Rumen Bacterial Competition in Continuous Culture: Streptococcus bovis Versus Megasphaera elsdenii

Streptococcus bovis and Megasphaera elsdenii were grown in continuous culture with maltose as the limiting substrate at dilution rates of 0.36, 0.22, and 0.12 h. After each steady-state turnover, the pH was decreased by adding concentrated HCl to the medium reservoir. Relative counts of the two species at each dilution rate indicated that when the pH was high (6.6 to 6.0), higher dilution rates selected for a higher ratio of S. bovis to M. elsdenii. At intermediate pH (6.0 to 5.4), higher dilution rates once again selected for greater numbers of S. bovis in relation to M. elsdenii, but the increase in S. bovis numbers was much less at the 0.36-h dilution rate. Decreasing the pH below 5.4 caused the ratio of S. bovis to M. elsdenii to increase greatly, and no M. elsdenii cells were seen below pH 5.1. The ratio of the two species could be explained by their relative affinities for maltose if pH was greater than 6.0, but the lower relative numbers of S. bovis in the 0.36-h, intermediate-pH (6.0 to 5.4) incubations could not. Analysis of lactate production by S. bovis in pure culture showed that l-lactate was produced only if the pH was less than 5.2 at dilution rates of 0.22 and 0.12 h and less than 6.0 at a rate of 0.36 h. The lower numbers of S. bovis relative to M. elsdenii in the incubations with a dilution rate of 0.36 h and intermediate pH thus could be explained by utilization of l-lactate by M. elsdenii. The very high numbers of S. bovis at pH less than 5.4 were consistent with the greater tolerance of this organism to low pH.