Impact of an urban effluent on antibiotic resistance of riverine Enterobacteriaceae and Aeromonas spp.

In order to evaluate the impact of an urban effluent on antibiotic resistan ce of freshwater bacterial populations, water samples Here collected from t he Arga river (Spain), upstream and downstream from the wastewater discharg e of the city of Pamplona, Strains of Enterobacteriaceae (representative of the human and animal commensal flora) (110 isolates) and Aeromonas (typica lly waterborne bacteria) (118 isolates) were selected for antibiotic suscep tibility testing. Most of the Aeromonas strains (72%) and many of the Enter obacteriaceae (20%) were resistant to nalidixic acid. Singly nalidixic acid -resistant strains were frequent regardless of the sampling site for Aeromo nas, whereas they were more common upstream from the discharge for enteroba cteria, The most common resistances to antibiotics other than quinolones we re to tetracycline (24.3%) and beta-lactams (20.5%) for Enterobacteriaceae and to tetracycline (27.5%) and co-trimoxazole (26.6%) for Aeromonas. The r ates of these antibiotic resistances increased downstream from the discharg e at similar degrees for the two bacterial groups; it remained at high leve ls for enterobacteria but decreased along the 30-km study zone for Aeromona s. Genetic analysis of representative strains demonstrated that these resis tances were mostly (enterobacteria) or exclusively (Aeromonas) chromosomall y mediated. Moreover, a reference strain of Aeromonas caviae (CIP 7616) cou ld not be transformed with conjugative R plasmids of enterobacteria, Thus, the urban effluent resulted in an increase of the rates of resistance to an tibiotics other than quinolones in the riverine bacterial populations, desp ite limited genetic exchanges between enterobacteria and Aeromonas, Quinolo ne resistance probably was selected by heavy antibiotic discharges of unkno wn origin upstream from the urban effluent.