| A strain of pathogenic bacteria (strain X-l) isolated from diseased crab was identified and the pathogenic mechanism was also studied by using bacteriological and molecular biological methods.First,according to morphology,physiology and biochemistry,serological reaction,and numerical system,the bacterium was identified as Pseudomonas.Second,the bacterium was pathogenic to mouse and crab in animals experiments and the mortalities were 100% and 87.5% respectively,and the bacterial LDsowas 2.9 X107 CPU per mouse. The effects of aquatic environment factors on bacterial pathogenicity were also studied,At different water temperatures (30,25 and 20),the moralities for crabs were about 100%,80% and 60% respectively. When water pHs were 5.0 and 10.0,the moralities for crabs were all about 100% within 10 hours,however,at pH7.5 it was only 40%. With supplying enough air in water and not doing it,the moralities for crabs were 60% and 100% respectively within 10 hours. In addition,the bacterium could agglutinate erythrocytes of chicken,mouse,cow,Crucian crap,Silver carp,rabbit and dove,and adhere to Hela cell strongly.When the lipopolysaccharide (LPS) of endotoxin extracted by Trichloroacetic Acid (TCA) lysis was analyzed,the result indicated that LPS had pathogenicity,but no lethality to crab.There were about 3.107 mg/mL,3.952mg/mL and 3.372mg/mL to the concentrations of the preliminarily extracted Extracellular Products (ECPs) which were obtained from culture mediums at different culture temperatures (20 ,25 and 30). The mortalities of ECPs were 20% (20),60% (25 ) and 20% (30) to crabs. Except for a 54 000 protein band,most protein bands were alike among ECPs by analyzing the profiles of SDS-PAGE. The ECPs had hematolytic activities,but no cell toxicities or casein enzymatic activities.All results in this study suggests that the main pathogenic mechanism of the lowly pathogenic Pseudomonas (strain X-l) might be the cooperation of adhesin,endotoxin and toxoproteins in ECPs to crab.
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