**3.2 Function of the microbiota in salmonids**

Although the function of microbiota in the physiology of salmonids has not been deeply studied, some evidence in Atlantic salmon and rainbow trout and other teleost fishes shows that it may play a similar role as described in mammals [48]:

a.*Antagonist against pathogens*: In salmonids and other fishes, the mucosa covers the entire organism; thus, it is estimated that microbiota colonizing it forms a main defense line against pathogens, by mechanisms that include competition for space or nutrients [71], interruption of communication signals, and production of inhibitory and antimicrobial substances, such as reported in mammalian [72]. However, few studies have focused on understanding the role of microbiota in salmonids. Some of them, performed in rainbow trout, show that bacteria from microbiota are able to protect against bacterial and fungal

pathogens specially in egg or larval stages when the immune system is not fully developed [69, 73, 74]. On the other hand, some probiotic bacteria that have direct antagonism against salmonid pathogens confer protection in challenge assays, for example, *Clostridium butyricum* protects rainbow trout against infection with *Vibrio* sp. [75].


In Atlantic salmon or rainbow trout, it is unknown if normal microbiota modifies the immune response, but some indirect evidence suggests:


As mentioned earlier, evidence shows that microbiota should play several roles in the salmonid physiology. However, functional association between metagenomic characterization and physiological parameters is an amazing challenge poorly explored. Likewise, the identification of the microorganism or consortium unique to fishes that could be administrated as probiotics to improve the performance of salmonid aquaculture is, therefore, a challenge ahead.
