Q-226. Population Structure and Genetic Relatedness of Salmonella Associated with Cladophora in Lake Michigan

M. N. Byappanahalli¹, S. Ishii², R. Sawdey², D. A. Shively¹, J. Ferguson², R. L. Whitman¹, M. J. Sadowsky²;
¹US Geological Survey, Porter, IN, ²Univ. of Minnesota, St. Paul, MN.

The bacterial pathogens Shigella, Salmonella, Campylobacter, and shiga toxin-producing E. coli (STEC) were recently found to be associated with Cladophora growing in southern Lake Michigan. Preliminary results indicated that the Salmonella strains associated with Cladophora were genetically identical. However, because of the small sample size (n=37 isolates) and a lack information on spatial-temporal relationships, the nature of the association between Cladophora and Salmonella remained speculative. In the present study, we investigated the population structure and genetic relatedness of Salmonella isolates (n=180) derived from Cladophora, stream and lake water, aquatic plants, beach sand, and sediments. Salmonella isolates were collected during the 2005-2007 swimming seasons (May-August) from Lake Michigan beachsheds in Illinois and Indiana. The genetic relatedness of Salmonella isolates was examined by using the horizontal, fluorophore-enhanced rep-PCR (HFERP) DNA fingerprinting technique. While the Salmonella isolates associated with Cladophora exhibited a high degree of genetic relatedness (≥ 92% similarity), the isolates were not all genetically identical. Spatial and temporal relationships were evident in the population examined, with tight clustering of the isolates both by year and location. Biochemical tests and 16S rRNA gene sequence analysis of representative strains confirmed that the isolates derived from Cladophora were indeed Salmonella strains (99.9% sequence identity with S. enterica subsp. enterica serovar Typhimurium strain LT2). These findings suggest that the relationship between Salmonella and Cladophora is likely casual and related to input types (e.g., wastewater, runoff, birds). Results of our studies indicate that Cladophora is likely an important reservoir for Salmonella and other enteric bacterial pathogens in Lake Michigan beachsheds, which in turn may influence nearshore water quality.