Enumeration and isolation of lactate-utilizing bacteria from the rumen of sheep.

A highly specific medium was developed for the enumeration of lactate-utilizing bacteria in the rumen of sheep. This medium, which contained 2.0% lactate, 2.0% Trypticase, 0.2% yeast extract, and volatile fatty acids, hemin, and trace elements in place of rumen fluid, enabled high counts (42 x 10 to 190 x 10/g of ingesta) of lactate-utilizing bacteria to be made with a high degree of specificity (96%). The medium also supported the growth of all species of predominant lactate-utilizing bacteria reported to occur in the rumen and thus is of importance for ecological studies where the incidence and influence of the different species on lactate metabolism under changing conditions in the rumen cannot be predicted. The survival rate of isolates was increased from 60 to 96% by addition to the modified maintenance medium of 40% rumen fluid in place of the volatile fatty acids, hemin, and trace elements used in the counting medium. These results, together with the slow growth of colonies in roll bottles, showed that, although highly selective, the counting medium was not optimal for the types selected.