Role of
gastrointestinal microbiota in fish (REVIEW ARTICLE)
Sorry about the length of this post, this is a review of a
review so I just tried to put in the main points. I would DEFINITELY recommend giving
this paper a read as it is a brilliant overview of the current (pre-2010)
research done on fish GI microbiota, and I found it really helpful.
Development, establishment and composition of GI
microbiota of fish
The microbial colonization, establishment, composition and
diversity in the GI tract of fish is believed to be linked to the microbial
composition of the rearing water, diet and environment. Critical factors such
as the developmental stage of the fish, gut structure, the surrounding
environment (e.g. external temperature), rearing/farming conditions and stress
(e.g. pollution) may affect initial colonisation and subsequent establishment
process. Generally the total bacterial load of fish at the larval stage is low
before active feeding, and the initial load is mostly derived from the water by
larvae to maintain osmotic balance.
In fish two distinct groups of bacteria are usually found in
the GI tract: allochthonous (transient) and autochthonous (adherent).
Autochthonous bacteria are able to tolerate low pHs and may therefore colonise
epithelial surfaces of the stomach, small and large intestines; however
allochthonous bacteria are only present transiently in the GI tract due to no,
or a easily out-competeable ability to colonise epithelial surfaces.
The microbial composition and density vary in different
regions of the GI tract depending on the physio-chemical conditions of the gut.
Another generally reported rule in fish is that there is a progressive increase
in culturable bacterial levels when progressing from the stomach to the
posterior intestine. However, this trend may be refuted once more data is
gained via the use of modern non-culturable techniques. Interesting some fish
species have been documented to show seasonal and day-to-day fluctuations in
the GI bacteria.
Many early studies done on fish GI microbial communities
have been derived from the homogenates of intestinal content and/or faecal
matter using culture-based techniques (using selective or non-selective
isolation media), followed by a phenotypic characterization using a series of
conventional morphological and biochemical assays. These culture-based studies
do not show us the full picture, and therefore a lot of the past work loses
value. As of 2010 we have several novel molecular technologies currently
available, which are sure to dramatically increase our current understanding of
the GI microbiotia (e.g. Denatured gradient gel electrophoresis aka DGGE is a genetic
fingerprint method of molecular detection, based on PCR amplification of 16S
rDNA which has been used to successfully study and characterize the microbes in
the GI tracts of many animals including fish.)
Usually the GI microbiota of fish consists mainly of
aerobic, facultative anaerobic and obligate anaerobic microorganisms (e.g. Bacteroides and Fusabacterium). As seen in many of the papers currently on the
blog, bacterial composition varies from FW to marine fish e.g. Aeromonads are commonly associated with
FW fish.
General effects of
the GI microbiota
Epithelial
differentiation:
·
Microbes have been shown to up-regulate genes
involved in DNA replication and cell division for epithelial proliferation
·
The GI microbiota is key in the metabolism and
trafficking of cholesterol
·
Microbial regulation of glycoprotein production
in the GI tract has been previously reported
·
Germ-free fish often do not show a differential
of the gut epithelium
·
Several studies show germ-free fish display a decreased level of alkaline
phosphatase activity (a marker of epithelial maturation/differentiation)
mucous-secreting goblet cells and hormone-secreting enteroendocrine cells
Fish immunity
·
The gut immune system (aka GALT, the
gut-associated lymphoid tissue) provides defence against infectious agents and
regulates immunity in the GI tract. GI microbes play a critical role in
development and maturation of this GALT, which in turn, will mediate a variety
of host immune functions.
·
In fish GALT consists principally of
lymphocytes, eosinophil granular cells, and several types of granulocytes and
plasma cells
·
Microbiota have been found to up-regulate the
genes involved in innate immunity parameters
·
The early exposure of the intestine to live
bacteria and subsequent colonization is very important for the development of
the gut barrier
Nutrition
·
The GI microorganisms of fish have been
previously reported to synthesise vitamins, animo acids, metabolites and
digestive enzymes (e.g. the presence of a high concentration of Aeromonas in the GI tract can play an
important role in digestion as Aeromonas
species secrete several proteases types).
·
Recent studies indicate that anaerobic bacteria might
play a role in the digestion and absorption of nutrients, alongside
contributing to fish nutrition by supplying the host with volatile fatty acids.
·
GI bacteria have been well documented in their
ability to produce the vitamin B12 (the production of which varies
with species, but is generally correlated with the abundance or anaerobes as opposed
to aerobes)
Disease outbreak/health
management
In a healthy organism some microbiota are established
whereas others are merely transient in the intestine. There is a delicate
balance between these microbes and if this balance is disturbed pathogens (present
in a transient state) may establish lethal infections.
The gut microbiota can protect the host by depriving
invading pathogens of nutrients and/or secreting a range of antimicrobial
substances. Many fish species harbour a wide range of intestinal bacteria that
have the ability to inhibit pathogens. Some are specific, whereas others show a
broad-spectrum inhibitory activity (e.g. Carnobacterium
spp., a species often isolated from the GI tracts of salmonids, has been shown
to inhibit several pathogens)
Manipulation
We may maniple the GI microbial community via the addition
of probiotics, prebiotics or synbiotics (nutritional supplements containing
both probiotics and prebiotics).
Probiotics
Usually, supplying probiotics through feed (rather than
water additives) results in a more successful establishment and colonisation in
adult fish, whereas larvae often response better to water additives. Probiotics
may increase survival, production, growth, immunity, protection from pathogens.
Growth may be increased due to probiotics enhancing feed conversion efficiency.
Protection from harmful bacteria may result from competitive exclusion,
production of organic acids, antibiotics, bacterioncins or lysozymes (or by
triggering the immune system)
Prebiotics
Prebiotics are usually non-digestible oligosaccharides used
as food ingredients to enhance the composition of certain endogenous
health-promoting bacteria in the GI tract of the host. They may help by
generating specific microbiota, and have been shown to stimulate the growth of
specific intestinal bacteria in fish. Past studies have shown they may lead to
improved growth, immunity and disease resistance. However, there are some
concerns when using prebiotics as several pathogens (as well as opportunistic
bacteria) may metabolise the prebiotics, leading to their widespread
proliferation – causing health issues for the host.
Nayak, S. K. (2010). Role of gastrointestinal microbiota in fish. Aquaculture Research, 41(11), 1553-1573.
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