Characterization of Escherichia coli Isolates from an Urban Lake Receiving Water from a Wastewater Treatment Plant in Mexico City: Fecal Pollution and Antibiotic Resistance

The presence of enteric bacteria in water bodies is a cause of public health concerns, either by directly causing water- and food-borne diseases, or acting as reservoirs for antibiotic resistance determinants. Water is used for crop irrigation; and sediments and aquatic plants are used as fertilizing supplements and soil conditioners. In this work, the bacterial load of several micro-environments of the urban lake of Xochimilco, in Mexico City, was characterized. We found a differential distribution of enteric bacteria between the water column, sediment, and the rhizoplane of aquatic plants, with human fecal bacteria concentrating in the sediment, pointing to the need to assess such bacterial load for each micro-environment, for regulatory agricultural purposes, instead of only the one of the water, as is currently done. Resistance to tetracycline, ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole was common among Escherichia coli isolates, but was also differentially distributed, being again higher in sediment isolates. A distinct distribution of chloramphenicol minimum inhibitory concentrations (MIC) among these isolates suggests the presence of a local selective pressure favoring lower MICs than those of isolates from treated water. Fecal bacteria of human origin, living in water bodies along with their antibiotic resistance genes, could be much more common than typically considered, and pose a higher health risk, if assessments are only made on the water column of such bodies.