Multiple Drug-resistant Strains of Aeromonas hydrophila Isolated from Tilapia Farms in Thailand

Fifty-five Aeromonas hydrophila isolates, which were collected from infected tilapia Oreochromis niloticus at aquaculture farms in Thailand, were tested for their antimicrobial susceptibilities against 11 drugs including ampicillin (ABPC), kanamycin (KM), streptomycin (SM), chloramphenicol (CP), florfenicol (FF), tetracycline (TC), sulfamonomethoxine (SMMX), trimethoprim (TMP) and three quinolones (QNs). All strains showed resistance to one or more drugs except FF, and more than half of the strains showed resistance to five drugs and more. These isolates were conjugated with Escherichia coli for detection of the transferable R-plasmid, and only one transferable R-plasmid, which encoded resistance to the combination of ABPC, CP, SM, SMMX and TC, was found from one isolate. This R-plasmid contained blaOXA-35 (ABPC resistance), cat2 (CP resistance), aadA1 (SM resistance), sul1 (SMMX resistance) and tetA (TC resistance).