Discovery of probiotic bacteria from fish gut with potential for use in marine aquaculture

The aquaculture industry has been growing rapidly for the last 30 years. In the last decade marine aquaculture intensified because of increasing need to for fill the world’s protein requirements. In southern Europe the culture of sea bass (Dicentrarchus labrax), gilthead seabream (Sparus aurata) and sole (Solea Solea) are of great importance, while the culture of meagre (Argyrosomus regius) has been introduced in recent years. Accompanying the increase in intensive aquaculture is a concurrent increasing disease, which result in large economic losses. Vibrio anguillarum is a well characterised fish pathogen that causes acute haemorrhagic septicaemia and the control strategies currently employed include vaccination and chemotherapy (Austin and Austin, 2007). Common practice among fish farms is the use of antibiotics to prevent the spread of infection, but concerns regarding impact on the environment and human health have resulted in the search for new methods of controlling infection. To date a wide range of bacteria have been proposed for their application as probiotics (Kesarcodi-Watson et al., 2008: review), but not all potential probiotics are viable in reality, so the search continues.

Sorroza and colleagues (2012) isolated gut bacteria from sea bass, gilthead bream and sole for determination of probiotic potential. Gut samples were cultured and the antagonistic potential of the isolates was tested against 10 known pathogenic strains of V. anguillarum, V. aloginolticus, P. damselae, Yesina ruckeri, Lactococcus garvieae, streptococcus iniae and those with antagonistic effects were identified using 16s rRNA gene partial sequencing by BLAST analysis. To determine how the antagonistic isolates may interact in the gut their ability to adhere to gut mucus and grow was compared to pathogenic strain Vibrio anguillarum. The purpose of this was to see if the antagonistic strain could out compete the pathogenic strain for binding sites (competitive exclusion). The adherence potential was characterised by fluorescence of nucleic acid after staining with fluorescence protein. Adhered bacteria were measured after incubation and rinsing (to remove bacteria that had not adhered). After determination of antagonism and ability to survive in the gut, the isolates were added to feed to determine their action in live fish. Of the 50 strains that were isolated, only one strain, Vagococcus fluvialis, showed antagonistic effects against the pathogenic strains. Antagonistic effects were observed against V. anguillarum, P. damselae and Yersinia ruckeri. V. fluvialis showed a 10% reduction in the growth of V. anguillarum after 48h, but this was not significant. V. fluvialis showed significant adhesion to mucus compared to bovine serum albumin and polystyrene. In the competitive assay the adhesion capability of V. anguillarum was significantly reduced (54%) after exposure of intestinal mucus to the antagonistic strain. Addition of V.fluvialis to feed showed significant increases in survival of sea bass with addition of the probiotic, compared to V. anguillarum stresses fish.

V. fluvialis is a lactic acid bacteria, bacteria commonly used in probiotics. The study demonstrated well that competition for binding sites inhibits the ability of a pathogen to adhere. The authors describe the ability of V. fluvialis to outcompete V. anguillarum for binding sites, but not the possibility of the production of secondary metabolites as antimicrobials. This might be an interesting area to explore, if isolated; the compound could be added directly to the feed at lower concentrations. Function aside, V. fluvialis clearly serves its purpose. There is definitely potential here for V. fluvialis to become a major probiotic in marine aquaculture.        

Sorroza. L., Padilla.D., Acosta. F., Roman. L., Grasso. V., Vega. J., Real. F., (2012). Characterisation of the probiotic strain Vagococcus fluvialis in the protection of European sea bass (Dicentrarchchus labrax) against vibriosis by Vibrio anguillarum. Veterinary Microbiology. 155: 369-373.

Austin. B., Austin. D. A., (2007). Bacterial fish pathogens: Diseases of farmed and wild fish, 4^th (revised) ed. Spriger-Praxis, Godalming.