Are Gorgonian Corals a Source of Novel Antibiotics?

Quorum sensing (QS) is a method of cell-to-cell communication employed by a variety of bacteria and such signals are often used to facilitate colonisation of host organisms. Gorgonian corals are already known to possess compounds that mediate cellular interactions and may be used to regulate microbial colonisation. The researchers in this study used extracts of twelve species of gorgonian corals sampled from the Caribbean to search for compounds that exhibited antibacterial activity that could potentially be developed for bacterial control purposes.

The original coral samples were collected at depths of 5-10m from La Parguera, Puerto Rico and Looe Key Reef, Florida Keys, USA. The following species were sampled: Briareum sp., Eunicea laciniata, Eunicea tourneforti, Plexaura flexuosa, Plexaura homomalla, Pseudoplexaura porosa, Pseudopterogorgia americana, and Pseudopterogorgia acerosa, for Puerto Rico, and Gorgonia ventalina, Plexaurella sp., Muriceopsis flavida, and Eunicea mammosa for Florida Keys. In the Texas-based lab, ethanol extracts were prepared and antibacterial assays performed.

A variety of gram-negative and gram-positive bacteria of both marine and non marine origin were used for the assays, including known coral pathogens such as Serratia marcescens and Vibrio alginolyticus. Human pathogens were also used in the antibacterial assays. The assay itself was a bacteria turbidity assay which was conducted in a 96-well flat bottom plastic microplate, an assay chosen as it requires less of the coral sample than others, thus less damage to the reefs. The assays were simultaneously replicated three times with the controls of bacteria only, bacteria and antibiotic and bacteria and ethanol. Growth inhibition of bacteria was measured by drawing comparisons between the growth rates of the bacteria with the coral antibiotic extracts and the bacterial with ethanol as a control.

Bacteria chosen for the assays had to be isolated and identified, and shown to be culturable. Firstly, mucus samples were collected and cultured on glycerol agar. Bacteria that had grown was then subcultured in a marine broth an identified by species. PCR was used to amplify DNA and amplified sequences were compared with data from the GenBank database. Vibrio parahaemolyticus was the only bacteria identified and it was isolated from Pseudoplexaura porosa.

Bioassays to detect long chain N-acylhomoserine lactones (AHLs) in the ethanol extracts were carried out using Pseudomonas aeruginosa as a QS reporter strain. Inhibition or stimulation of quorum sensing was tested for, and P. aeruginosa is known to be sensitive to the long chain AHLs that result in these responses, making it an ideal reporter strain to reveal the compounds. Fluorescent signals that indicate transient gene expression were measured using spectrophotometry, this allowed for real-time QS detection.

Another biosensor strain, Chromobacterium violaceum, was used as an indicator organism in experiments to measure AHL presence by quantifying violacein synthesis. The synthesis of this pigment indicates a bacterium under QS control. The bioassay in this experiment was based again on spectrophotometry to compare absorbances at A₅₉₀ of the bacteria exposed to antibacterial compounds and the bacteria with ethanol as a control.

Their main findings were that the gorgonian corals possessed compounds that could inhibit and stimulate quorum sensing, suggesting selection on behalf of the corals of the bacteria that could colonize. P. americana, P. acerosa, and P. flexuosa displayed the highest QS inhibition and P. porosa displayed the highest QS stimulation. These differences in effects on QS hint at corals actively selecting the bacteria that can colonize the coral holobiont. More antimicrobial activity was noted against non-marine strains than marine strains of bacteria, suggesting that corals employ several mechanisms of bacterial control. Bacillus subtilis, vancomycin resistanct Enterococcus and methicillin-sensitive Staphylococcus aureus were most sensitive to compounds from the gorgonian extracts and can all be pathogenic to humans. Gram-negative strains of bacteria were less susceptible to gorgonian compounds than gram-positive strains from both marine and non-marine origins, suggesting that gorgonian antimicrobial activity is not broad-spectrum, this may be expected as they would have evolved mechanisms to deal with the bacteria present in the coral holobiont.

The researchers closed by calling for further work on the Caribbean gorgonians which exhibited strong antibacterial action against human pathogens, as these represent a potential for much needed novel antibiotics.

Reference

Hunt., L.R., Smith, S.M., Downum., K.R. and Mydlarz, L.D. (2012) Microbial regulation in gorgonian corals. Mar. Drugs 10: 1225-1243.