The application of probiotics in aquaculture has received concerted research efforts but the localised
intestinal immunological response of fish to probiotic bacteria is poorly understood. Therefore, a study
was conducted to evaluate the probiotic effect of Pediococcus acidilactici on Nile tilapia (Oreochromis
niloticus) with specific emphasis on intestinal health and probiotic levels as well as system level responses
such as growth performance, feed utilization and haemato-immunological parameters under
non-challenged conditions. Fish (9.19 0.04 g) were fed either a control diet or a P. acidilactici supplemented
diet (at 2.81 106 CFU g1) for six weeks. At the end of the study the probiotic was observed
to populate the intestine, accounting for ca. 3% (1.59 105 CFU g1) of the cultivable intestinal bacterial
load. Real-time PCR indicated that the probiotic treatment may potentiate the immune-responsiveness of
the intestine as up-regulation of the gene expression of the pro-inflammatory cytokine TNFa was
observed in the probiotic fed fish (P < 0.05). Light microscopy observations revealed elevated intraepithelial
leucocyte (IEL) levels in the intestine of P. acidilactici fed tilapia after six weeks (P < 0.05) of
feeding and a trend towards elevated goblet cells was also observed after six weeks feeding (P ¼ 0.08).
Concomitantly at week six, along with elevated IELs and elevated TNFa mRNA levels in the intestine, an
increased abundance of circulating neutrophils and monocytes were observed in fish fed the probiotic
supplemented diet (P < 0.05). This haemopoietic expansion of innate immune cells could be reflective of
an elevated state of immuno-readiness. Together these results suggest that the probiotic has a protective
action on the intestinal mucosal cells, stimulating the innate immune response after feeding for a period
of six weeks. These immunological modulations did not impair growth performance or the remaining
haematological and zootechnical parameters compared to the control group (P > 0.05).