Quorum sensing in coral associated vibrios

Corals are hosts to a diverse population of microbes associated with their tissue and mucus, these microbes are different from those found in the water column and usually distinct to the coral species. This has led to the idea that corals are ‘holbionts’, consisting of the coral itself and the associated zooxanthellae and microbes. This relationship is thought to have an important impact on the health of the coral ecosystem and the ability of the corals to deal with environmental stress.

Shifts in the coral associated bacteria have been seen during coral disease. It is thought that Vibrios are a main cause of coral disease, and a shift of the coral associated bacteria to a population dominated by Vibrios has been observed during times of disease. There is an idea that the bacterial population shifts to one dominated by Vibrios during times when the holobiont is under stress, although this is an idea that has been criticised. The paper gives several examples of coral diseases that a caused by Vibrio species being associated in times of thermal stress, one of these is taken from the paper by Ben Haim et al., 2003, where coral bleaching by V.coralliilyticus was associated with elevated sea temperatures.

The aim of the paper was the asses the idea that coral diseases may be attributed to the Vibrios utilising quorum sensing to outcompete the coral associated bacteria in times of environmental stress. A range of Vibrio strains from healthy and diseased coral were analysed for the quorum sensing molecules AHL’s (N-acylhomoserine lactones) and AI-2 (autoinducer – 2) at a range of different temperatures.

While increases in temperature have been shown to cause a shift towards Vibrio dominated populations, it was not known if increases in temperature caused an increase in AHL production. For V.harveyi R-21466 and V.Shiloi LMG 19703 AHL production decreased with increasing temperature. The AHL production remained constant at the different temperatures for the other Vibrio strains.

As quorum sensing is known to control the production of enzymes in Vibrios, so the effect of temperature on enzymes activity was also investigated. Generally enzyme activity was higher at 25⁰c and 30⁰c than at 18⁰c. There was only one strain where enzyme activity was correlated with AHL production, this had a negative correlation.

This is apparently the first paper that has tried to analyse the effect of temperature on quorum sensing in Vibrios, although the results from the paper did not seem to indicate any significant results, which may explain the lack of a discussion it is an important building block. Furthermore the authors state that many Vibrio species, such as V.harveyi and V.cholorea among others converge their various signals the transcriptional activator LuxO. Therefore investigation into LuxO would be an ideal development of this study.

The final part of the study was concerned with the inhibition of AHL production by V.harveyi R-21446, although this part of the paper is interesting I feel that the reasoning behind conducting this part of the experiment was poorly explained and is the first mention of it throughout the paper. A purple pigment violacein is regulated by QS. I assumed that the pigment was produced by the C. violaceum as the names are similar; the author only stated that it was produced by a strain, not saying which one. It was found that when R-21446 was overlaid with C. violaceum a non-pigmented zone was created round the R-21446 grown at higher temperatures. Similar results were seen with two other species, this suggests that R-21446 is capable of interfering with QS signals from other species. It has been shown that other bacteria can both produce and degrade signal molecules with this is the first time it has been shown in Vibrios.

The study concludes that the inhibitory activity of V.harveyi may give the species a competitive advantage and may explain why it is represented among Vibrios in the holobiont during times of thermal stress.

A review of: Tait, K., Hutchinson, Z., Thompson, F.L., Munn, C.B. 2010. Quorum sensing signal production and inhibition by coral-associated vibrios. Environmental Microbiology Reports. 2(1), pp. 145-150.