Phenotypic and genotypic characterization of tetracycline- and oxytetracycline-resistant Aeromonas hydrophila from cultured channel catfish (Ictalurus punctatus) and their environments.

Oxytetracycline-resistant (OTcr) and tetracycline-resistant (Tcr) Aeromonas hydrophila were isolated commonly from catfish intestinal contents and the water and sediment of catfish culture ponds, but less frequently in market catfish. Isolates demonstrated two resistance phenotypes, Tcr OTcr and Tcs OTcr, when plated directly on to MacConkey agar containing 30 micrograms of tetracycline or oxytetracycline per ml. Tcs OTcr isolates expressed Tcr after induction by 1 h of growth in tryptic soy broth containing 1 microgram of tetracycline per ml. Over 90% of the resistant aeromonads hybridized with DNA probes for class A or class E Tcr determinants; class E was twice as prevalent as class A. The distribution of class A and E Tcr determinants varied with the Tcr phenotypes. Prior to induction, 86% of isolates with class A determinants were Tcr as well as OTcr, while only 16% with class E determinant were Tcr. Three of 5 Tcr aeromonads without class A or class E determinants and 1 of 14 with class E determinants transferred Tcr to Escherichia coli.