Selection of antibiotic-resistant standard plate count bacteria during water treatment

Abstract

Standard plate count (SPC) bacteria were isolated from a drinking-water treatment facility and from the river supplying the facility. All isolates were identified and tested for their resistance to 6 antibiotics [carbenicillin, kanamycin, chloramphenicol, streptomycin, sulfanilamide, tetracycline] to determine if drug-resistant bacteria were selected as a consequence of water treatment. Among the isolates survivng test procedures, there was a significant selection (P < 0.05) of gram-negative SPC organisms resistant to .gtoreq. 2 test antibiotics. These bacteria were isolated from the flash mix tank, where Cl, alum and lime are added to the water. Streptomycin resistance in particular was more frequent in this population as compared with bacteria in the untreated river water (P < 0.01). SPC bacteria from the clear well, which is a tank holding the finished drinking water at the treatment facility, were also more frequently antibiotic resistant than were the respective river water populations. When 15.8 and 18.2% of the river water bacteria were multiply antibiotic resistant, 57.1 and 43.5%, respectively, of the SPC bacteria in the clear well were multiply antibiotic resistant. Selection for bacteria exhibiting resistance to streptomycin was achieved by chlorinating river water in the laboratory. Evidently, selective factors operating in the aquatic environment of a water treatment facility can act to increase the proportion of antibiotic-resistant members of the SPC bacterial population in treated drinking water.