SCREENING AND IDENTIFICATION OF TWO PROBIOTIC BACILLUS STRA

22 strains of Bacillus with different characters were isolated from soil and shrimp ponds. Approximately 5 ×108 logarithmic phase cells of every strain were administered into 500 mL seawater culturing Liptopenaeus vannamei nauplius m larvae respectively. Based on the effect of Bacillus on the survival rate of L. vannamei larvae metamorphosis from nauplius m to zoea Ⅱ, 10 strains which had very significant (p<0.01 ) effects on larval metamorphosis were screened initially. Then from the 10 strains screened, two strains, zou4 and zou8 which had more significant (p<0.05) effects on larval metamorphosis than other strains were selected as potential probiotic Bacillus for L. vannamei larvicuhure. Combining phenotypic and physiological characteristics, zou4 and zou8 were identified initially as B. firmus and B. cereus respectively. To investigate the phylogenetic posi-tion of strains zou4 and zou8, about 1.5kb 16S rRNA genes were sequenced and compared with that of related strains, which re-vealed that strains zou4 and zou8 exhibited the highest levels of similarity to B. firmus and B. cereus respectively, similarity value were all above 99 %. Phylogenetic tree was constructed based on genetic distance analysis, which also showed that strains zou4 and zou8 clustered as B. firmus and B. cereus respectively. Based on the results obtained, strains zou4 and zou8 were identified as B. firmus and B. cereus respectively.