The Gut Microbiota of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 80

microbial fermentation to the total energy requirements of herbivorous fishes. Mountfort et al.
(2002) reported SCFAs turnover rates comparable to those of ruminants for three herbivorous
species, indicating that hindgut processes are an important contributor to the energy needs of
the host fishes. With regard to microbiota composition, culturable bacteria may be dominated
by Vibrio spp. in some fishes (Clements and Bellwood 1988; Clements etal. 1989). Meanwhile,
'giant' Epulopiscium spp. have only been observed in surgeonfish guts, which suggests that
some fish gut-associated bacterial phylotypes are specific symbionts (Angert etal. 1993). How-
ever, a more diverse microbiota has been described in the digestive tracts of numerous species
of herbivorous fishes representing several mainly tropical families (Fidopiastis et al. 2006).
The high-nutrient assimilation efficiency and high levels of fermentation end products in the
gut of kyphosids (Choat and Clements 1998) suggest that microbial fermentation may play an
important role in the digestion of algal polysaccharides in these herbivores. Using a molecular
approach based on 16S rDNA cloning, Fidopiastis et al. (2006) described the microbiota of
the zebraperch ( Hermosilla azurea ), which has a strictly macroalgae diet and a relatively long
digestive tract with an enlarged hindgut and an associated blind caecum (HC). These authors
reported that bacterial counts and also the SCFAs concentration were significantly higher in
HC contents compared to anterior gut regions. In the HC section, the microbiota composition
was dominated by the Proteobacteria Enterovibrio and Desulfovibrio ; other minor components
were Bacteroides and Faecalibacterium from the phyla Bacteroidetes and Firmicutes, respec-
tively. Contrasting results were described by Moran et al. (2005) about the microbiota of the
herbivorous Kyphosus sydneyanus , a species from the same family as Hermosilla azurea .Phy-
logenetic analysis of sequences retrieved showed that most formed a clade within the genus
Clostridium (Firmicutes), with one clone associated with the parasitic mycoplasmas. In subse-
quent studies (Skea et al. 2005; 2007; Clements et al. 2007) the microbiota of three temperate
marine herbivorous fish species ( Kyphosus sydneyanus , Odax pullus and Aplodactylus arcti-
dens ) was investigated using molecular cloning. In all of these herbivores, close to 50% of
the cloned sequences corresponded to Clostridia, including different taxonomic members of
this bacterial group. Clostridia are mostly polymer degraders, using polysaccharides and pro-
teins as substrates and yielding alcohols and SCFAs as fermentation products. A bacterial
community dominated by clostridial species is therefore consistent with the ratios of SCFAs
previously reported in K. sydneyanus , O. pullus and A. arctidens (Mountfort et al. 2002).
Data on the composition of microbiota in fish intestines are controversial. According to
some authors, the composition is similar to that of integuments and gills, and most intestinal
bacteria are aerobic or facultative anaerobic (Cahill 1990). On the other hand, there are data
showing that the intestines of fishes (especially herbivorous species) contain both facultative
and obligate anaerobes (Clements 1997). Interpretation and comparison of relevant results
obtained by different authors are complicated by the fact that a wide variety of differing
techniques have been used and some of them distinguished between allochthonous bacteria
and bacteria closely associated with the intestinal mucosa (autochthonous) (e.g. Hansen and
Olafsen 1999; Ringø et al . 2006; Bakke-McKellep et al . 2007; Olsen et al. 2008; Ringø et al .
2008; Zhou et al . 2011; Hartviksen et al . 2014; Ringø et al . 2014). However, some general
remarks about the bacterial composition of the microbiota of fish can be made using current
information. Based on the review of Izvekova et al. (2007), bacteria observed in different
fish were grouped using several criteria (structural and metabolism) to obtain graphical
representation of the most commonly reported microbes in marine and freshwater fish.
Figure 4.3 shows the distribution of aerobic microbes grouped into Gram-negatives (A) and