Seminar Paper Megan Kerry & Oliver Ashbery.

 Characterization of quorum sensing signals in coral-associated bacteria

Globally the significance of bacteria has recently come to light, not just in their discoveries but also in studies looking at what symbiotic relationships occur between the bacteria and a host. Bacteria communicate by means of quorum sensing. It is of interest as to whether this communication is limited to specific species or genus of bacteria, and whether it is essential for a successful symbiotic relationship to occur.

The purpose of the study was to gain a better understanding of the ecological role of Acyl homoserine lactones (AHL), a type of quorum sensing molecule secreted by coral-associated bacteria.

Samples were taken from the mucus of 7 healthy, stony corals, and 1 healthy soft coral, from a reef in the Gulf of Eilet in Israel, over varying depths of 3-10m. Tenfold dilutions were created and all were incubated, including the original sample, at 22oC for 2-3 weeks. (bacteria isolated)

Isolated colonies of the bioreporter strains Escherichia coli and Agrobacterium tumefaciens were used for the detection of quorum sensing molecules, specifically AHLs with carbon chains with lengths of 4-12 (E. coli) and 10-12 (A. tumefaciens). These were used in a quorum sensing induction assay and thin layer chromatography, to identify the type of AHL, if any, being produced. The quorum sensing induction assay involved introducing the sample bacteria to the bioreporter strain and measuring bioluminescence. Luciferase is an enzyme that causes luminescence, and is produced in the presence of quorum sensing molecules, hence bioluminescence is an indicator to quorum sensing activity. The supernatants of the cultures were extracted and subjected to C18 thin layer chromatography to identify the samples that responded to the bioreporter A. tumefaciens. The bacterial DNA was extracted and subjected to PCR. The resulting sections of 16S rRNA were compared with the GeneBank database to identify the specific strains.  

The results showed that out of the 120 examined and tested isolates 41 showed the ability to induce quorum sensing activity, roughly 30%. Induction factors were used to show how successful a bacterium is at inducing this activity and most of the 30% showed factors of around 1.5-2 whereas some of the bacteria which are related to the Vibrio genus showed induction values of 50 plus, demonstrating higher activities of quorum sensing.

The DNA extraction and identification allowed a phylogenetic tree to be created. This showed that 77% of Vibrio, 75% Thalassomonas, 66% of Erythrobacter, and 50% of Phtotobacterium, Ruegeria and Pseudoalteromonas strains identified exhibited quorum sensing capabilities.

This paper suggests that specific groups of bacteria practice quorum sensing. A majority of Vibrios found exhibit the ability to quorum sense, and these are omnipresent in the marine environment. These could be connected: quorum sensing is beneficial to the survival of marine bacteria. From these results the conclusions were made that for quorum sensing the bacteria used are generally part of a specific group of species’ and these bacteria tend to fit into mainly the Proteobacteria group which are common in the coral holobiont, and as mentioned above the QS inducing strains were closely related to the Vibrio genus.

In my opinion the paper provides was good, however the method was a little confusing to keep up with, but if you break it down to the bare bones then it does make sense and the results that are very clear and concise.

An improvement would be to use several bioreporter strains that detect for carbon chains of a smaller range that enable more specific AHLs to be identified. The samples were taken from the mucus only, presumably to prevent unnecessary damage to the corals. Now that an initial experiment has been carried out it can be decided whether samples from different parts of the coral should be investigated. My only improvements if I were to do a study like this would be to perhaps use a few more bio reporter strains and also to take the bacteria not just from the mucus but from other parts of the coral as well just to see if there is a depth or area difference between activity,

It would be interesting to investigate the change is quorum sensing molecules and bacteria present in damaged corals, to observe if coral bleaching happened or if different strains colonised the corals, for example. and finally I would also have taken some bacteria samples from damaged coral as then you can see in addition to time and temperature effects whether QS is reduced in efficiency (and if so by how much) in damaged corals.

The url is below, any questions please feel free to ask:

http://www.ncbi.nlm.nih.gov/pubmed/21523464.