Alternative schemes of butyrate production in Butyrivibrio fibrisolvens and their relationship to acetate utilization, lactate production, and phylogeny

Butyrivibrio fibrisolvens strains D1 and A38 produced little lactate, but s train 49 converted as much as 75% of its glucose to lactate. Strain 49 had tenfold more lactate dehydrogenase activity than strains D1 or A38, this ac tivity was stimulated by fructose 1,6-bisphosphate, and had a pH optimum of 6.25. A role for fructose 1,6-bisphosphate or pH regulation of lactate pro duction in strain 39 was, however, contradicted by the observations that ve ry low concentrations (< 0.2 mM) of fructose 1,6-bisphosphate gave maximal activity, and continuous cultures did not produce additional lactate when t he pH was decreased. The lactate production of strain 49 was clearly inhibi ted by the presence of acetate in the growth medium. When strain 49 was sup plemented with as little as 5 mM acetate, lactate production decreased dram atically, and most of the glucose was converted to butyrate. Strain 49 did not possess butyrate kinase activity, but it had a butyryl-CoA/acetate CoA transferase that converted butyryl-CoA directly to butyrate, using acetate as an acceptor. The transferase had a low affinity for acetate (K-m of 5 mM ), and this characteristic explained the acetate stimulation of growth and butyrate formation. Strains D1 and A38 had butyrate kinase but not butyryl- CoA/acetate CoA transferase, and it appealed that this difference could exp lain the lack of acetate stimulation and lactate production. Based on these results, it is unlikely that B. fibrisolvens would ever contribute signifi cantly to the pool of ruminal lactate. Since relatives of strain 49 (strain s Nor37, PI-7, VV1, and OB156, based on 16S rRNA sequence analysis) all had the same method of butyrate production, it appeared that butyryl-CoA/aceta te CoA transferase might be a phylogenetic characteristic. We obtained a cu lture of strain B835 (NCDO 2398) that produced large amounts of lactate and had butyryl-CoA/acetate CoA transferase activity, but this strain had prev iously been grouped with strains A38 and DI based on 16S rRNA sequence anal ysis. Our strain B835 had a 16S rRNA sequence unique from the one currently deposited in GenBank, and had high sequence similarity with strains 39 and Nor37 rather than with strains A38 or D1.