Livestock producers no longer should have to take all of the blame when Escherichia coli (E. coli) bacteria are found in an area. U.S. Department of Agriculture (USDA) scientists have recently identified urban sources of E. coli. Studies by Agricultural Research Service (ARS) scientist Mark Ibekwe suggest that in some parts of California, pathogens in local waterways are more often carried there via runoff from urban areas, not from animal production facilities.

Even though most strains of E. coli are non-pathogenic, the bacterium is monitored by public health officials as an indicator of water quality. Cows are often seen as the culprits when E. coli is found in local lakes, rivers and other bodies of water.

Ibekwe, who works at the ARS U.S. Salinity Laboratory in Riverside, CA, and his colleagues collected 450 water and sediment samples from 20 sites throughout California's middle Santa Ana River Watershed. The collection sites included urban areas, livestock feeding areas, parks, National Forest lands and three wastewater treatment plants.

Then the scientists extracted E. coli bacteria from each sample and identified 600 different isolates of E. coli in their samples, many of which could be placed into six clonal populations. They found the greatest variety of different types of E. coli in runoff discharged from areas dominated by urban development or human activities.

Ibekwe also tested all the E. coli isolates for resistance to various antibiotics. He found that from 88 to 95% of the isolates were resistant to rifampicin, and that around 75% were resistant to tetracycline. Tetracycline resistance was by far the most common type of resistance observed in E. coli isolates collected near wastewater treatment plants.

The scientists also found that 24% of E. coli collected in sediment samples associated with urban runoff—a total of 144 isolates—showed resistance to as many as seven antibiotics. 

Learn more in the November/December 2012 issue of Agricultural Research magazine.