Probiotic Applications in Temperate and Warm Water Fish Species - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 257

liver, vertebral axis, muscles, and part of the intestine) evidenced differences between controls
and larvae fed yeast. At 23 dph, a peak in the expression of SOD and GPX genes and increased
activity of the SOD enzyme were observed in the control group, indicating the presence of
oxidative stress; this was not observed in the group fed with the yeast. Changes in enzyme and
gene expression patterns were attributed to the presence of yeast, assuming a possible involve-
ment of superoxide anion retention in fish larvae, which could represent importance to the host
in increasing cell or tissue responsiveness to growth- and/or differentiation-enhancing factors.
No changes of heat shock protein 70 (HSP70) were observed in sea bass larvae fed with D.
hansenii at 23 and 48 dph.
Illustrating the importance of utilizing the correct dosage, the administration of D. hansenii
at 7 × 10 5 CFU g -1 led to a significantly lower growth rate with respect to the control group
(Tovar et al . 2002), 1 × 10 6 CFU g -1 led to significant improvements (Tovar- Ramírez et al .
2004) and 6 × 10 6 CFU g -1 led to growth equal to that of the control (Tovar- Ramírez et al .
2004).
10.2.2 Effects of probiotics on European sea bass
immune system
Manipulation of the larval digestive system in aquaculture, by the addition of probiotics
through either culture water or live food, is an important tool to manage fish bacterial com-
munities and pathogen-host interactions (Verschuere et al . 1999; 2000a; 2000b). However,
the mechanisms of probiotic action in fish are not fully understood, although it is widely
accepted that selected probiotic bacteria are able to stimulate the host immune system (Nayak
et al . 2010; Dimitroglou et al . 2011). The first work that assessed the interaction between
probiotics and the mucosal immune system of sea bass larvae and post-larvae tested a strain
of Lb. delbrueckii subsp. delbrueckii (AS13B), isolated from the intestinal microbiota of adult
sea bass (Picchietti et al. 2009). Lb. delbrueckii subsp. delbrueckii was administered live to
developing sea bass (during gut metamorphosis), using Brachionus plicatilis and Artemia
salina as carriers. The results evidenced that in treated fish gut integrity was unaffected, while
the density of acidophilic granulocytes in the intestinal mucosa was significantly higher than
in controls. The probiotic induced T cell infiltration into the sea bass intestinal mucosa, which,
concomitant with increased total body TcR-β transcripts, was indicative of a stimulatory
effect of beneficial bacteria on the sea bass immune system. Furthermore, probiotics induced
significantly lower IL-1β transcripts and a trend towards lower IL-10, Cox-2 and TGF-β
transcription in the treated groups compared with the control; these events were interpreted
as the direct or indirect effect of beneficial bacteria on fish physiology. It is clear that future
studies of the genomic analysis of cytokines and chemokines would help to elucidate the
pathways which are involved in mediating the host immunostimulatory effects in order to find
suitable probiotic treatments for sustainable aquaculture to reduce the use of antibiotics and
guarantee a healthy environment for fish.
For this reason, recent studies have investigated some probiotic strains for use in aquacul-
ture as preventive measures against vibriosis in the sea bass larvae (Touraki et al . 2012) and
adults (Sorroza et al . 2012). In particular, an experimental challenge performed to evaluate the
protection offered by a B. subtilis strain against infection by Vibrio ( Listonella ) anguillarum
(Touraki et al . 2012) demonstrated that the mortality of fish challenged with the pathogen
was reduced in the sea bass larvae treated with B. subtilis enriched Artemia nauplii compared
with the untreated group. In addition, the administration of the probiotic Vagococcus fluvialis