UNDERSTANDING INTERSPECIES COMPETITION WITHIN PROBIOTICS AND DEBUNKING THE POND SIDE GROW OUT MYTH

Although highly susceptible to disease which can result in huge economic losses, shrimp farming has become one of the fastest growing food production areas.  Probiotics have become an attractive alternative to chemotherapeutics for use in aquatic disease management and there are numerous scientific references to probiotic use in shrimp aquaculture.   Probiotics can improve disease resistance and survival of shrimp due to antagonism to disease causing agents, stimulate the immune system, support digestive activities, and reduce stress by improving water quality.  Most commercial products are combinations of bacteria that have been selected for their individual activities.  However, there has been increasing speculation that probiotics in combination are capable of either working together or against each other in the ponds.  Questions regarding probiotic interactions, dosing strategies and grow out practices need to be answered to improve disease management and water quality in shrimp farming operations.  Therefore, we examined interactions among probiotics commonly used in shrimp farming by testing different combinations of bacteria in agar well diffusion assays.  Bacteria from overnight cultures (100 µl) were spread on TSAII agar plates.  Cell free supernatants (CFS) were prepared from overnight cultures of different Bacillus subtilis strains (A-D).  Zones of inhibition were observed and measured (Table 1).  

Interestingly, all four B. subtilis strains react differently in the inhibition assay.  Three of the four strains are antagonistic to B. amyloliquefaciens and B. methylotophicus.   B. subtilis (A) is antagonistic to B. pumilus and possibly to B. licheniformis, while B. subtilis (C) appears to improve the growth of B. methylotrophicus.  To further examine these interactions, equal amounts of B. subtilis (A) and B. pumilus were grown in liquid culture and the combined culture was examined overtime with B. subtilis (A) overtaking B. pumilus within 6 hours.  These data suggest that pond side grow outs could reduce the efficacy of the commercial product.