Identification of pets and raccoons as sources of bacterial contamination of urban storm sewers using a sequence-based bacterial source tracking method.

In urbanized areas, contaminated storm sewers can feed high bacterial levels into free-flowing streams and rivers. Although illicit connections sometimes cause contamination, urban wildlife and free-roaming domesticated or feral pets may be another source. After eliminating illicit connections as sources of high levels of Escherichia coli in two storm sewers tributary to the Huron River in southeast Michigan, the roles of urban wildlife, pets, humans, and birds were investigated using a sequence-based bacterial source tracking technology. After enumeration, E. coli were isolated from water samples collected during spring to fall, 2005. Sequences in the gene beta-glucuronidase of each isolate were compared to sequences of reference strains from humans, raccoons, pets (cats and dogs), and birds. The highest percentage source for six of ten events was pets (ANOVA, p=0.005). Among isolates attributed to pets, strains from cats occurred more frequently on seven of nine events in which pets had a non-zero probability. High raccoon percentages (up to 60%) occurred in late summer and fall, and varied significantly more than in the spring (F-test), possibly reflecting urban raccoon den-site mobility. The sequence-based bacterial source tracking method suggests that feces from pets and raccoons are important contributors to urban storm sewers.