Antibiotic resistant bacteria in wastewater processed by the Metropolitan Water Reclamation District of Greater Chicago system

The Metropolitan Water Reclamation District of Greater Chicago (District) initiated a research study to determine the total numbers and percentages of antibiotic resistant fecal coliform (FC) bacteria in raw sewage (RS) entering and final effluents (FE) discharged from its seven Water Reclamation Plants (WRPs). The density of FC was determined on m-FC agar containing ampicillin (ampR-16 μg/ml), gentamycin (genR-8 μg/ml), tetracycline (tetR-8 μg/ml), or all three antibiotics. The study was primarily undertaken to determine whether secondary sewage treatment at the District WRPs adequately reduces the numbers and percentages of FC_ampR, FC_tetR, FC_genR, FC_amp/tet/genR in the FE. The numbers of ampR, tetR, genR, and amp/tet/genR FC observed in RS ranged from 2.0 × 10⁵ to 1.1 × 10⁷, 9.5 × 10⁴ to 2.2 × 10⁶, 95 to 1.5 × 10⁴ and 90 to 9.5 × 10³ per 100 mL, respectively. Secondary sewage treatment without disinfection was shown to reduce the number of antibiotic resistant FC by two-three orders of magnitude. The numbers of FC_ampR, FC_tetR, FC_genR, and FC_amp/tet/genR observed in non-disinfected FE ranged from 2.0 × 10² to 6.4 × 10³, 2.2 × 10² to 4.1 × 10³, 9 to <20 and 9 to <20 per 100 mL, respectively. The relative percentages of antibiotic resistant FC observed in FE followed the same trend observed in RS: FC_ampR > FC_tetR > FC_genR > FC_amp/tet/genR. Only one FC_amp/tet/genR bacteria was found in this study indicating that multiple-antibiotic resistant FC was virtually eliminated by secondary sewage treatment. The results of multivariate regression analysis showed that the percentages of antibiotic resistant FC in the FE from all seven District WRPs were lower than the percentages of these organisms in RS (p<0.01). These results support the conclusion that secondary sewage treatment in the District effectively reduces the number of antibiotic resistant FC and that the environments of the District's seven WRPs are not conducive to the propagation or survival of antibiotic resistant fecal coliform bacteria.