Coral-mucus Vibrio Integrons are Evolutionary Hotspots

Coral-mucus-associated Vibrio integrons in the Great Barrier Reef: genomic hotspots for environmental adaptation

Aside from the well-known symbiotic associations between corals and dinoflagellate zooxanthellae (corals being largely dependent on zooxanthellae for certain nutrients), the role of prokaryotes in the coral microbiodome has been the target of recent study. The authors of this study (Boucher et al, 2011) focus on Vibrionaceae, a family of proteobacteria, which they obtained from the mucus of the scleractinian coral Pocillopora damicornis from the Great Barrier Reef and investigate their associations within the coral microbiodome.

Some corals exhibit an apparent immunity to pathogens, although this is not the same adaptive immunity observed in vertebrates, and the hypothesis aiming to explain this immunity is known as the coral probiotic hypothesis. The authors make use of the ideas behind this hypothesis to explore the relationship between Pocillopora damicornis and its associated Vibrios.

The coral probiotic hypothesis suggests that the prokaryotic microbiome provides pathogen resistance (notably to pathogens that may cause coral bleaching) to the host coral and that this immunity insinuates a dynamic pattern of antimicrobial production by non-pathogenic (commensal) bacteria, enabled by horizontal gene transfer.

The genetic element behind this immunity and its transfer (and also facilitating antimicrobial resistance among pathogens), known as the integron, is a system consisting of an integrase gene (intI) and an associated integration site (attI), where an integrase protein (IntI) catalyzes the insertion and removal of gene cassettes. Gene cassettes typically consist of a single gene and recombination site (attC) and there may be up to 100 cassettes per integron array.

Integrons are vital in the spread of both antimicrobials and antimicrobial resistance among commensal bacteria and pathogenic bacteria. Their importance is evidenced by a rise in proportion of Vibrio 16s rRNA genes sequenced by both commensal and pathogenic bacteria during coral bleaching events.

The authors cultivated coral mucus samples and amplified, cloned and sequenced the genes present in the samples using PCR. They then acquired datasets from which to draw comparisons via a variety of methods including screening the colonies for IntI, constructing fosmid libraries, sequencing Vibrio housekeeping genes, performing a taxonomic assignment of Vibrio cultivars by recA phylogeny and more as detailed in the article.

            Their main findings were that a diverse variety of Vibrio species were contained within the mucus of P. damicornis and that the mucus Vibrio-cassette arrays were found to be highly dynamic (in that around 90% of their integron associated gene cassettes were being actively shared, leaving around 10% at most in common between cultivars). This meant that mucus Vibrio-cassette arrays could evolve more rapidly in comparison to chromosomal genes, and the high mobility of genes allows the integrons of mucus Vibrio to be viewed as strong evolutionary hotspots in genomes. Comparisons to free living Vibrios exemplified this observed high gene mobility. A direct link between the Vibrio cultivars, Vibrio coral pathogens and human pathogens was demonstrated by the exchange of a subset of integron associated gene cassettes (associated with antimicrobial resistance), exemplifying the extensive scale of cassette sharing between microbial niches. This link may be useful in further understanding how resistance antimicrobial and antibacterial drugs spreads in a medical context.

            The diversity of cassette genes discovered in the coral mucus may show a cooperative sharing of resources and a mutually beneficial association within the coral microbiodome. This diversity and association allows quick adaptation in response to threats from pathogens that may be viewed as immunity as mentioned earlier. While beneficial to the coral microbiodome, this rapid adaptation to threats does act as a selective pressure to pathogens, facilitating an evolutionary arms race on both sides.

Boucher, Y. et al., 2011. Coral-mucus-associated Vibrio integrons in the Great Barrier Reef: genomic hotspots for environmental adaptation. ISME Journal (2011) 5, pp.962–972. Available at: http://www.nature.com/ismej/journal/v5/n6/full/ismej2010193a.html