OMG, Vibrios produce OMVs

The production and release of outer membrane vesicles or OMVs is an important trait which all domains of life have conserved. OMVs are used mainly in transporting cell signalling compounds, DNA/RNA and toxins.  However, they have been shown to contain the content of the cytoplasm. OMVs allow the biochemical compound to be delivered to the target location at high concentrations with relatively little chemical from a safe distance. This allows bacteria to initiate disease processes in their targets without coming into contact with potentially fatal antimicrobial compounds. Despite this their ecological importance has just begun to be studied. Vibrio shilonii is an important marine pathogen and a well studied gram negative bacteria. It is most well studied in is ability to induce coral bleaching in Oculina patagonica by damaging the zooxanthellae symbionts. This study tests whether Vibrio shilonii AK1 are able to produce and release OMVs and if so address the fitness advantage for V.shilonii and their influence on the coral holobiont.  Examining the effect of temperature on OMV release in V.shilonii at 20 and 30 degrees centigrade.

The confirmation of OMVs was done using a transmission electron microscope. The OMVs were then isolated and purified. The majority of OMVs had a single membrane but occasionally vesicles with two membranes were observed. There was no significant difference observed between the amount of OMVs at the two different temperatures. Using MaxQuant analysis the authors discovered 1,405 different proteins present in the OMVs. Proteins which were identified in multiple replicates were then selected for further analysis. Intensity based absolute protein quantification or iBAQ was used to quantify the amount of each protein present. The results ranged from 10^6 to 10^9. Of the 50 most abundant proteins to be identified, these were predicted to be membrane proteins. 598 common proteins were collected at the two temperature ranges and 91 showed significant difference. 81 of which were more abundant at the lower temperature and were predicted as cytoplasmic membrane proteins. 334 were exclusive to the lower temperature whilst only 27 were exclusive to the higher temperature. Most of the proteins exclusive to the lower temperature also showed the lowest abundance in the iBAQ.  Gene ontogeny analysis showed that the majority of the proteins found at both temperatures were related to catalystic, substrate binding and transporter activity.

Screening using the agar diffusion method revealed significant quorum sensing activity in the supernatant both with and without the OMVs at both temperatures. Of all the enzymes measured, V.shilonii AK1 showed high activity. There was an increase of 2-4 fold in cultures grown at the higher temperature. What is very interesting is lipase activity increased 100 fold at the higher temperature but the lipase activity of the OMVs was similar at the two temperatures.

This study is conclusive evidence that AK1 produce OMVs but unlike similar studies there was not a significant increase in OMVs at an elevated temperature. This is interesting as an increase in sea temperature may not necessarily mean that AK1 are able to cause more disease to coral. One way in which temperature stressed bacteria rid themselves of misfolded proteins is through OMV release. So an increase in temperature could lead to more stressed pathogens which are unable to rid themselves of waste. This could potentially positively effect disease of corals. I would be interested to see how this relates in further studies.

Li, J., Azam, F. and Zhang, S. (2016). Outer membrane vesicles containing signalling molecules and active hydrolytic enzymes released by a coral pathogen Vibrio shilonii AK1. Environmental Microbiology, 18(11), pp.3850-3866.