Different isolates of Bacillus subtilis from gastrointestinal tract of wild caught black tiger shrimp, Penaeus monodon to improve a locally produced probiotic/bioaugmenter for controlling pathogenic Vibrio in Sri Lankan shrimp culture systems

Abstract

Bacillus species are the major probiotic bacteria used in both hatchery and grow-out systems of tropical shrimp culture industry to control pathogenic Vibrio species. Our previous works have confirmed that the use of a locally produced probiotic/bioaugmenter containing a locally isolated strain of Bacillus subtilis could contribute significantly in producing healthy post larvae of Penaeus monodon and in obtaining a profitable harvest from grow-out ponds compared to systems that did not use the product. Therefore, present study was planned to isolate different isolates of Bacillus subtilis to improve that probiotic or to formulate a new probiotic with improved performances. Samples of apparently healthy black tiger shrimp, P. monodon residing in estuaries in the North Western Province were transported to the laboratory. Gastrointestinal tract samples of the shrimp were first subjected to heat treatment (to kill non-spore forming bacteria) and isolation, purification and characterization of different isolates of Bacillus subtilis were carried out following standard microbiological procedures. Isolates were identified as different strains of Bacillus subtilis by biochemical tests in API 20E and API 50 CHB test kits (bioMerieux, France). Out of the nine different isolates of B. subtilis, only three displayed high growth rate while tolerating wide range of salinity (5gL-1 to 35gL-1) and pH (6 to 10). Antagonistic properties of those three isolates of B. subtilis on five species of pathogenic Vibrio of cultured shrimp in Sri Lanka (isolated and identified in our previous work) were investigated. The diameter of inhibitory zones produced by the new isolates of B. subtilis on the pathogenic Vibrio species ranged from 2 to 4 mm; they could be used to improve the locally produced probiotic/bioaugmentor. The selected three isolates of B. subtilis have been sent for the confirmation of strain type by DNA analysis.