Probiotic Modulation of the Gut Microbiota of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 192

probiotic was present in all samples, suggesting it to be indigenous to the fish in the experiment
(indeed, it was originally isolated from the GI tract of juvenile grouper), the authors concluded
that dietary administration stimulated further establishment of B.pumilus in each compartment
of the digestive tract, which was based on their observation that the abundance, as a propor-
tion of the total band intensities, of the probiotic was higher in the probiotic replicates than the
control replicates.
Taken together, it is interesting to note that several studies have failed to observe detectable
levels of Bacillus probionts in the GI tract of fish after dietary exposure: B. toyoi fedtoEuro-
pean eel (Chang and Lui 2002), B.subtilis C-3102 fed to koi carp (He etal . 2011) and B.clausii
fed to grouper (Yang et al . 2012). Despite this, however, these studies reported to some extent
that probiotic administration modulated the gut microbiota and/or induced host benefits. This
therefore suggests that low (i.e. non-detectable using the methods so far employed: DGGE and
plate counts on general purpose media), or non-viable, populations of Bacillus cells/spores in
the GI tract of fish can be sufficient to affect the host microbiota to some extent. Further studies,
using more sensitive methods such as qPCR and metagenomics, are warranted to more accu-
rately determine the level of probiotic presence and impact on the host when using Bacillus
strains as probiotics for fish. It is evident from the literature that probiotic viability, and levels,
within the GI tract of fish can be important factors and thus it is recommended that well docu-
mented Bacillus strains which can populate the GI tract be considered as preferential candidate
probiotics.
Recently, Ran et al . (2012) investigated the persistence of 21 Bacillus strains, isolated from
the intestine of channel catfish which displayed antibacterial activity against fish pathogens,
and a B. subtilis type strain, on the intestine tissue of channel catfish using a culture based
approach. Catfish were fed Bacillus spore-supplemented feeds (ca. 10 9 CFU g -1 ) for 7 days
followed by a normal non-supplemented feed for 3 days. No probiotic Bacillus were detected
in the control fed fish but relatively high ( log 5 CFU g -1 ) Bacillus levels were observed in all

of the probiotic fed fish 3 days after the cessation of dietary provision. For six probiotic strains
(AB01, AP76, AP77, AP79, AP143 and AP254),

>

10 7 CFU g -1 of introduced Bacillus was

>

observed in the gut.

8.3 LACTIC ACID BACTERIA (LAB)

LAB are a group of Gram-positive rods and cocci that are non-sporing, lacking catalase and
oxidase (cytochrome c), and are fermentative in Hugh-Leifson medium. Readers with a spe-
cific interest in LAB in the gut of fish are referred to the reviews of Ringø and Gatesoupe
(1998), Ringø and Birkbeck (1999), Ringø (2004), Ringø et al . (2005; 2010), Michel et al .
(2007), Gatesoupe (2008), Lauzon and Ringø (2012) and Chapter6 . Bacteria belonging to this
group often produce bacteriocins and other chemical compounds that may inhibit colonization
of pathogenic bacteria in the GI tract (for review see Ringø et al . 2005; Ringø 2008). The most
commonly used LAB probionts for applications with fish so far belong to the Carnobacterium ,
Enterococcus/Streptococcus , Pediococcus , Lactobacillus , Lactococcus and Leuconostoc gen-
era (Table 8.2) but some information on Vagococcus fluvialis is also available (Roman et al .
2012; Sorroza et al . 2012).