The in-vitro metabolism of D,L-lactic acid by rumen microorganisms

Bacterial and protozoal fractions, isolated by centrifugation and filtration from the rumen of sheep given various diets, were used to examine the in-vitro metabolism of both the D(-) and L(+) isomers of lactic acid. The rate of disappearance of both the D- and L-lactic acid isomers in protozoal incubations was up to 15 × higher than the rate of disappearance in bacterial incubations, which was relatively constant at between 0.04 and 0.073 lactate per g protein per h for cells recovered from animals receiving various diets. However, the rate of lactate disappearance in the protozoal fraction varied from 0.133 g per g protein per h with a barley/hay diet to 1.12 g per g protein per h with a silage diet. L-Lactate disappeared more rapidly than the D-lactate in all protozoal incubations. It was confirmed that the disappearance of lactate associated with the protozoal fraction did not originate from adherent or associated bacteria. Further fractionation of the protozoal population by differential filtration showed that lactate only disappeared when incubated with entodiniomorphid protozoa and not with holotrich protozoa. Endogenous propionate and butyrate production by the entodiniomorphid ciliates was stimulated by 100% in the presence of lactate. It was calculated that on certain diets up to 30% of the volatile fatty acids formed from lactate could be protozoal in origin.