Interactions between probiotic, pathogen and protein sources

Probiotics are an integral part of aquaculture practices to increase yields of fish by limiting loss through disease. Modulation of the immune system is a commonly reported benefit, with high potency to stimulate immunity seen under in vitro and in vivo conditions. There are an assortment of common bacteria used including Carnobacterium. The probiotic is naturally occuring in the gut microbiota of salmonids and has displayed antimicrobial abilities, but to be a favourable probiotic it must be able to adhere and grow in the mucus or on the enterocyte surface of the digestive tract.

Hartviksen et al 2015 studied the adherence of a species of this genus, Carnobacterium divergens, and whether it could exclude or displace the common fish pathogen Aeromonas salmonicida using ex vivo methods. This has been utilised for it combines the best of both in vitro and in vivo methods. The use of the intestine environment means results are more applicable to reality, and the removal of it from the organism allowing for flushing etc. enables greater control over variables. There are limitations, for the tissue viability will decrease over time, leading to the use of only one hour incubation as any longer may result in its degradation. Intestines were exposed to either saline (the control) the probiotic, the pathogen, first the probiotic then pathogen or the reverse. The effects of diet, which the fish were given for nine weeks, were also investigated the four variants being: fishmeal (FM), pea protein concentrate (PPC), extracted sunflower (ESF) and feather meal (FeM). Diet was considered as it is known to influence intestinal microbiota and structural integrity.

General exposure to C. divergens showed improvement of the intestinal structure with lower frequencies of intraepithelial leukocytes and debris in the lumen, as well as higher frequency of healthy mitochondria. Intestines exposed to A. salmonicida had greater debris in the lumen with damaged microvilli and enterocytes across all feed types, with FeM having the greatest changes observed up to 7 micrographs per individual showing damage. Having the probiotic before the pathogen showed similar intestinal structuring to the control but the reverse showed in all dietary groups an increase in tissue oedema. Fish fed FeM had an apparent decrease in the prevalence of healthy mitochondria. In the control, diet seemed to have very little effect on the levels of both probiotic and pathogen.

Concerning adherence to the intestine in all treatments where C. divergens was present it increased compared to the saline exposed control; diet had no influence nor was there an interaction between diet and treatment. Levels of the probiotic would lower on exposure to the pathogen in comparison to when solitary, illustrating that A. salmonicida is an opportunistic bacterium that can displace the endogenous probiotic. However pre and post exposures of C. divergens would cause lower levels of the pathogen compared to its singular treatment, so the probiotic had some desired effect.

Alternative protein sources in the form of the varying diets can be seen to not significantly affect the uptake and adherence of both bacteria species. Nor was there any dramatic alterations to intestinal structure following the control or probiotic exposure. However the combination of feed especially FeM with A. salmonicida enhanced structural changes and damage. The mechanism is unknown however as with the control FeM caused no alterations to structure. It is therefore important to consider feed type not only in its appropriateness to the animal but also on whether it increases susceptibility to disease. However the use of C. divergens as a probiotic does seem promising, especially in that the protein source had no effect on its ability to establish so could possibly be supplemented alongside any feed type.

Paper reviewed:

Jose L Gonzalez Vecino, M. (2015). Probiotic and Pathogen Ex-vivo Exposure of Atlantic Salmon (Salmo Salar L.) Intestine from Fish Fed Four Different Protein Sources. Journal of Aquaculture Research & Development, 06(05).