Indigenous Lactic Acid Bacteria in Fish and Crustaceans - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 137

expression in the head kidney and intestine (Pérez-Sánchez et al . 2011b). Upon lactococco-
sis challenge, elevated intestinal mucosal antibody (IgT) mRNA levels and survivability were
observed compared to the control group (Pérez-Sánchez et al . 2011b).
A large stumbling block in extrapolating inferences from probiotic studies is the paucity of
information pertaining to natural Lactobacillus levels in salmonid fish. Some studies provide
a brief insight into the quantity of these Lactobacillus spp. within the GI tract of salmonid
species. Heikkinen et al . (2006) observed that Lactobacillus spp. accounted for 4.7-13.7%
of the culturable bacterial community isolated from the intestinal contents of rainbow trout.
Azizpour (2009) reported isolation of Lb . plantarum from the whole intestine of rainbow trout,
identified by morphology and biochemical tests, and indicated that the bacterium accounted for
9% of the isolates tested. Although extensive information pertaining to the proportion of Lacto-
bacillus spp. from culture-independent analysis is lacking, Huber et al . (2004) enumerated the
bacterial community structure of the GI tract of the same fish species using FISH and demon-
strated that 10-12% of the total microbiota was identified as belonging to the Gram-positive
bacteria with low GC content. However, as the low CG Gram-positive group includes a num-
ber of genera, this does not give a reliable estimation of the level of Lactobacillus spp. Indeed,
according to a recent pyrosequencing based study the abundance of lactobacilli, identified as
Lb . sakei , Lb . crispatus , Lb . delbrueckii subsp. lactis , Lb . johnsonii , Lb . fermentum , in the dis-
tal gut of rainbow trout constituted 1% of the sequences obtained and were absent in many

replicates (Desai et al . 2012). Further such studies are required to determine Lactobacillus
levels in the GI tract of salmonids.
Little is known of the factors affecting the indigenous Lactobacillus levels of salmonids, but
Moffit and Mobin (2006) observed that dietary erythromycin treatment elevated Lactobacillus
levels in the intestine of Chinook salmon ( Oncorhynchus tshawytscha ) which were otherwise
absent or at non-detectable levels before and after treatment. Dietary soybean meal has been
reported to reduce the prevalence of culturable Lactobacillus spp. in the intestinal contents of
rainbow trout from ca. 14% of the cultivable population to 5% (Heikkinen et al . 2006).

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6.3.3 Lactococcus
The genus Lactococcus was proposed by Schleifer and colleagues in 1985 to reclassify some
species of the genera Lactobacillus and Streptococcus . Members of this genus have been iso-
lated from different sources, such as dairy products, plants and animals. Although Lactococcus
species are generally considered non-pathogenic, some species have been associated with fish
diseases. Among them, Lactococcus garvieae is the aetiological agent of lactococcosis, an
emergent disease, which affects cultured freshwater and marine fish with special incidence in
rainbow trout (Eldar and Ghittino 1999; Vendrell et al . 2006).
Some studies have demonstrated that Lactococcus spp. are members of the normal fish
microbiota. Heikkinen et al . (2006) observed that lactococci accounted for 5.9-14.0% of the
culturable bacterial community isolated from the intestinal contents of rainbow trout, but Desai
et al . (2012) reported that lactococci, such as Lc . lactis , were not present in the distal gut of
the majority of rainbow trout sampled and when present their abundance was low (i.e. 0.1%

of the total sequence reads). Hovda et al . (2007) detected Lactococcus species in the PI, MG
and DI of Atlantic salmon by DGGE analysis, but Lactococcus spp. were not detected by
culture-dependent methods. Skrodenyte-Arbaciauskiene et al . (2008) sampled the intestinal
contents of two salmonid species, freshwater Atlantic salmon and sea trout, and observed that
a bacterial strain which could not be differentiated between Lc . garvieae and Enterococcus

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