Probiotic Applications in Crustaceans - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 301

as suggested by many authors, the specific mechanisms of probiotics resulting in the observed
host benefits are probably of a very diverse nature and unlikely to arise from independent mech-
anisms (Gatesoupe 1999; Merrifield et al. 2010). This complicated synergistic multi-factorial
relationship makes the demonstration of probiotic modes of action complicated, especially
under in vivo conditions and especially when multi-strain probiotics are considered. Further-
more for crustaceans, the diversity of the culture systems (e.g. extensive, intensive, super
intensive, closed recirculation, static biofloc, flow through etc.) must also be considered as
they may influence the probiotic effects.
Today our understanding in crustaceans is mostly derived from humans, terrestrial ani-
mals, or more recently studies in fish. This section will focus on the proposed mechanisms of
action of probiotics (direct effects on the host) for shrimps and other crustacean species, with
specific focus on the localized mechanisms and effects on the digestive tract. Proposed, and
investigated, modes of action in crustaceans include the production of inhibitory compounds,
competition for chemicals or available energy, competition for adhesion sites, interference
with quorum sensing, modulation of the host immune response, provision of nutrients and
contributions to digestive enzyme activity.
11.3.1 Modulation of the gut microbiota
It is today accepted that the effects of probiotics in aquatic species arise from an initial mod-
ification of the GI microbiota. This microbial shift leads to changes in intestinal conditions,
resulting in effects at different levels such as immune modulation or modulation of digestive
functions. This initial modification, seen as the primary effect of probiotics on the host, can
go from a simple introduction of a new microorganism species in the microbial ecosystem of
the digestive tract to an entire shift of the microbial communities shown in fish. This shift is
the result of specific or non-specific antagonisms toward autochthonous microbes. Unfortu-
nately, this primary effect of probiotics has often been overlooked in crustacean studies and
many assumptions are made through our experiences in other animals. Indeed most of the
papers reporting the effect of probiotics on the physiology and performance of crustaceans do
not investigate what is actually happening within the digestive tract. Some investigators do
however enumerate or track the probiont in the gut microbiota, but rarely have studies looked
at the probiotic effect on the composition of the indigenous microbiota. A number of stud-
ies have reported the effect of probiotics on the total bacterial count and/or cultivable Vibrio
sp. in the digestive tract of crustaceans (Rengpipat et al. 2003; Venkat et al. 2004; Castex
et al. 2008; Li et al. 2009; Zhang et al. 2009; Boonthai et al. 2011; Zhang et al. 2011); how-
ever, to our knowledge only a limited number of documents report the effect of probiotics,
or even prebiotics, on the microbiota of crustaceans with application of molecular techniques
(Daniels et al. 2010; Thompson et al. 2010; Liu et al. 2011a). Thompson and colleagues con-
sidered the use of molecular techniques to investigate the effect of Gram-negative probionts
in Pacific white shrimp, L. vannamei . In this study, terminal restriction fragment length poly-
morphism (T-RFLP) was used to analyse bacteria within faecal and mid/hindgut samples.
A limited bacterial diversity and distinct changes in microbial communities were observed
when a Vibrio gazogenes was used as dietary probiotic over an 8 week period. PCR-DGGE
of the 16S rRNA V3 region has been used to study the effects of Bacillus spp., and prebi-
otic MOS, on the GI microbial communities of H. gammarus larvae (Daniels et al. 2010).
Microbial community profiles were observed to shift away from control lobsters in those fed
Bacillus spp., in combination with prebiotics, with a reduction in species richness and diversity