Marine
invertebrates, and the symbiotic and mutualistic microbial communities living
inside them, have traditionally been used for discovering novel natural
products, although the microorganisms are the true producers of the majority of
compounds. These chemicals are significant in the pharmaceutical industries for
manufacturing new drugs. Previously unexplored bacterial niches and whole
genome sequences may harbour new compounds and activate silent biosynthetic
gene clusters, respectively. This particular study was influenced by the human
microbiome project to determine whether marine vertebrates can harbour a
potential source of new types of marine Actinobacteria. Intestinal bacterial
diversity is higher than was first thought, but the diversity of Actinobacteria
can vary depending on which discrete environmental niche they are located in.
Vibriosis
is one of the most dominant diseases that affects both wild and farmed fish,
and is induced by many different Vibrio
species, which are all fast-growing, opportunistic bacteria that quickly colonise
the gut in fish. In some cases, antibiotics can cause organisms (i.e. humans) to
become more susceptible to infection by bacteria, so commensal bacteria are
important in controlling the onset of disease.
Vertebrates
have been largely ignored in studies aiming to discover natural microbial
products, so this is the first to focus on the fish microbiome acting as a
source of bacterial diversity, and to isolate culturable and taxonomically-distinct
Actinobacteria for new compounds. The commensal microbes being tested in this
study use a large number of biological activities to inhibit the growth, and
effects, of both Gram-positive and Gram-negative pathogens on fish.
Intact
whole dead fish were bought from commercial vendors and dissected. Three main
phyla, Actinobacteria, Firmicutes and Proteobacteria, were found in the
phylogenetic tree that was created after the stomach and intestinal contents
were analysed. As all marine fish are susceptible to at least one Vibrio species, there are limited types
of treatment that can be applied specifically for fish pathogens, which is why
traditional antibiotics have been over-used in aquaculture.
The discovery
and development of compounds for drug synthesis can be used against protozoan
parasites and bacteria, and the capacity of these isolates to produce bioactive
natural products against them was tested by producing large-scale liquid
cultures. Five out of the 21 extracts inhibited the growth of one or more Gram-positive
strains, and eight of the nine stopped one or more Gram-negative strains from
growing, so some compounds may have broad spectrum efficacy for treating
vibriosis in aquaculture.
One certain
isolated strain, FI-1004, showed even lower sequence identity to published
sequence data (97.8%), which implied that they had significantly diverged from
known cultured isolates. Secondary screening of fractions of FI-1004 showed
that only sebastenoic acid, a novel bioactive lipid, was responsible for the
bioactivity. Branched chain fatty acids are universally found in all bacteria and
are common substrates for phospholipid synthesis for bacterial cell membranes.
Although saturated anteiso fatty acids containing 15 carbon atoms are relatively
rare, sebastenoic acid was discovered with a unique unsaturation pattern (3 particular
sub-units) for this compound class. Sebastenoic acid displayed moderate
activity against the three Gram-positive test strains. In addition, because
strain FI-1004 can produce an antibiotic to act against common pathogens, it may
be a type of probiotic within the fish microbiome.
Maintaining
a healthy microbiota has been linked to maintaining the general health of the
fish host, as they are in direct contact with opportunistic pathogens in the
marine environment, and probiotic bacteria are thought to provide more benefits
to the health of fish in aquaculture than commercially available products
typically used for humans. Therefore, it is critical in understanding the microbial
composition of commercial fish to determine the appropriate probiotics for
protection against harmful bacteria such as many Vibrio species, as it is species-specific and it may prove to be a
more effective management strategy.
This
research represents the first study into the discovery of natural products from
the fish microbiome. The authors have managed to decipher a large gap in the
literature in terms of the role of the microbial community in vertebrates to
produce novel bioactive secondary metabolites, so this should be used as a
springboard for further investigations. However, they also used a range of fish,
such as lantern fish, mackerel and Canadian rock cod from different regions like
Moss Landing, California and Seattle, Washington, when they already mentioned
that the diversity of gut microbiomes in fish are known to vary widely between
species and distributions. Therefore, they seem to imply that these novel
bioactive compounds are ubiquitous across many or all fish taxa, but the probiotic
effect on common pathogens may differ between species, if for example, higher
concentrations of these compounds are needed to act against bacteria in some
fish species more than others.
No comments:
Post a Comment