| Vibrio is one of the primary pathogenic bacteriums of ocean reared creature, which distributes over sea water and organism nearby the coast ,and its pathogenicity depends on parasitifer'physiological functions and water quality environmental condition, etc. It hazardous fishes, Shrimp and any another shellfishes,and seriousiy contaminates aquaculture in china. Accordingly, in order to encourage mariculture's continue development, Vibriosis must be prophylaxised and controlled.Hard clam Meretrix meretrix is a important commercially mollusc species along the coast in China.Recently ,as cultivation formula's expansion, Meretrix meretrix's large scale decease have posed great economy expenses because of bad water quality and epidemic.Thereby, we must preclude and control disease which happens on hard clam Meretrix meretrix to ensure its cultivation develop healthly. Vibriosis'epidemicity infection is mainly reason of its death,some pathogenic bacterium have been isolated and proved its pathogenicity from diseased hard clam Meretrix meretrix, such as Vibrio alginolyticus, Vibrio furnissii and Vibrio parahaemolyticus, and some scholars have studied on pathogenicity and the prevention and cure of Vibrio parahaemolyticus.This experiment isolate ascendant Pathogenic bacterium(WT01) form diseased Clam, and was confirmed to be the pathogens of the epidemic by artificial infection experiment. The morphological, physiological and biochemical characteristics and 16S rRNA sequence were analyzed. And the relationship between reproduction of the bacterium and NaCl concentrations, pH and temperature were also determined. The results showed that the strain was gram-negative, glucose fermented without gas production, oxidase positive, required sodium ions for growth, no pigment, non-luminescence; It grews well on TCBS-plate as yellow colonies and was sensitive to the vibriostratic agent O/129. Therefore, it was confirmed that the strains belonged to the genus of Vibrio. The sequence analysis of 16S rRNA gene of WT01 isolation and comparison with that of other related vibrios-showed that WT01 was very close to Vibrio natriegen, The similarities was 99%.Concolusionly, strain WT01 belongs to Vibrio natriege, and the LD50 of the bacterium to Meretrix meretrix was 5.5×106CFU/mL. The isolate could grow well at temperature of 10-42℃, pH from 5to 10 and NaCl of 0.5-8% The sensitive test shows that the pathogen is sensitive to drugs such as norfloxacin (NOR) ,bactrim (SXT) ,ceftriaxone (CRO), cefobid (CFP), streptomycin (STR), chloromycetin (CMP)and cidomycin(GEN).For the sake of determining the effects of microecologics on the non-specific response of Meretrix meretrix, they were exposed to microecologics (Bacillus sp.) at environmental levels (1×105cfu/L,1×106cfu/L,1×107cfu/L) for1d,3d,7d,14d,21d,28d respectively and recovered for 14 days,at experimental condition. The effects on catalase (CAT), superoxide dismutase (SOD) activities, antibacterial activities and bacteriolytic activities the digestive gland were evaluated. Besides, the Meretrix meretrix was injected peritoneally with Vibrio natriegen in 28 d for determine the effect of microecologics on the disease resistance. The results showed that all the experiment group can remarkably enhance Meretrix meretrix's nonspecial immune factors'activities, the antibacterial activities and SOD activities were enhanced significantly, and immune factors activities have contrast in different concentration groups. Contrast to control group, bacteriolytic activities and CAT activities increased significantly in 7d, while antibacterial activities and SOD activities increased significantly in 14d. transferred Meretrix meretrix to normal sea water for 20 days, bacteriolytic activities and SOD activities were recovered to the level corresponding to the control group, while antibacterial activities and CAT activities still much higher than control group.The result of peritoneal injection indicated that relative percent survival values of Meretrix meretrix in test group s reached 22.2%, 27.8%, and 44.4%, respectively, which exhibits that the used microecologics can enhance the nonspecial immune function of Meretrix meretrix. The results indicated that microecologics improved the water quality apparently according to the detection.
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