The world is your oyster.... unless you get food poisioning

Vibrio vulnificus and V. parahaemolyticus are natural components of some molluscan shellfish holobionts, and these two species are responsible for around 80% of all foodborne diseases. This high percentage is most likely a direct result of people’s fondness for raw or lightly cooked oysters. Several techniques are used to reduce the level of pathogenic bacteria in bivalve molluscs, although some only remove human and animal derived pathogens, not Vibrio species. These involve temperature, UV light, X-rays, pressure, and acid treatments. Most of these are very expensive, or change the taste of the shellfish. Phage treatment has also been suggested but bacterial resistance can make the method risky and inefficient.

Porsby and Gram looked at the possibility of using Phaeobacter inhibens to prevent the establishment of Vibrio species in live oysters (Ostrea edulis), because they produce tropodithietic acid (TDA) which is an antibacterial compound. P.inhibens supernatant was tested to ascertain its antibacterial effect on the two Vibrio species. V.anguillarum was also tested because it is very sensitive to TDA and therefore was used as a proxy for the presence of the acid. V.vulnificus was sensitive to the TDA supernatant, but V.parahaemolyticus was not inhibited at all. Pure TDA does inhibit V.parahaemolyticus so it is likely that the concentration of TDA used, or that is produced by T.suecica, is not high enough.

The first experiment co-cultured P.inhibens and V.vulnificus with and without the presence of the microalgae Tetraselmis suecica in an ‘oyster juice’ medium. In monoculture V.vulnificus grew from 10³ cfu/ml to 10⁶ cfu/ml, whereas in co-culture with P.inhibens it did not grow but stayed at around 10³ cfu/ml. The same pattern was seen when in the presence of T.suecica.

The second experiment used live oysters to determine whether P.inhibens can establish itself in oyster tissue, in order to hopefully prevent establishment of Vibrio species. P.inhibens did colonise the oysters and persist for at least five days, but they did not do so indefinitely and also did not prevent colonisation of V.vulnficus.

Although colonisation of the oysters by P.inhibens was not fully achieved in this study, maybe this is a good thing because changing the holobiont of the oyster may have fitness implications that we do not know about. TDA supernatant did however inhibit growth of one of the Vibrio species, meaning It could possibly be used post-harvest as a more cost effective way to reduce V.vulnificus numbers and therefore reduce the likelihood of food poisoning when eaten. The authors could maybe look at more Roseobacter clade species in the future to see if any can inhibit V.parahaemlyticus.

Porsby C.H., Gram L.(2016) Phaeobacter inhibens as a biocontrol agent against Vibrio vulnificus in oyster models. Food Microbiology. 57. 63-70.