Increase in the prevalence of oxolinic acid resistant Acinetobacter spp. observed in a stream receiving the effluent from a freshwater trout farm following the treatment with oxolinic acid-medicated feed

Abstract

Acinetobacter spp. were used as bacterial indicators to monitor antimicrobial resistance in a freshwater trout farm before and after treatment with oxolinic acid (OA)-medicated feed. The level of susceptibility to four antimicrobial agents was determined in 331 Acinetobacter isolates obtained by a selective procedure from a pond (n=100), the inlet channel (n=105), and the stream receiving the farm effluent (n=126). Before treatment, OA resistance was detected only among isolates from the pond (40%), in which the last medication with OA was dated back to 6 months before. Following treatment, a high prevalence of OA resistance was observed among isolates from both the pond (33–53%) and the effluent recipient (21–55%). In contrast, no OA resistant strains were isolated from the inlet channel. OA resistant strains were significantly more resistant to oxytetracycline (OT) compared with OA sensitive strains (PAcinetobacter isolates from the pond and the stream receiving the farm effluent in comparison with isolates from the inlet channel. Since the two biotypes predominant in the pond and the effluent recipient included the vast majority of OA resistant isolates, the low diversity of Acinetobacter spp. observed at these sites could have been enhanced by exposure to the drug. The results of this study indicate that the use of OA in inland farms can affect the levels of antimicrobial resistance and diversity in the microflora of natural aquatic habitats situated downstream. Since the ecological consequences deriving from changes of the indigenous aquatic microflora are unknown, the careful consideration of the use of OA in aquaculture was suggested.