BackgroundThe probiotic bacterium Phaeobacter inhibens strain S4Sm, isolated from the inner shell surface of a healthy oyster, secretes the antibiotic tropodithietic acid (TDA), is an excellent biofilm former, and increases oyster larvae survival when challenged with bacterial pathogens. In this study, we investigated the specific roles of TDA secretion and biofilm formation in the probiotic activity of S4Sm.ResultsMutations in clpX (ATP-dependent ATPase) and exoP (an exopolysaccharide biosynthesis gene) were created by insertional mutagenesis using homologous recombination. Mutation of clpX resulted in the loss of TDA production, no decline in biofilm formation, and loss of the ability to inhibit the growth of Vibrio tubiashii and Vibrio anguillarum in co-colonization experiments. Mutation of exoP resulted in a ~60 % decline in biofilm formation, no decline in TDA production, and delayed inhibitory activity towards Vibrio pathogens in co-colonization experiments. Both clpX and exoP mutants exhibited reduced ability to protect oyster larvae from death when challenged by Vibrio tubiashii. Complementation of the clpX and exoP mutations restored the wild type phenotype. We also found that pre-colonization of surfaces by S4Sm was critical for this bacterium to inhibit pathogen colonization and growth.ConclusionsOur observations demonstrate that probiotic activity by P. inhibens S4Sm involves contributions from both biofilm formation and the production of the antibiotic TDA. Further, probiotic activity also requires colonization of surfaces by S4Sm prior to the introduction of the pathogen.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0617-z) contains supplementary material, which is available to authorized users.

Bacterial pathogens, including several Vibrio spp. and Roseovarius crassostreae, cause severe mortality of larval and juvenile eastern oysters. The introduction of beneficial bacterial isolates in oyster hatcheries and nurseries for the biocontrol of bacterial diseases is a good alternative to the use of antibiotics. The goal of this study was to screen and characterize marine bacterial isolates as potential agents to prevent larval and juvenile mortality by the oyster pathogens Vibrio tubiashii and R. crassostreae. Screening of bacterial isolates from Rhode Island marine organisms and environment using agar-based assay methods for detection of antimicrobial activity against oyster pathogens led to the isolation of candidate probionts Phaeobacter sp. S4 and Bacillus pumilus RI06-95. Pretreatment of larval and juvenile oysters for 24 h with 102-106 cfu/mL Phaeobacter sp. S4 or B. pumilus RI06-95 protected larval oysters against mortality resulting from challenge with R. crassostreae and V. tubiashii (relative percent survival (RPS) range, 9%-56%). These probiotics also protected juvenile oysters against challenge with V. tubiashii (RPS, 37%-50%). Probiotic isolates had no negative impact on oyster survival. Protection conferred to larvae against bacterial challenge was short-lived, lasting for only 24 h after removal of the probiotics from the incubation water. These results suggest the potential of marine bacterial isolates Phaeobacter sp. S4 and B. pumilus RI06-95 to serve as biocontrol agents to reduce the impact of bacterial pathogens in the culture of Crassostrea virginica.

Bacteria under the operational group Bacillus amyloliquefaciens (OGBa) are all Gram-positive, endospore-forming, and rod-shaped. Taxonomically, the OGBa belongs to the Bacillus subtilis species complex, family Bacillaceae, class Bacilli, and phylum Firmicutes. To date, the OGBa comprises four bacterial species: Bacillus amyloliquefaciens, Bacillus siamensis, Bacillus velezensis and Bacillus nakamurai. They are widely distributed in various niches including soil, plants, food, and water. A resurgence in genome mining has caused an increased focus on the biotechnological applications of bacterial species belonging to the OGBa. The members of OGBa are known as plant growth-promoting bacteria (PGPB) due to their abilities to fix nitrogen, solubilize phosphate, and produce siderophore and phytohormones, as well as antimicrobial compounds. Moreover, they are also reported to produce various enzymes including α-amylase, protease, lipase, cellulase, xylanase, pectinase, aminotransferase, barnase, peroxidase, and laccase. Antimicrobial compounds that able to inhibit the growth of pathogens including non-ribosomal peptides and polyketides are also produced by these bacteria. Within the OGBa, various B. velezensis strains are promising for use as probiotics for animals and fishes. Genome mining has revealed the potential applications of members of OGBa for removing organophosphorus (OPs) pesticides. Thus, this review focused on the applicability of members of OGBa as plant growth promoters, biocontrol agents, probiotics, bioremediation agents, as well as producers of commercial enzymes and antibiotics. Here, the bioformulations and commercial products available based on these bacteria are also highlighted. This review will better facilitate understandings of members of OGBa and their biotechnological applications.

