MOLECULAR IDENTIFICATION AND DRUG RESISTANCE OF PATHOGENIC AEROMONAS SOBRIA ISOLATED FROM MISGURNUS ANGUILLICAUDATUS

Outbreaks of mass mortality among cultured loach Misgurnus anguillicaudatus occurred on some farms of Dunshang county, Jiangsu province. A predominant strain (designated as NQ090701) was highly virulent to loach; all fish injected with strain NQ090701 died from the 2th to 3th day post-injection, and all the moribund/dead loach exhibited the same gross symptoms as that observed in natural outbreaks. Morphological, physiological and biochemical characteristics of strain NQ090701 were very similar to Aeromonas sobria. Sequencing analyses of 16S rRNA, gyrB and rpoD also re- vealed strain NQ090701 to be highly homologous to A. sobria; in the phylogenetic trees constructed using the neighbor-joining method based on 16S rRNA, gyrB and rpoD gene, strain NQ090701 all clustered with the A. sobria. Therefore, strain NQ090701 was identified as A. sobria based on the phenotypes and molecular characteristics. In addition, ten antimicrobial drugs were examined for their minimum bactericidal concentrations to strain NQ090701 by the tube dilu- tion. Norfloxacin had the preferable bactericidal effects, because all the tested drug concentrations can kill the strain NQ090701; and sarafloxacin hydrochloride is the second preferable drug, of which the minimal bactericidal concentration was 4mg/L. Strain NQ090701 was resistant to thiamphnicol and sulfamethoxazole in the range of 1mg/ml to 30mg/ml. Results in this study reveal that norfloxacin and sarafloxacin hydrochloride can effectively control the disease of loach caused by pathogenic Aeromonas sobria in aquaculture.