The invitro efficacy of oxytetracycline against re-isolated pathogenic Aeromonas hydrophila carrying the cytolytic enterotoxin gene through hybrid catfish, Clarias macrocephalus (Gunther, 1864) x Clarias gariepinus (Burchell, 1822) in Thailand

Aeromonas hydrophila is a pathogen infecting farmed hybrid catfish, Clarias macrocephalus (Gunther, 1864)xClarias gariepinus (Burchell, 1822) which incurs substantial economic losses in Thailand. The study aimed at a genetic tracking of A.hydrophila infection and the invitro assessment of the efficacy of antibiotics against its virulent strains. Five clinical strains from catfishes and Nile tilapia were employed. They were 3-passage re-isolated through healthy hybrid catfish and the cytolytic enterotoxin gene (AHCYTOEN) of individuals was traced. Each of the re-isolates at a dose of similar to 6.67x10(5)CFU/g was intraperitoneally injected into similar to 15 g-healthy hybrid catfish and their pathogenicity were observed for 7days. It was found that AHCYTOEN was carried over whereas typical signs of motile aeromonas septicaemia were found in the specimens. The bacterial strains of Nile tilapia origin did not induce mortality but those of catfish origins (80%-100% rate of mortality). The strains were susceptible to the tetracycline antibiotics, and oxytetracycline produced MIC50 and MBC as low as 0.007-0.031g/ml and 1-8g/ml respectively. As oxytetracycline specifically inhibited pathogenic A.hydrophila invitro, it is recommended that an appropriate dosage regimen of the drug should be established.