All the probiotic-supplemented diets resulted in growth higher than that of the control
diets, suggesting that the addition of probiotics mitigated the effects of the stress factors. This
resulted in better fish performance, with better growth results in the diets supplemented with
the yeast. Similar results were observed by Va´zquez-Jua´rez et al. (1993) when yeast isolated
from the intestines of wild rainbow trout was introduced into the digestive tracts of domestic
rainbow trout, producing a significant increase in the growth of the cultured trout. In
contrast, the use of the bacterial mixture in the optimum protein diets at either density caused
no significant growth increases when compared to the control and yeast treatments. These
results may be explained by the greater adaptive capacity of yeasts in aquatic environments
in contrast to bacteria such as Lactobacillus and Streptococcus. It is also necessary, however,
to consider the possibility of interspecies differences, as suggested by Noh et al. (1994), who
studied the effect of supplementing common carp feeds with different additives, including
antibiotics, yeast (S. cerevisiae) and bacteria (S. faecium). They observed better growth
response with probiotic-supplemented diets, but obtained the best growth with a bacterium,
not a yeast. Similar results were reported by Bogut et al. (1998), who fed common carp diets
supplemented with S. faecium, reporting that the bacterium has a better probiotic additive for
carp than yeast, clearly in contrast to the present results for tilapia.