Frequency of transferable multiple antibiotic resistance amongst coliform bacteria isolated from a treated sewage effluent in Antofagasta, Chile

Antibiotic-resistant coliform bacteria from raw and treated water from activated sludge of a sewage treatment plant were isolated on eosin methylene blue agar (EMB) plates in the separate presence of ampicillin, tetracycline, chloramphenicol and streptomycin. Antibiotic resistant coliforms obtained from inflow and effluent waters were studied to determine influences of treatment on the species composition, resistance patterns and transferable resistance in these bacteria. The bacterial counts obtained showed a high proportion of antibiotic resistant coliforms in the treated effluent. The resistant coliform species were eliminated at different rates by the treatment. Escherichia coli decreased, whereas the proportion of Klebsiella sp increased. The percentage of multiple-resistant isolates increased at the end of the treatment process. The high frequency of resistance was to ampicillin, followed by tetracycline and sulphatrimethoprim. Moderate incidence of resistance was to streptomycin, chloramphenicol and cephradine. The lowest rate of resistance was to kanamicin and nalidixic acid. Mating experiments of multiple- resistant coliforms with an E. coli K-12 donor confirmed the capability of transferring its resistance. The results of this study suggested that wastewater treatment could reduce the total number of enteric bacteria in sewage, but may increase the proportion of antibiotic resistant coliforms in effluent water. Environmental release and agricultural use of treated sewage effluent containing bacteria having elevated antibiotic resistance levels presents a potential public health risk, as well as the risk of transferring the antibiotic resistance to bacterial populations in the environment.