The Gut Microbiota of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 86

studies which have identified isolates as Bacteroides by biochemical and physicochemical
means may in fact have been Cetobacterium spp. Further research should be dedicated
to understanding the distribution of Cetobacterium spp. in the gut of aquatic animals and
quantifying their vitamin contributions to the host.
Some authors have proposed a possible contribution of the fish gut microbiota to host nutri-
tion by providing enzymatic activities complementary to the host (Ray et al. 2012). It has been
suggested that fish gut microbiota might have positive effects for the digestive processes of fish
and indeed an extensive range of enzyme-producing microbiota have been isolated and identi-
fied in the GI tract of fish. In addition to Bacillus , Microbacterium , Micrococcus , unidentified
anaerobes and yeast are also suggested to be possible contributors. Nonetheless, in contrast
to endothermic animals, it is difficult to conclude the exact contribution of the GI microbiota
because of the complexity and variable ecology of the digestive tract of different fish species,
the presence (or absence) of a stomach and pyloric caeca, and the relative intestinal length.
Readers with an interest in this topic are referred to the comprehensive review of Ray et al.
(2012) which reports numerous examples of amylase-, protease-, lipase-, chitinase-, cellulase-
and phytase-producing bacteria isolated from the GI tract of fish.
While it is difficult to estimate the contribution of specific bacteria to the function of the
whole gut ecosystem, it is reasonable to expect that the overall microenvironment would be
strongly influenced by the predominant populations of organisms. It is expected that modern
molecular approaches and new sequencing technologies will significantly improve our knowl-
edge of the fish gut microbiota and the factors that influence its composition and its effects on
the host.
4.3 COMPOSITION OF THE MICROBIOTA IN EARLY LIFE
STAGES
The early developmental stages of fish and other vertebrates typically occur within the chorion,
a germ-free environment (Roeselers et al. 2011). After eclosion, vertebrates are exposed to
the microorganisms present in their respective local environment. The external surfaces of the
vertebrate body are subsequently colonized with microbes, with the majority of these microbial
residents assembling into dense communities, particularly in the GI tract.
The early studies of Strøm and Ringø (1993), Berg et al . (1994), Munro et al . (1994), Berg
(1995), Ringø et al . (1996) and Ringø and Vadstein (1998) revealed colonization in larval gut
after hatching and the bacterial colonization seems to follow a two-step pattern. However, lit-
tle is known about its stability, especially after dietary changes (live food, artificial food) or
treatment with antibiotics or disinfectants, which are routine practices in larval aquaculture.
Understanding some aspects of microbial ecology in aquaculture systems, such as knowing
the types, numbers and sources of bacteria commonly associated with different developmen-
tal stages, could be useful for manipulating microbiota as a strategy to prevent pathogenic
infection or to improve nutrition, especially in early life stages.
Some investigations have reported that bacteria present in the hatchery environment may
influence the composition of GI microbiota (Cahill 1990; Ringø and Birkbeck 1999). Based
on a culture-dependent approach, these results suggest that bacteria present in the GI tract gen-
erally seem to be those from water or the diet, and which can survive and multiply (Olafsen
2001). Furthermore, larvae may ingest substantial amounts of bacteria by grazing on suspended