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 to each other. However, because of the small sample size (n = 37 isolates) and a lack of information on spatial-temporal relationships, the nature of the association between Cladophora and Salmonella remained speculative. In this study, we investigated the population structure and genetic relatedness of a large number of Cladophora-borne Salmonella isolates from Lake Michigan (n = 133), as well as those isolated from stream and lake water (n = 31), aquatic plants (n = 8), and beach sands and sediments (n = 8) from adjacent watersheds. Salmonella isolates were collected during 2005-2007 between May and August from Lake Michigan beachsheds in Wisconsin, 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 populations examined, with tight clustering of the isolates both by year and location. These findings suggest that the relationship between Salmonella and Cladophora is likely casual and is related to input sources (e.g. wastewater, runoff, birds) and the predominant Salmonella genotype surviving in the environment during a given season. 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.
Bibliographical noteFunding Information:
We thank Donna Ferguson, Joy Marburger, and Douglas Wilcox for their critical review of this manuscript. We thank Gregory Kleinheinz, Julie Kinzelman, and Ken Hyde for providing Cladophora samples. We thank Valerie O'Bannon, Norbert Tavares, Hung Vu, and Tao Yan for their technical help. This work was supported, in part, by grants from the Minnesota Sea Grant College Program, NOAA Office of Sea Grant, United States Department of Commerce under grant no. NA03-OAR4170048 (to M.J.S. and R.E.H.), and from the University of Minnesota Agricultural Experiment Station (to MJS). This paper is journal reprint no. JR 558 of the Minnesota Sea Grant College Program and is Contribution 1514 of the USGS Great Lakes Science Center.
- Beach water quality
- Cladophora-Salmonella association
- Enteric bacteria
- Environmental survival
- Lake Michigan
- Public health