| Epidemiologic investigation from 2008 to 2009 showed that freshwater fish bacterial disease commonly occurred in some fish farms of Jiangsu, Sichuan, Guangdong, Jiangxi and Hubei Province. The disease fish typical symptom was filled with blood in fins, skins, intestines and organs. Generally speaking the serious infected fish had a blooding muscle, body surface ulcers, incomplete fin, incomplete tail and abdominal dropsy. This bacterial disease infected a lot of important freshwater fish farmed in China, Such as silver carp, crucian carp, grass carp, blunt snout bream and channel catfish. The 65 strains were isolated from the diseased fish under natural conditions. Healthy crucian carp were infected with some strains of the isolated. These strains induced 10%-100% mortality. According to the result of many morphological, physiological and biochemical characteristics tests, comparison to the data of Bergeys manual of determinatine bacteriology 9th, DFJY-001, XZBL-011, CYBH-009 and others total of 54 strains belonged to Aeromonas hydrophila, HACY-007, NCTTF-013 and others total of 11 strains belonged to Aeromonas sobria. A. hydrophila and A. sobria share were 83.1% and 16.9%. A. hydrophila was considered as the main pathogenic bacteria. The freshwater fish can be infected by A. hydrophila or A. hydrophila and A. sobria.Recent years, with the rapid development of aquaculture, the freshwater fish bacterial disease which caused by A. hydrophila commonly occurred in China. It caused large economic damages to aquaculture production. However, the phenomenon of irrational using antimicrobial frequently occurred, leading to the bacteria resistant to antimicrobial agents increasing rapidly. The antibacterial resistance problem has caused widespread attention home and abroad. The 52 strains of A. hydrophila isolated in freshwater fish collected from four areas of China. Their susceptibilities to many antimicrobial agents studied by using agar disk diffusion method and curette dilution method. The result showed the following four aspects.(1) Two methods'results were similar in the general trend. But the proportion of bacteria resistant to antimicrobial agents by using agar disk diffusion was higher than the other. This difference may be caused by two reasons. On the one hand, the standard of bacteria resistant to antimicrobial agents was different from two methods. On the other hand, the antimicrobial agents of two methods came from different company.(2) All the strains collected from four areas of China were resistant to penicillin, ampicillin, midecamycin, spiramycin, sulfisoxazole and terimethoprim. But the different frequency of bacteria resistant to aminoglycoside antibiotics, tetracyclines antibiotics and quinolones was found. The 100% of the strains collected from Jiangsu province were resistant to nice antimicrobial agents. On the contrary, the strains isolated in grass fish collected from Guangdong province were sensitive to various antimicrobial agents. The others strains had the resistance to many general antimicrobial agents, and a high proportion of bacteria resistance came from Sichuan province.(3) The phenomenon of multiple antibacterial resistance was widely existed in all the strains, especially serious occurred in eight strains collected from Jiangsu province. The 100% of bacterial strains isolated from Jiangsu with simulataneous resistance against 16-17 antimicrobial agents. For example, gentamycin, kanamycin, streptomycin, neomycin, tetracycline, doxycycline and so on. Whereas the strains collected from Guangdong, Sichuan and Hubei province separate showed resistance against 8-9,9-13, 7-14 antimicrobial agents.(4) The effect of many common antimicrobial agents were reduced than before, but the isolated strains showed high suspectibilities to quinolones. The minimum inhibitory concentration of few strains were 3.13μg/mL, and majority of the strains collected from different areas except Jiangsu province were 0.39μg/mL to 1.56μg/mL. The results indicated that Levofloracin has good effect to A. hydrophila. Key words:Aeromonas hydrophila; Aeromonas sobria; freshwater fish; antimicrobial agents; antibacterial resistance; multiple antibacterial resistance...
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