Bacillus sp. S11, used as probiotic in penaeus monodon , produced antimicrobial substance in log phase of growth cycle. In this research, the optimum conditions for production of this antimicrobial substance are medium containing 2% (w/v) yeast extract. 0.25%(w/v) dipotasslum phosphate pH 7.0, 2.0% (v/v) inoculum, and aeration at 200 rpm at 40°C. The antimicrobial activity of cultured broth decreased after incubation at 70 and 80°C for 15 minutes and could not be detected after 60 minutes. its activity also ndisappeared after incubating at 100 °C for 10 minutes or autoclaving at 121°C for 20 minutes. Antimicrobiat substance showed its activity at the conditions of wide range of pH (4-10) and 0-5% (w/v) sodium chloride, Prolease could reduce the activity of the substance whereas no effect of α-amylase and lipase could be detected. The solvents such as acetone, acetonitrile, ethanol and methanol could not reduce antimicrobial activity of cultured broth whereas chloroform, diethylether and toluene could reduce this activity. Bactericidal activity of antimicrobial pellet from 0-30% saturated ammonium sulfate precipitation was tested with B. cereus ATCC 11778 and V. harveyi 639. The results showed that B.cereus ATCC 11778 was killed within 5 hours by this antimicrobial concentration of 4.10 AU/ml. However, V. harveyi 639 was not killed within 5 hours by the same substance at concentration below 204.8 AU/ml. The purification of this substance was performed by the steps of precipitationg with 0-30% saturated ammonium sulfate. Running through Sephadex G-50 column chromatography and DEAE. Sephadex A-25 column chromatography. The specific activity of this substance increased from 28.57 AU/mg protein of cultured broth to 1256.44, 2133.33 and 3200 AU/mg protein, respectively The results from SDS-PAGE and Sudan Black B staining showed that this molecular weight was approximately 3.5 kDa and consisted of lipid.
