A new probiotic for aquaculture, Phaeobacter gallaeciensis reduces abundance of a pathogenic Vibrio species.

Vibrio anguillarum is a marine pathogen that causes the disease ‘vibrosis’, a lethal haemorrhagic septicaemic disease that affects various marine and freshwater fish, bivalves and crustaceans. The pathogen is responsible for severe economic losses in aquaculture worldwide due to its high mortality rates.
In this study, Phaeobacter gallaeciensis of the Rhodobacteraceae was shown to antagonize V. anguillarum, ultimately reducing mortality levels significantly in cod larvae (Gadus morhua), highlighting the potential of P. gallaeciensis as a probiotic for marine fish larvae and their feed cultures.

The typical first feed for marine fish larvae are rotifers (Brachionus plicatilis), which are themselves fed or enriched with microalgae such as Tetraselmis suecica and Nannochloropsis oculata. These species can harbour high concentrations of the pathogens, which can be combated using antibiotic prophylactics. However, their use must be avoided as it can lead to the production of resistant strains. The use of a P.gallaeciensis as a probiotic bacteria to reduce Vibrio concentrations is a far more natural way to prevent the disease, capitalising on the competitive exclusion caused by this superior coloniser.

The antibiotic mechanism of P.gallaeciensis was suggested by a previous study to be the production of the compound tropodithietic acid (TDA). This study compared the effect of a TDA-negative mutant of P.gallaeciensis with the TDA-positive type strain to confirm if this is the mechanism. Axenic cultures of the rotifer and microalgal species were used in the different treatments to study the probiotic effect of P.gallaeciensis.

In Tetraselmis microalgae, wild-type P. gallaeciensis reduced the numbers of Vibrio cells a thousand-fold, and complete elimination of Vibrio was achieved in 3 out of 4 replicates. The presence of the TDA-negative mutant reduced Vibrio numbers ten-fold, though it was only significant for two of the four replicates, it still suggests however that there is another mechanism of antagonism.
In Nannochloropsis cultures, Vibrio was completely elmintated by wild-type P. gallaeciensis in 1-2 days in high algal density.
In the rotifer species Brachionus plicatilis, wild-type P. gallaeciensis reduced Vibrio numbers a thousand-fold, whereas the effect of the TDA-negative mutant was non-significant.

In cod larvae, those challenged with V. anguillarum in the absence of P. gallaeciensis saw 100% mortality, whilst larvae challenged in the presence of wild-type P. gallaeciensis saw only 12.5% mortality. Which, bizarrely, was less than the control larvae grown in the absence of both bacteria, which saw 37.5% mortality. The TDA-negative mutant reduced mortality to 68.8% mortality, which again suggests a second mechanism of antagonism. Larvae that were not challenged by V. anguillarum had a lower mortality in the presence of P. gallaeciensis, indicating that it has additional beneficial effects beyond its action against V. anguillarum.

P. gallaeciensis is therefore able to colonise two aquaculture-relevant algae species and a rotifer species without compromising their growth. It strongly reduces or eliminates V. anguillarum numbers within those species and reduces mortality significantly in cod larvae.
The findings from this study are likely the first stage in the process of manufacturing a new probiotic. Now that P. gallaeciensis has been shown experimentally to effectively reduce pathogenic Vibrio numbers in cod larvae, its plausibility as a probiotic will need to be tested.

D’Alvise, P. W., Lillebø, S., Prol-Garcia, M. J., Wergeland, H. I., Nielsen, K. F., Bergh, Ø., & Gram, L. (2012). Phaeobacter gallaeciensis reduces Vibrio anguillarum in cultures of microalgae and rotifers, and prevents vibriosis in cod larvae. PloS one, 7(8), e43996.