| Aeromonas hydrophila is a main pathogen in aquaculture, which cause diseases in aquatic animals. In recent years, the infection of A. hydrophila is more and more serious to Megalobrama amblycephala. M. amblycephala is a main species of cultured freshwater fishes in China. In this study, A. hydrophila isolated from M. amblycephala cultured in Hubei and Jiangsu Provinces. Pathogenicity experiment was carried out to confirm the infection of A. hydrophila to M. amblycephala. In addition, drug-resistance characteristics of different isolated A. hydrophila strains were investigated. The research results were summarized as follows:1. The investigation of A. hydrophila infects to M. amblycephalain in Hubei and Jiangsu ProvincesThe epidemiology investigation of infection of A. hydrophila to M. amblycephala cultured in8cultured areas in Hubei and Jiangsu Provinces were carried out during July to September in2012. The results showed that the mortality rates exceeding30%recorded in cultured M. amblycephala which caused by A. hydrophila has caused serious economic loss. It was difficult to control even in the early stages of infection. The occurrence peaked from July to September, and the water temperature being above25℃. It was more serious in polluted and higher temperature conditions. It was largely attributed to a large growth of A. hydrophila at pollutional water and high temperature, the immune function decreasing caused by the germplasm resources degradation.2. The pathogenicity of A. hydrophila strains to M. amblycephalaEight A. hydrophila strains isolated from M. amblycephala were selected to be subjects in this study. To investigate the relationship between virulence genes and pathogenicity of A. hydrophila, five virulence genes, namely aerolysin(aerA), serine protease (ahpA), hemolysin(hlyA), enterotoxin(altA), anti-metalloproteinases (astA), were surveyed and detected by PCR amplification. The hemolytic and proteolytic were detected by the multiple proportion dilution method. The lethal dose50%(LD50) was calculated from experiment. Moreover, the study results suggested that the frequency of virulence genes, aerA, ahpA, hlyA, altA and astA was100%(8/8),75%(6/8),87.5%(7/8),100%(8/8) and75%(6/8), respectiely. Two genes, ahpA and altA were not observed in the strain E6; and three genes, ahpA, hlyA, altA were not detected in strainX22. Among these six strains recorded with all virulence genes, five strains showed the ratio of hemolytic and proteolytic of1:128,1:128, respectively; and the remaining strain showed the ratio of hemolytic and proteolytic were up to1:128,1:64. The hemolytic and proteolytic ratio of E6and X22were lower than others which were1:64,1:64; and1:32,1:64, respectively. The LD50for M. amblycephala of the6strains were from5.0x105to1.0x106cfu/ml. The LD50of another two strains E6and X22was2.4x106cfu/ml and4.0x106cfu/ml, respectively. It found that aerA, ahpA, hlyA should be considered as key virulence genes, in which, altA and astA were less infectious.3. The drug-resistance characteristics of A. hydrophilaTo research the drug-resistance characteristics of8strains of A. hydrophila,25kinds of antibiotic were chosen to test using disk diffusion method. The results showed that8strains were highly resistant to penicillins, sensitive to the antibiotics of the third and fourth cephalosporin, and certain extent resistance to aminoglycosides, fluoroquinolones, sulfonamides, and tetracyclines. The different strains were different in drug resistance. Strain X22was the most resistant. This may be related to the culture conditions and overuse of antibiotics. Strain W2was low resistant, but the trend of resistance was recorded. The strains isolated from Hubei Province showed more resistance to Cephalothin V, Kanamycin, Novobiocin than the strains isolated from Jiangsu Province. The strains isolated from Jiangsu Province showed more resistance to Enrofloxacin, Tetracycline than the strains isolated from Hubei Province. The resistance trend of strains isolated from Hubei Province showed more profoundly to Novobiocin. The main reason was the difference in choosing and using the drugs. |
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