We previously reported that the macrophytic green alga Cladophora harbors high densities (up to 106 colony-forming units/g dry weight) of the fecal indicator bacteria, Escherichia coli and enterococci, in shoreline waters of Lake Michigan. However, the population structure and genetic relatedness of Cladophora-borne indicator bacteria remain poorly understood. In this study, 835 E. coli isolates were collected from Cladophora tufts (mats) growing on rocks from a breakwater located within the Indiana Dunes National Lakeshore in northwest Indiana. The horizontal fluorophore enhanced rep-PCR (HFERP) DNA fingerprinting technique was used to determine the genetic relatedness of the isolates to each other and to those in a library of E. coli DNA fingerprints. While the E. coli isolates from Cladophora showed a high degree of genetic relatedness (≥92% similarity), in most cases, however, the isolates were genetically distinct. The Shannon diversity index for the population was very high (5.39). Both spatial and temporal influences contributed to the genetic diversity. There was a strong association of isolate genotypes by location (79% and 80% for lake- and ditch-side samplings, respectively), and isolates collected from 2002 were distinctly different from those obtained in 2003. Cladophora-borne E. coli isolates represented a unique group, which was distinct from other E. coli isolates in the DNA fingerprint library tested. Taken together, these results indicate that E. coli strains associated with Cladophora may be a recurring source of indicator bacteria to the nearshore beach.
Bibliographical noteFunding Information:
We thank Douglas Wilcox, US Geological Survey, Donna Ferguson, Orange County Public Health Laboratory, and Alex Da Silva, Indian Department of Environmental Management, for their critical review of the manuscript. This work was supported, in part, from a grant from the Minnesota Sea Grant College Program, awarded by the NOAA Office of Sea Grant, United States Department of Commerce, under Grant no. NA03-OAR4170048 (to M.J.S.). This article Contribution number is 1417 of the Great Lakes Science Center.
- E. coli population structure
- Fluorophore-enhanced rep-PCR
- Great Lakes
- Indicator bacteria
- Recreational water quality