers and DNA probes derived from this ECII-specific region-18 can readily differentiate ECII strains from other serotype 4b strains. This facilitates the detection and monitoring of these strains belonging to ECII clonal group in foods, clinical samples, and the environment. Genetic characterization of wap by the construction of deletion mutants suggested that the ECII wap mutant but not the ECI wap mutant may be involved in specific environmental adaptations such as surface adherence and possibly biofilm formation in ECII strains. Mutational and functional analyses showed that the deletion mutant of region-18 in ECII had an enhanced death rate during post-stationary incubation at 42?,suggesting that the ECII-specific region- 18 may be implicated in post-stationary stress responses. Two c72.44-negative variants of epidemic-associated L. monocytogenes serotype 4b strains were isolated from laboratory cultures. Naturally occurring c74.22-negative variants that exist under laboratory conditions without any noticeable phenotypic differences from their original forms may complicate the analysis of phage sensitivity and pathogenic characteristics. Genetic Characterization of Genes Specific to Listeria monocytogenes Epidemic-Associated Serotype 4b Strains by