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In this study, African catfish (Clarias gariepinus) (9.68 ± 0.16 g, mean ± SE) were reared with biofloc technology (BFT) with three different carbon sources; raw rice bran (RRB) or when incubated (24 hr) with Bacillus species under aeration (cellular respiration, ResRB), or without aeration (fermentation, FerRB). The proximate composition, water solubility, and total soluble sugars of RRB, ResRB, and FerRB were measured. A control included fish cultured in a recirculating system. Water quality, biofloc production/proximate composition and subsequent effects to growth, feeding efficiencies, body proximate composition, and selected plasma biochemical parameters to triplicate groups of C. gariepinus were measured after 42 days. FerRB had the lowest crude fibre and higher total soluble sugars and water solubility compared to ResRB or RRB. Biofloc produced with FerRB had significantly higher (p < 0.05) crude protein. Ammonia‐N was significantly lower (p < 0.05) in the first 3 weeks in both control and FerRB treatments. Using FerRB significantly improved (p < 0.05) growth, feeding efficiencies, and whole‐body crude protein in African catfish compared to the control and ResRB. Therefore, using FerRB with BFT can be a highly effective strategy to create a zero‐exchange culture system while also significantly improving growth and feeding efficiencies of African catfish juveniles.

Bacterial pathogens are a major cause of mortality in bivalve hatcheries, and outbreaks can result in shortages of seed supply to the grow-out industry. The use of probiotic bacteria is a potential preventative measure to limit the impact of bacterial diseases. Previous research showed that the marine bacteria Phaeobacter inhibens S4 (S4) and Bacillus pumilus RI06-95 (RI) protect larval eastern oysters (Crassostrea virginica) when challenged with the pathogens Vibrio tubiashii RE22 (now Vibrio coralliilyticus RE22) and Roseovarius crassostreae CV919-312 T. In this study, these probiotic bacteria were tested under hatchery conditions. Daily addition of S4 and RI (10 4 colony forming units (CFU)/ml) to 100-l culture tanks resulted in a significant decrease in the levels of total Vibrios in water and tank surfaces (P < 0.05), but not in oysters. Larval growth and survival was unaffected by the probiotic treatments. Larvae treated with probiotics in the hatchery showed significantly less mortality than larvae from control tanks when exposed to 10 5 CFU/ml of V. coralliilyticus RE22 for 24 h in a laboratory challenge. These results suggest that S4 and RI are safe and potentially effective tools to limit disease outbreaks in oyster hatcheries.

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Global aquaculture production continue growing in order to meet up with ever increasing fish demand, especially as fish from capture has levelled off and makes an increase in aquaculture production as the only hope to meet the demand for fish. Aquaculture industry involves culture of many fish species of either fresh or brackish water origin and among the important freshwater fish species is North African catfish which is also called African catfish (Clarias gariepinus). Its production is increasing as the total aquaculture production also increases and its culture is spread globally, with Nigeria officially reported as producing the highest annual amount of the fish. There is a substantial culture of the fish in Malaysia, though not a native fish species but its presence in Malaysia is getting closer to over three decades and for almost a decade now, it has been the highest finfish produced in either fresh or brackish water aquaculture in Malaysia. This review discussed African catfish aquaculture in Malaysia and Nigeria, compared the production and prospects of the industry in the two countries.