Probiotic Applications in Crustaceans - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 307

exclusion and immunomodulation) probiotics have several advantages over the application of
pure immunostimulants, precisely for the prevention of bacterial or viral infection.
11.3.6 Direct or indirect nutritional effects
As explained by Verschuere et al. (2000a), it is often unclear whether a probiotic effect on
growth is attributed to better health (i.e. biocontrol) or if it is a consequence of the nutritional
effects of the probiont, acting as food source or contributing to the digestive function. It is
obvious that interaction may exist in some cases between the two effects. This is where con-
fusion can arise between zootechnical additives, acting mainly in maintaining an organism's
health and favouring growth, and veterinary additives acting strictly as biocontrol agents. A
combination of nutritional contribution and disease control will probably yield the best probi-
otic effect.
Very few studies report that probiotic bacteria are good candidates for improving the diges-
tion of nutrients and the growth of crustaceans, and the mechanisms behind such benefits
remain unclear (Venkat etal. 2004; Lin etal. 2004; Wang 2007). Interestingly, growth improve-
ment is mainly reported when probiotics are administered in the feed (Castex 2009). Even if
little is known about the role of the intestinal microbiota on nutritional processes in marine
invertebrates (Harris 1993), the microbiota may serve as a supplementary source of food
and nutrients such as vitamins, exogenous enzymes or essential amino acids (Dall and Mori-
arty 1983). Thus the manipulation of the intestinal microbial communities could be a way to
improve the energetic pool provided by the microbial biomass through a direct production of
nutrients in the mucosa or a better feed digestibility. For instance, Erasmus etal. (1997) demon-
strated that enteric bacteria play an integral role in abalone nutrition by hydrolysing complex
polysaccharide components of macroalgae to simple polymers and smaller units which are
rapidly assimilated by the host. Indeed it was reported that abalone enteric bacteria produced
enzymes capable of degrading agar, carrageenan, laminarin and alginate and that 70-90% of
the enzyme activities were extracellular, suggesting that bacterial enzymes were secreted into
the lumen of the gut.
It is accepted that probiotics may improve digestive activity by synthesis of vitamins, by
cofactors or by improving enzymatic activities (Fuller 1989; Gatesoupe 1999). For instance,
Gorospe et al. (1996) explained the nutritional contribution of Pseudomonas sp. in Artemia
culture as the bacteria being an additional source of protein, and Yu et al. (1998) demonstrated
that vitamin B12 producing bacteria can be used as a nutritive complement for rotifers Bra-
chionus plicatilis . More globally, probiotics may favour growth by a number of mechanisms,
either alone or in interaction, by: (1) increasing the pool of digestive enzymes in the GI tract
or stimulation of digestive enzyme production by the host, (2) increasing nutrient availability
to the organism for absorption in the GI tract (for example by pre-digesting some elements
of the feed), and/or (3) providing additional bacterial biomass which the host can use as an
additional nutrient source. A nice example is provided by Doeschate and Coyne (2008) who
suggested three levels of actions for the probiotic Pseudoalteromonas sp.strainC4onthe
nutrition of farmed abalone Haliotis midae : (1) pre-digestion of alginate in kelp based feed,
(2) increased alginate lyase activity in the abalone digestive tract, and (3) utilization of strain
C4 as a protein source.
In crustaceans, the information available is restricted to the production of enzymes con-
tributing to digestion and/or the stimulation of digestive enzyme activities of the host by the
probiont. Ochoa-Solano and Olmos-Soto (2006) selected three Bacillus strains and showed