Sources and Sinks of Escherichia coli in Benthic and Pelagic Fish

Escherichia coli and fecal coliform bacteria were isolated from five benthic and four pelagic fish species to determine their role in the fecal contamination of recreational waters. All fish were collected during fall 2006 from Southworth Marsh in the Duluth-Superior Harbor, a public beach that is commonly posted to minimize water contact due to high E. coli levels. Although fecal coliform bacteria were isolated from each fish species, they were only isolated from 66% and 72% of the individual benthic and pelagic fish, respectively. While 42% of the fecal coliforms from benthic fish were E. coli, only 4% of these bacteria from pelagic fish were E. coli. Cluster analysis showed different fish species harbored identical strains of E. coli and some fish contained multiple E. coli strains. The potential source for 65% of the E. coli isolates obtained from fish were identified by using the HFERP DNA fingerprinting method and libraries of E. coli DNA fingerprints from warm-blooded animals and environmental isolates collected in the area. The E. coli strains whose source could be identified were most similar to strains isolated from sediments, Canada geese, mallard ducks, and wastewater. None of the fish E. coli had DNA fingerprints matching those from any water or beach sand isolates. Although our results demonstrate that benthic fish contain E. coli, it may be more appropriate to consider these fish as a vector of E. coli from other sources, rather than a new source of E. coli contamination in aquatic environments.