Aim:  To assess Pediococcus acidilactici as a dietary supplement for on‐growing red tilapia (Oreochromis niloticus). Methods and Results:  Tilapia were fed either a control diet or control diet supplemented with Ped. acidilactici at 107 CFU g−1 for 32 days. Ped. acidilactici colonized the intestinal tract and significantly affected the intestinal microbial communities. PCR‐DGGE revealed direct antagonism of gastric Ped. acidilactici with an endogenous uncultured bacterium during a period of reverting to nonsupplemented feeding. Light microscopy revealed that gut integrity and leucocyte levels were unaffected by Ped. acidilactici; however, blood leucocyte levels and serum lysozyme activity were elevated after 14‐days' feeding. No significant improvements in growth performance were observed at the end of the trial (day 32), but survival was significantly higher in the probiotic group. Conclusions:  The study demonstrates that oral supplementation of Ped. acidilactici modulates intestinal bacterial communities in on‐growing red tilapia and also stimulates some aspects of the nonspecific immune response. Significance and Impact of the study:  To our knowledge this is the first study assessing the effects of probiotics on the gut microbiota of tilapia using culture‐independent methods. Such methods are crucial to understand the mechanisms which underpin and mediate host benefits.

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The aim of the present study was to assess the effect of a commercial alginic acid source (Ergosan) on tilapia Oreochromis niloticus intestinal microbial balance, intestinal morphology, and growth parameters. Fish were fed a basal control diet or the basal diet plus a source of alginic acid (5 g kg(-1) Ergosan; Schering-Plough Aquaculture, UK) for 9 weeks. At the end of the trial, light and electron microscopy demonstrated that the morphology of the intestinal tract at the gross and ultra-structural level was not affected by dietary alginic acid inclusion. Both groups of fish displayed healthy, normal morphology with no signs of disease, cell or tissue damage. Intestinal epithelial leucocyte infiltration was not affected by dietary alginic acid. Molecular bacterial profiles derived from PCR-DGGE illustrated highly similar microbial communities (both within the lumen and associated with the intestinal mucosa) in the respective treatment groups. Microbial ecological parameters (e.g. species diversity and richness) also remained unaffected. Although not significant, trends towards elevated survival and body protein content were observed in the alginic acid-fed fish. These results are suggestive that alginic acid does not adversely impact the indigenous gastrointestinal microbial balance and subsequently does not impact upon the epithelial brush border integrity. Validation of non-detrimental impacts of immunostimulatory products on gastric microbiota and epithelial integrity should be pursued in future studies as maintaining microbial balance and epithelial integrity is essential for proper gut functionality.

Both hypoxia and hyperoxia, albeit in different magnitude, are known stressors in the aquatic environment. Adopting an integrated approach, mirror carp (Cyprinus carpio L.), were exposed chronically (i.e. 30 days) to hypoxic (1.8 ± 1.1 mg O(2) l(-1)) and hyperoxic (12.3 ± 0.5 mg O(2) l(-1)) conditions and resultant biological responses or biomarkers were compared between these two treatments as well as with fish held under normoxic conditions (7.1 ± 1.04 mg O(2) l(-1)). The biomarkers determined included the activities of glutathione peroxidase (GPx), measurement of oxidative DNA damage (using modified Comet assay employing bacterial enzymes: Fpg and Endo-III), haematological parameters, histopathological and ultrastructural examination of liver and gills. Specific growth rate (SGR) of the fish, as an important ecotoxicological parameter was also determined over the exposure period. The study suggested that while the levels of hepatic GPx were unaffected, there was a significant difference in activity in the blood plasma under different exposure conditions; the hyperoxic group showed increased GPx activity by approximately 37% compared to normoxic group and the hypoxic group showed a decrease by approximately 38% than the normoxic group. Interestingly, oxidative DNA damage was significantly higher in both hypoxic and hyperoxic by approximately 25% compared to normoxic conditions, Fpg showing enhanced level of damage compared to the Endo-III treatment (P< 0.001). The haematological parameters showed enhanced values under hypoxic conditions. Transmission electron microscopic (TEM) studies revealed damage to liver and gill tissues for both the treatments. Interestingly, SGR of fish was significantly lowered in hypoxic by approx. 30% compared to normoxic condition and this was found to be correlated with DNA damage (R = -0.82; P = 0.02). Taken together, these results indicate that prolonged exposure to both hypoxic and hyperoxic conditions induce oxidative stress responses at both DNA and tissue levels, and hypoxia can result in compensatory changes in haematological and growth parameters which could influence Darwinian fitness of the biota with wider ecological implications.

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