A continuous coculture of four ruminal bacteria, Megasphaera elsdenii, Selenomonas ruminantium, Streptococcus bovis, and Lactobacillus sp. strain LB17, was used to study the effects of the
ionophores monensin and
tetronasin on the changes in ruminal microbial ecology that occur during the onset of
lactic acidosis. In control incubations, the system simulated the development of
lactic acidosis in vivo, with an initial overgrowth of S. bovis when an excess of
glucose was added to the
fermentor. Lactobacillus sp. strain LB17 subsequently became dominant as pH fell and
lactate concentration rose. Both
ionophores were able to prevent the accumulation of
lactic acid and maintain a healthy non-
lactate-producing bacterial population when added at the same time as an excess of
glucose.
Tetronasin was more potent in this respect than
monensin. When
tetronasin was added to the culture 24 h after
glucose, the proliferation of lactobacilli was reversed and a non-
lactate-producing bacterial population developed, with an associated drop in
lactate concentration in the
fermentor. Rises in culture pH and
volatile fatty acid concentrations accompanied these changes.
Monensin was unable to suppress the growth of lactobacilli; therefore, in contrast to
tetronasin,
monensin added 24 h after the addition of
glucose failed to reverse the
acidosis. Numbers of lactobacilli and
lactate concentrations remained high, whereas pH and
volatile fatty acid concentrations were low.