What Makes Bacteria “Friendly”?

Marine aquaculture, as a sector, is constrained by disease. Rearing animals in high stock densities increases the likelihood of colonisation by pathogenic bacteria, and therefore disease, which can have huge economic repercussions. One explicit example of the economic costs of disease in marine aquaculture is in Norway where, to date, over US$77 million has been spent on the management of fish disease. In the past decade there has been an intensive development in probiotics for aquaculture to reduce the incidence of disease, as well as other benefits such as increased growth rate. By definition, probiotics are “living bacteria that stably colonize the guts of animals with a positive impact on the health of the host.” Whilst research efforts in probiotics have increased, the mechanisms in which probionts reduce pathogeneity are still poorly understood. In the reviewed paper, Rendueles et al. (2012) used the zebrafish (Danio rerio) model system to better understand the molecular mechanisms with confer host advantage for various potential probiotics.

Rendueles et al. (2012) raised germ free zebrafish larvae for use in all their experiments. Firstly Edwardsiella ictaluri was identified as a virulent bacterial strain which induced a strong inflammatory response and rapid mortality. Using E.ictaluri as a pathogenic challenge, the authors investigated the action of 37 potential probiotic bacteria strains which were selected for experimental use as they are widely exploited as probiotics in food and aquaculture industries. Zebrafish larvae were pre-cultured with the potential probiotics prior to initiation of the pathogenic challenge, pre-colonized treatment mortality rates were then compared to pathogen only controls; from this experiment three protective probionts were identified. One strain which provided significant protection against the pathogen challenge was Escherichia coli MG1655 and it was therefore investigated further to better understand the mechanisms in which it provided protection. Experiments were conducted which convincingly ruled out direct toxicity to pathogen and immune system activation as mechanisms. Importantly, E.coli MG1655 is a biofilm forming bacterium, and therefore authors hypothesised that the mechanism of protection could be adhesion and competition for space with the pathogen. To test this hypothesis zebrafish were pre-cultured with a mutant E.coli MG1655 strain which had type 1 fimbriae (adhesion organelles) knocked-out, the type 1 fimbriae deficient mutants had an impaired ability to confer protection against the pathogenic challenge.

The reviewed paper has made an important step forward in understanding the molecular mechanisms in which probionts provide protection against pathogens. Not only has the paper made an interesting finding about the importance of adhesion factors in pathogen protection, it also provides a novel system in which to study probiotic protection mechanisms, and host-pathogen interactions per se. I believe that unpicking host-microbe interactions is important for a wide range of biological fields, including evolutionary theory (See Matt’s Creator of Species Blog on the hologenome); however there are also clear economic applications for the use of probiotics in aquaculture.

Rendueles, O., Fre, M., Be, E., Herbomel, P., Ferrie, L., Levraud, J., & Ghigo, J. (2012). A New Zebrafish Model of Oro-Intestinal Pathogen Colonization Reveals a Key Role for Adhesion in Protection by Probiotic Bacteria, PlosPathogens, 8(7).

http://www.plospathogens.org/article/info%3Adoi%2F10.1371%2Fjournal.ppat.1002815