The Gut Microbiota of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 84

of a low yeast population (10 4 CFU g -1 ) through feed can induce beneficial effects in the host
(Tovar etal. 2002; Tovar-Ramírez etal. 2004). Indeed, the volume of a yeast cell may be larger
than those of bacteria (200-300 μm 3 for brewer's yeast versus 1μm 3 for Pseudomonas ; Gate-
soupe 2007) and therefore yeasts may be of physiological importance even when accounting
for less than 1% of the total microbial isolates. Therefore, an apparently low yeast load may
correspond to a population size sufficient to act upon the host.
Yeast identified from the fish intestine belong to two phyla: Ascomycota, among which
Saccharomycetaceae are probably the most important family, and Basidiomycota, which
include the genus Rhodotorula (red yeast commonly detected in the microbiota of both
marine and freshwater fish) (Newman et al . 1972; Andlid et al . 1995). The Ascomycota yeast
Metschnikowia zobelii and Candida tropicalis and the Basidiomycota yeast Trichosporon
cutaneum are dominant in some marine fish. The Ascomycota Debaryomyces hansenii ,
Candida spp., Saccharomyces cerevisiae , and the Basidiomycota Leucosporidium sp. have
been frequently isolated as dominant yeast from the rainbow trout intestine. Cryptococcus ,
Pichia , and Saccharomycodaceae have also been occasionally isolated (Gatesoupe 2007). A
summary of descriptions is shown in Table 4.1.
It has been reported that yeast isolated from the intestine of rainbow trout may adhere to
and grow in intestinal mucus (Andlid etal. 1998). Some yeast can colonize the intestine of fish
when introduced through feed (Waché et al. 2006). This colonization ability may be related to
cell surface hydrophobicity (Vázquez-Juárez et al. 1997) and the ability of the strains to grow
on mucus (Andlid et al. 1998). Furthermore, yeast have immunostimulatory properties due to
components such as ß-glucan, mannoproteins, chitin (as a minor component) and nucleic acids
(Ortuño et al. 2002). Studies have shown the beneficial effect of S. cerevisiae when added to
fish feed. Fish diets supplemented with this yeast act as immunostimulants that enhance the
growth, feed efficiency, blood biochemistry, survival rate, and non-specific immune response in
several fish species (Welker et al. 2007; Chiu et al. 2010; Harikrishnan et al. 2010; Tukmechi
et al. 2011). Recently, Hoseinifar et al. (2011) reported that dietary inactive brewer's yeast
might affect microbiota composition; although the total intestinal bacterial counts were not
affected, the levels of LAB were significantly elevated in fish fed dietary yeast. Most published
studies have been performed with Saccharomyces cerevisiae ; however, promising results have
also been obtained with Debaryomyces hansenii , which has been assessed in grouper and gilt-
head sea bream (Reyes-Becerril et al. 2008a; 2008b).
4.2 THE IMPORTANCE OF THE MICROBIOTA
It is generally recognized that the GI microbiota of animals serves several functions, namely
aiding digestion and the development of the mucosal system, angiogenesis, and as a protective
barrier against disease (Rawls et al. 2004; Ringø et al. 2007). An important study by Rawls
et al. (2004) showed that the GI microbiota can regulate the expression of 212 genes in the
digestive tract of zebrafish, some of them related to the stimulation of epithelial prolifera-
tion and promotion of nutrient metabolism and innate immune response. Studies performed
in model vertebrates - mice and zebrafish - also provide insights into the microbial-host
molecular dialogues that impact on several functions of the host, including nutrition, immu-
nity and development (Rawls et al. 2006; Round and Mazmanian 2009). An important aspect
of these results was the specificity of the host response, which depends on the bacterial species