Changes in the microbial population of a rumen-simulating fermenter in response to yeast culture

Yeast increased the total viable count of bacteria on a medium containing rumen fluid and soluble sugars by 38% (P < 0.05) and the cellulolytic population increased by 49%, although the increase was not statistically significant (P > 0.05). Microscopic counts indicated a 25% increase in total bacteria (P < 0.05). Bacterial counts with mannitol as sole energy source also increased (P < 0.05), indicating an increased Selenomonas population; that was not in itself sufficient to account for the increase in the total bacterial count. The protozoal population was significantly lower in yeast supplemented vessels (P < 0.001), which may account partly for the increased bacterial numbers.