BIOSCIENCES BIOTECHNOLOGY RESEARCH ASIA, August 2014. Vol. 11(2), 507-515 * To whom all correspondence should be addressed. Tel.: +98-9171140799; Fax: +98-7913331019; E-mail: Kafilzadeh@jia.ac.ir Putative Bacterial Flora Assessment in Juvenile Litopenaeus vannamei as Probiotics by using Antibacterial Activity Tests Farshid Kafilzadeh 1 *, Mina Ziarati 1 and Mehran Avakh Keysami 2 1 Department of Biology, Jahrom Branch, Islamic Azad University, Jahrom, Iran. 2 Department of Aquaculture, MirzakochekKhan Jihad Agriculture Technical and Vocational Higher Education Center, Rasht, Iran. doi: http://dx.doi.org/10.13005/bbra/1300 (Received: 30 May 2014; accepted: 07 July 2014) The objective of this study was to find an effective probiotic of putative bacterial flora, using in vitro antagonism test in Litopenaeus vannamei. Twelve groups of putative bacterial flora, isolated from juvenile Litopenaeus vannameiPersian Gulf Jihad Agriculture Center Hatchery unit, were used for antibacterial investigation against six pathogenic bacteria including Vibrio parahaemolyticus, Aeromonas hydrophila, Pseudomonas aeroginosa, Escherichia coli, Salmonella sp. and Vibrio cholerae. The three groups of putative bacteria (Bacillus sp., Micrococcus sp. and Corynebacteriumsp.), collectively showed antagonistic activity against the six pathogens. Bacillus sp. showed greatest inhibition of pathogens both in well and disc diffusion assays. Bacillus sp. also inhibited all six pathogens tested by the cross-streak method. Addition of 10 ml of cell- free supernatant of Bacillus sp. to pathogens resulted in complete suppression within 12 h. Maximum growth and maximum zone of inhibition (16 mm), against A. hydrophila were accrued at pH 8. Very slow growth was observed at pH 6 and pH 9. There was no growth at pH 4 and pH 5. Maximum growth was observed at 30 o C and 37 o C with maximum production of the antibacterial component. TSB media with 1% NaCl was optimum for growth and the zone of inhibition was 15 mm. The culture reached a stationary phase at 12 h but maximum antibacterial activity was observed only at 30 h. These results suggest that Bacillussp. has potential applications for control of pathogenic bacteria in aquaculture systems. Key words: Putative bacteria, Inhibition zone,Bacillus sp., Antibacterial activity, Litopenaeus vannamei. Pathogenic microorganisms population in larvae and rearing tank water has been reported to reduce the survival rate of larvae and post larvae 1 . In the present day diseases have frequently affected shrimp culture all over the world. Implicated in these outbreaks were viruses, bacteria, rickettsia, mycoplasma, algae, fungi and protozoan parasites 2 . Bacteria species are microorganisms that live in the shrimp’s environment, often as part of the normal microflora inhabiting the surface of their cuticle or colonizing areas of the gut or hepatopancreas 3 . For preventing and controlling from infectious microbial diseases a host of antibiotics and other chemicals have been used 2 . The use of antibiotics during shrimp cultivation has led to problems of bacterial drug resistance 4 . Now, researchers are trying to use probiotic bacteria in aquaculture to improve water quality by balancing bacterial population in water and reducing pathogenic