Probiotic Applications in Crustaceans - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 304

that the V. harveyi quorum sensing system regulated the virulence of the bacterium towards
gnotobiotic Artemia franciscana and Br. plicatilis , respectively. The use of quorum sensing
disrupting compounds has been suggested as a promising strategy in order to inhibit the
virulence of pathogenic bacteria in aquaculture (Defoirdt et al. 2004). For instance, when
added at optimal concentrations, halogenated furanones, produced by a marine red algae
and previously shown to disrupt quorum sensing signal molecules, protected Artemia from
several pathogenic Vibrio spp. (Defoirdt et al. 2006). Therefore, steering the microbial
communities toward microorganisms able to degrade these signalling molecules can be seen
as a promising strategy.
Interestingly, quorum sensing disrupting compounds affect virulence of pathogenic Vibrio
spp. but do not affect their growth, thus limiting the risk of resistance development compared
to antibiotics. Hence, the search for new probiotic bacteria which can act as signal molecule
inactivating bacteria is a future challenge in aquaculture systems.
11.3.4 Competition for nutrients and adhesion sites
Other studies suggest competitive exclusion by probiotic bacteria competing for resources
such as nutrients and adhesion sites. The latter has been often suggested but unfortunately
little evidence to demonstrate it under in vivo conditions can be found in the literature (Lee
etal. 2000). Studies on this subject are limited because little is known about the colonization of
the crustacean's digestive tract by bacteria and the existence of specific attachment sites (Harris
1993). It is accepted that crustaceans have a less developed GI microbiota than fish and that the
microbiota would be predominately transient in the foregut and the hindgut regions. Bacterial
concentration and composition are thus likely to be influenced by a variety of parameters,
such as the culture system, temperature, bacterial communities of the culture medium, feeding,
moulting, intestinal transit and defecation (Moss et al. 2000). Also while most research has
focused on bacteria that are able to attach to intestinal mucus, transient bacteria have also
been demonstrated to exert beneficial effects in crustaceans (Castex et al. 2008; 2009; 2010).
Therefore, although it may potentially be beneficial, it is not necessary for a probiotic to display
'colonization' to exert a local effect during transient passage through the GI system. In this
case, regular administration of the probiotic via the feed can be a means to artificially sustain
their presence in the digestive tract (Castex 2009; Merrifield et al. 2010).
When, or if, adhesion and colonization of the mucosal surfaces by probiotic bacteria do
occur, the attachment of pathogens can be prevented or their removal can be achieved from an
infected digestive tract (Vershuere etal. 1999; Vine etal. 2004). In crustaceans, one of the best
examples was provided by Gullian etal. (2004) who demonstrated colonization of the digestive
gland of L. vannamei by several probiotic strains and highlighted competitive interaction of
these strains with pathogenic V. harveyi . For instance, the probiotic strain Vibrio P62 was able
to reduce the colonization of V. harveyi S2 by 60%, and to displace the indigenous microbiota
of the digestive gland. Of course one must be aware that the adhesion of a probiotic to the
gut wall does not necessarily imply that competition for adhesion sites is the only mode of
action involved.
Several studies also suggest competition for nutrients or energy sources (Rico-Mora et al.
1998; Verschuere et al. 1999). Rico-Mora et al. (1998) showed that a probiotic strain, isolated
for its high capacity to grow in organic-poor substrates, was able to prevent the establishment
of V. alginolyticus in a diatom culture, whereas no inhibitory effect of the strain was detected
toward other Vibri o spp. Others have also suggested that the competition for iron could be a