Advances in techniques in molecular biology to analyse the effect of live yeast on ruminal microflor

Rumen pH is one of the most critical determinants of rumen function particularly for the cellulolytic rumen bacteria which fail to grow at pH 6.0 and below. Rumen pH falls as a result of enhanced fermentation due to increasing concentrate in the diet, this fall inhibits degradation of the fibrous components of the diet and is the cause, in part at least, of the negative associative effects between forages and concen- trates. It has been suggested that feed additives based on Saccharomyces cerevi- siae could help alleviated this post feeding drop in rumen pH resulting in more stable rumen fermentation. I will examine in more depth the effect of S. cerevisiae on rumen pH in particular in regards to the ability of S. cerevisiae to prevent ruminal acidosis. Acute rumen acidosis occurs when a ruminant ingests a large quantity of rapidly fer- mented carbohydrates. The microbial changes in the rumen associated with such an event have been visualized as a spiral in which the availability of rapidly fermen- table carbohydrate results in the production of volatile fatty acids by a wide range of rumen microorganisms with an associated drop in rumen pH, thereafter, in some case, an overgrowth of Streptococcus bovis leads to the rapid accumulation of lactic acid and a further drop in pH, as the pH declines lactobacilli start to predominate leading to a further accumulation of lactic acid and a yet further drop in rumen pH. However, whilst acute acidosis can, and in cases does, lead to death due to asso- ciated metabolic acidosis), sub-acute ruminal acidosis (SARA), also known as chro- nic or sub-clinical acidosis, is perhaps a more common a well-recognized digestive disorder that is an increasing health problem in most dairy herds. SARA is a disorder of ruminal fermentation that is characterized by extended periods of depressed ru- minal pH below 5.5–5.6. As before this drop in ruminal pH is a result of the break- down of dietary carbohydrates particularly from cereal grains leading to the production of volatile fatty acids and lactic acid but unlike acute acidosis the rumen pH does not spiral below pH 5 and indeed may recover to above pH 6 latter in the feeding cycle. Thus cattle experiencing SARA often do not exhibit any clear overt clinical symptoms with the most common clinical sign associated with SARA a reduced or erratic feed intake

Yeast cultures based on S. cerevisiae are widely used in ruminant diets. Available products vary widely in both the strain of S. cerevisiae used and the number and viability of yeast cells present. We have noted that not all strains of the yeast are capable of stimulating digestion in the rumen. Certain strains of S. cerevisiae can help prevent the decrease in rumen pH associated with feeding a cereal based diet and this appears to be associated with a decrease in rumen lactate concentrations

What is not clear is how the yeast prevents the post feeding decline in rumen pH. S. cerevisiae had no influence on the buffering capacity of rumen fluid suggesting that the pH stabilization is a secondary rather than direct effect. Similarly it seems unlikely that the yeast directly removed acidic end products from the rumen and although there is some limited evidence that S. cerevisiae might out compete the rumen microorganisms for soluble sugars most studies on the effect of S. cerevisiae on rumen pH have focused on the effects of the yeast on rumen bacterial populations.

It has been suggested that when added in co-culture S. cerevisiae decreased the ability of S. bovis to produce lactate acid however it is likely that this is an artifact of the in vitro conditions used and in practice what was observed was a reversal of the ability of S. bovis to uncouple growth from fermentation in low nitrogen high energy environments and that in practice S. cerevisiae supplementation in vivo would not provide sufficient nitrogen in the rumen for such an effect to be observed. Thus most studies have related the effect of S. cerevisiae on rumen pH and lactate accumulation to a selective stimulation in the growth and metabolism of lactate utilizing bacteria in the rumen such as Megasphaera elsdenii and Selenomonas ruminantium Recent data from my laboratory using modern molecular techniques has shown that the addition of S. cerevisiae both prevented the post feeding drop in rumen pH and increased the numbers of Megasphaera and Selenomonads that could be recovered from rumen fluid.

In conclusion live cultures of S. cerevisiae can help prevent a post feeding drop in rumen pH in animals fed concentrate diets thus reducing the likely hood of both clinical and sub clinical acidosis, this appear to be due to the ability of the yeast to selectively stimulated the growth of lactate utilizing Megasphaera and Selenomonas in the rumen.