Identifying sources of surface water contamination by discriminant analysis of patterns of antimicrobial resistance in Escherichia coli

A repeated cross-sectional study was conducted to investigate whether discriminant analysis of antimicrobial resistance profiles of E. coli would be useful in identifying sources of fecal contamination of surface water from the Red Cedar (Michigan) watershed. Fecal samples were collected from livestock, companion animals, human septic tanks, farmed deer and wild geese from the watershed, and water was collected from several different sites on the Red Cedar River. Disc diffusion was used to test for susceptibility to neomycin, gentamicin, streptomycin, chloramphenicol, ofloxacin, trimethoprim/sulfamethoxazole, tetracycline, ampicillin, nalidixic acid, nitrofurantoin, cephalothin, and sulfisoxazole. Both resubstitution and cross-validation discriminant function analysis methods were applied to the data.;Resistance to at least to one antimicrobial was seen in isolates from domestic animals, wildlife, surface water, and human septic tanks. Overall, E. coli isolates from food animals showed resistance to the largest number of antimicrobials, followed by horses, companion animals, human septic tanks, farmed deer, wild geese and surface water. Discriminant analysis using the resubstitution method produced a higher but biased average rate of correct classification (ARCC), while cross-validation produced lower but valid ARCC. Also, the ARCC improved as the number of source categories decreased.