This study aimed to determine whether or not Caribbean gorgonian corals have detectable levels of antimicrobial and quorum sensing (QS) activity. They did this by testing dominant Caribbean gorgonian corals for the presence of QS activity and bactericidal activity, against an array of bacterial strains. The gorgonians represent dominant genera from two locations in the Caribbean; Puerto Rico and the Florida Keys.
The coral extracts were tested for antimicrobial activity against a suite of gram-negative/ gram-positive bacteria, both marine and non-marine to test a range of bacterial inhibition. Antimicrobial activity was measured using turbidity assays and plotting the growth curves.
QS activity was measured by looking at inhibition of QS in the presence of related AHL’s and QS induction. Long chain AHL’s were detected using a QS sensitive reporter strain, a Pseudomonas aeruginosa mutant which is unable to produce AHL signals which harbours a biosensor plasmid, which contains a virulence gene and green fluorescence protein. This allows the detetction of gene expression in real time. They then tested QS inhibition by adding synthetic AHL’s and QS stimulation without the AHL’s. For the detection of short chain AHL’s the authors used the bacterial strain Chromobacterium violaceum. Again, for QS inhibition synthetic AHL’s (short chain) were added and QS stimulation without the AHL’s.
All eight gorgonians showed significant, but varied, antibacterial activity. Pseudoplexaura porosa and Pseuodpterogorgia acerosa showed the highest inhibitory activity, against all strains. All gorgonian corals had greater inhibitory activity against gram-positive bacterial strains, showing greater activity to non-marine strains, with the exception of Briareum sp.
Differences in antagonistic effects in the long chain AHL assay were observed among the corals tested, the gorgonian corals; Psedopterogorgia americana, Pseudopterogorgia acerosa, Plexuara flexuosa and Gorgonia ventalina had the highest inhibitory effect on QS, suggesting some sort of anti QS compound. Other corals did not have a significant effect on QS (above controls). Except from Pseudoplexaura porosa and Gorgonia ventalina, none of the coral extracts induced QS activity in the long chain bioassay (P. porosa exhibited a 17-fold increase in QS activity of negative control). G ventalina exhibited a lower increase (2-fold) compared to P. porosa over the negative control.
This study’s findings support previous findings, that antibiotic activity in corals is significantly higher in non-marine species than marine strains. Showing that Caribbean gorgonian corals do not possess a broad spectrum of antimicrobial activity, which, as the authors point out, is contrary to what has been found in previous studies with other corals. A third of corals tested antagonised QS activity, but only 2 corals induced QS activity, hinting that Caribbean gorgonian corals possess compounds that both antagonise and stimulate QS. Inducing QS activity seemed to be genus specific, as the ability was restricted to two genera; while 3 species from Eunicea, along with Briareum and Mucriceopsis did not induce QS activity. However, the authors do state some limitations to the assays used in this study. Mainly, that corals with negative results may have had AHL concentrations below the threshold or non specific AHL’s.
The study of QS within coral associated bacteria is a topic, which has only just begun to be looked in to. Uniquely, this study (as far as I am aware) is the first one to look in to how corals may regulate QS, presenting a possible new mechanism for corals to control their associated bacteria through the regulation of their phenotype. This study was covered in the marine microbiology seminars but if anyone is interested in taking a look, here is a link to it.
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