Resistance patterns of E.coli strains isolated from domestic sewage before and after treatment in both aerobic lagoon and activated sludge

Abstract This work compares antibiotic resistance of 870 E. coli strains isolated from domestic raw sewage, in the effluent from aerobic lagoons and activated sludge plants. Efficiency of both treatment systems in removing faecal coliforms (FC) has also been evaluated. Reduction of FC was greater in the aerobic lagoon (99.99% in summer) than in the activated sludge system (91.30% in summer). Although the aerobic lagoon is efficient in removing faecal coliforms in comparisons with the activated sludge plant, it discharges these bacteria into receptor environments in larger (30.10 3 CFU/100 ml) or smaller quantities (9–10 2 CFU/100 ml) according to season. Among these FC, E. coli which was relatively high (on average 80%) constitutes a health problem when it is resistant to antibiotics. As for percentage of global antibiotic-resistance (evaluated by Gallery ATBG-method), the E. coli strains isolated from the effluent of the aerobic lagoon showed significantly higher antibiotic-resistance (34.66%) than those isolated from domestic sewage (23%). In the activated sludge, the percentage of antibiotic resistant strains (resistance to at least one antibiotic) showed seasonal changes in the inflow and outflow water samples. The increase of the percentage of antibiotic resistant strains of E. coli in the outflow of the lagoon system is probably related to the selection of antibiotic resistant strains by this kind of treatment. Transfer of antibiotic-resistance between E. coli strains can occur in the wastewaters treated by this system. Survival experiments on two strains of E. coli, E. coli resistant to seven antibiotics and E. coli O126:B16 sensitive to the 15 tested antibiotics, isolated from the environment, show the ability of resistant bacteria to survive better than sensitive ones in waste water treated in lagoons.