Live yeasts in the gut: Natural occurrence, dietary introduction, and their effects on fish health and development

This minireview summarizes the present state of knowledge concerning the importance of yeasts in fish gut. Yeasts have been commonly isolated in the gastrointestinal tract, and high population densities were sometimes noted in healthy fish, but the data were quite variable in terms of colony counts and taxonomical diversity. Rhodotorula sp. seemed relatively frequent in both marine and freshwater fish, and Debaryomyces hansenii has been found to be dominant in rainbow trout. Some other dominant strains have been described, such as Metschnikowia zobelii, Trichosporon cutaneum, and Candida tropicalis in marine fish, and Candida sp., Saccharomyces cerevisiae, and Leucosporidium sp. in rainbow trout. The natural proliferation of yeasts in fish mucus may be generally considered as commensalism, in spite of a few cases of pathological infections mainly due to opportunistic strains. Several strains were documented to settle and grow in fish intestine after experimental introduction, particularly S. cerevisiae and D. hansenii in rainbow trout. There have been a few instances of competition among yeasts in fish intestine, while the effect of yeast on associated bacteria is still unclear. Yeasts can stimulate the immune response in fish. ß-glucans is likely the most important compound in this regard, but some other cell-wall components or soluble factors may also play a role. Both cellular and humoral responses have been educed by dietary yeast, depending on the experimental conditions. Other benefits may be expected for the host, especially the intestinal colonisation of early feeding fry with yeast, which may have some effect on development, e.g. by accelerating the maturation of the digestive system. In older fish, dietary yeast may stimulate metabolism and growth. Such beneficial effects need to be further investigated, either in cases of natural colonisation or after dietary introduction, while trying to elucidate the mode of action, and determine whether cellular viability is a prerequisite for efficacy.