Mark Ibekwe, a scientist from the Agricultural Research Service (ARS), the USDA chief intramural scientific research agency, studied hundreds of E.coli isolates from the middle Santa Ana River Watershed to determine their source.
The region contains concentrated animal-feeding operations and a sizable human population, natural-resource managers are concerned about the diverse number of E.coli populations throughout the watershed, according to the research.
The bacteria can survive in surface water and sediment because of high nutrient content in manure from livestock facilities, runoff from residential areas, warm temperatures and other urban sources.
Cows are often seen as the culprits when E.coli is found in lakes, rivers and other bodies of water.
The researchers 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.
Ibekwe and his team extracted E.coli bacteria from each sample and identified 600 different isolates of 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.
E.coli isolates collected from water and sediment samples at the same location at the same time exhibited a considerable level of genetic diversity.
Ibekwe also tested all the E.coli isolates for resistance to various antibiotics.
They found that from 88% to 95% of the isolates were resistant to rifampicin, and that around 75% were resistant to tetracycline.
Tetracycline resistance was the most common type of resistance observed 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.
The range of different antibiotic-resistant isolates identified suggests that public-health officials who track water quality might need to increase their database of E.coli fingerprints.