Epi300tm

Minimum inhibitory concentrations (MICs) of AHL-degrading bacteria were determined using microtiter plate dilution assays in R2A broth with about 1 × 10⁵ cells/well, as previously described (Yajko et al., 1987 (link); Riesenfeld et al., 2004 (link)). MICs were determined after 1, 2, and 3 days of incubation at 30°C in the dark. MICs were read using a Multiskan^® Spectrum microplate spectrophotometer (Thermo Labsystems, Vantaa, Finland) and was defined as the lowest concentration of an antimicrobial agent at which the organism showed no visible growth (Yajko et al., 1987 (link)). E. coli strain EPI300^TM (Epicentre, Madison, WI, United States) was used as a negative control. The criteria used for the interpretation of antimicrobial susceptibility were based upon the achievable levels of antimicrobial agents. The tested antibiotics were PENG, AMP, AMO, carbenicillin (CAR), piperacillin (PIP), CEFL, and CEFD. All these antibiotics are in common use, and the environmental samples used in the present study were polluted with wastewater from PENG, AMP, AMO, CEFL, and CEFD production. The antibiotics were tested at concentrations of 8, 16, 32, 64, 125, 250, and 500 μg/mL.