Factors affecting L-lactate utilization by Selenomonas ruminantium.

Studies were conducted to evaluate factors that affect L-lactate utilization by the ruminal bacterium Selenomonas ruminantium HD4. L-Lactate uptake decreased over time both in the presence and absence of 10 mM L-malate. Compared with uptake in the absence of malate, 10 mM L-malate increased L-lactate uptake at 30 s and 45 min. Because L-malate had little effect on L-lactate uptake for cells grown on soluble carbohydrates compared to lactate-grown cells, it seems that the stimulation due to L-malate is inducible. Sodium concentrations between 25 and 100 mM stimulated long-term (45 min) L-lactate uptake in the presence of 10 mM L-malate, whereas uptake in the absence of L-malate was low regardless of the Na+ concentration. Monensin inhibited L-lactate uptake by 42%, and this is consistent with the involvement of Na+ in L-lactate utilization. Initial uptake (30 s) of L-lactate was not dependent on Na+. L-Lactate uptake was stimulated by 10 mM L-malate at extracellular pH values between 4.0 and 8.0. No inhibition of L-lactate uptake was observed in the presence of 10 mM glucose, maltose, sucrose, xylose, or D-lactate. The metabolic inhibitors carbonyl cyanide m-chlorophenyl-hydrazone, 2,4-dinitrophenol, and NaF inhibited L-lactate uptake (> 79%), suggesting that protons may be involved in L-lactate uptake by this bacterium. Collectively, these experiments show that L-lactate uptake by S. ruminantium HD4 is stimulated in the presence of 10 mM L-malate at pH values and Na+ concentrations commonly found in the rumen.