Probiotic Applications in Crustaceans - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 302

highlighting a stabilization in bacterial communities in the probiotic fed H. gammarus .Both
studies used traditional culture-dependent and more advance culture-independent methods
of microbial analysis to provide a more holistic picture of changes in bacterial communi-
ties. However the lack of knowledge about the presence, role and diversity of the intestinal
microbial communities in crustaceans limit interpretation of these findings. Additional stud-
ies on the crustacean microbiota using modern culture-independent techniques (PCR-DGGE,
qRT-PCR, FISH and next-generation sequencing) are required to address this issue and to
better identify and understand the basic probiotic effect in crustaceans. Furthermore, studies
looking at the microbial communities in different sections of the GI are also required. Indeed
the digestive tract of crustaceans is composed of three major sections - foregut, midgut and
hindgut - with different morphological characteristics and functions. These differences are
likely to have an effect on the associated microbial communities, as was demonstrated by
Johnson et al. (2008) in their study on the microbial communities in the foreguts and hindguts
of L. vannamei juveniles raised in a closed recirculating system. 16S rDNA sequences indi-
cated that the microbial communities in the rearing water were more diverse than those in
shrimp gut, and that the foregut and the hindgut harbour distinct communities. Indeed, DNA
sequence of selected DGGE bands indicated that the foregut contained sequences belonging to
Mycobacterium spp., Propionibacterium spp., Desulfocapsa spp., and chloroplast genes while
the hindgut showed a lower diversity with sequences exclusively related to Vibrio spp. Such
studies provide useful information to better understand the sensitivity of crustaceans to enteric
bacterial infections and to potential methods of microbial manipulation. For instance, cray-
fish, having a much shorter midgut compared to penaeid shrimps, are less susceptible to oral
Aeromonashydrophila infections, which could be related to preliminary reports which suggest
that the midgut is a preferential infection site for this bacterium (Jiravanichpaisal 2011).
11.3.2 Production of inhibitory substances
In a study of the probiotic Bacillus S11, Rengpipat et al. (2000) assumed competitive exclu-
sion in the shrimp's gut as they observed a reduction in V. harveyi concentration in the gut of
P.monodon fed the probiotic when challenged with the pathogenic bacteria. However, no infor-
mation was provided by the authors as to how the probiotic might compete with the pathogen,
though the production of inhibitory substances may be assumed since the bacteria was selected
for its in vitro antimicrobial activity against Vibrio spp. The antagonism against pathogens,
especially by the production of inhibitory substances, is probably the mode of action most fre-
quently suggested in current literature. This can be explained by the fact that the preliminary
screening processes of probiotics often rely on antagonism assays towards pathogenic bacteria
(Vershuere et al. 2000a). The methods used in such studies are based on the capacity of the
bacteria to produce some extracellular substances that inhibit other bacterial strains, as recently
detailed by Hai et al. (2007). Rengpipat et al. (1998) selected a Bacillus S11 strain, from over
350 isolates from samples of mud, water and black tiger shrimp, based on its high antimicro-
bial performance against pathogenic V.harveyi and V.parahaemolyticus . Decamp etal. (2008)
used a similar approach in order to select several Bacillus strains included in a commercial pro-
biotic. Antimicrobial effects of bacteria generally come from the following substances released
alone or in combination: antibiotics, bacteriocins, siderophores, lysozymes, proteases, hydro-
gen peroxide or organic acids reducing the pH of the medium, and/or ammonia (Verschuere
etal. 2000a). Gildberg etal. (1997) stated that among the extracellular substances produced by
probiotics, bacteriocins would be the most important. For instance, antimicrobial activities of