Behaviour in conventional sewage purification processes of coliform bacteria with transferable or non-transferable drug-resistance

Abstract

Transferable ( R +) and non-transferable ( R −) resistance of coliform bacteria in settled sewage and after biofiltration, secondary sedimentation, chlorination and sand filtration was investigated. The percentage of R − coliforms resistant to ampicillin (A), chloramphenicol (C) or streptomycin (S), but not kanamycin (K) or tetracycline (T), was slightly reduced through the purification system. On the average the percentage of R + coliforms resistant to one or more of the same drugs was reduced by about 50%. This reduction was mainly accomplished by biofiltration and sand filtration. The ratio of transferable to non-transferable resistance for A, C, K or T increased during secondary sedimentation and chlorination. R factor transfer in water may explain these findings. Rapid passage over stony surfaces in biological and sand filters is unfavourable for conjugation and may damage sex pilli while R factor transfer can be expected under the relatively stagnant conditions in sedimentation and chlorination tanks. The incidence of R factors conferring resistance to all five drugs simultaneously increased through the purification system. These R factors are probably large and may carry markers which determine resistance to sewage purification processes. Between 30 and 40% of all R factors mediated resistance to at least four of the five drugs studied. The incidence of Escherichia coli I among R + coliforms varied but did not exceed 50%. The finding that conventional sewage purification has a limited effect on the incidence of drug resistance in bacteria supports the view that sewage should be treated by more advanced methods prior to discharge into the environment.