The effect of the sulfonamide class of antibiotics, namely, trimethoprim and sulfamethoxazole (TMP–SMX) on growth and metabolism of a bacterial consortium isolated from an anaerobic digester of a sewage treatment plant, was investigated. The bacterial consortium was exposed to various concentrations of TMP–SMX. The results indicated that there was no effect of TMP and SMX on the bacterial consortium in terms of bacterial growth and carbon and nitrate removal even at the highest concentration tested at 250 mg/L. The carbon removal was more than 90%, and the nitrate removal was more than 87% in all culture conditions. A pure culture was isolated and identified as Leuconostoc mesenteroides from the consortium, which was able to use TMP and SMX as the sole source of carbon for growth. This pure culture has the sul1 gene to show resistance to these antibiotics. This research could be used in the anaerobic digestion process to degrade various classes of sulfonamide antibiotics before the treated sludge is discharged into the environment to reduce the concentration of TMP and SMX.