Six strains of anaerobic rumen bacteria were grown in a rumen fluid medium containing different concentrations of the urease inhibitor, acetohydroxamic acid. Bacteroides succinogenes S85 failed to grow in the presence of 1 × 10−2 M acetohydroxamic acid whereas growth by Selenomonas ruminantium GA192 was not affected when the initial concentration was 5 × 10−2 M. In cultures of the remaining strains, acetohydroxamic acid extended the lag phase without changing the growth rate in the log phase. The order of decreasing overall sensitivity to acetohydroxamic acid by the strains in terms of growth was Bacteroides succinogenes S85 [Formula: see text]Bacteroides ruminicola 23 > Butyrivibrio fibrisolvens D1 ≥ Butyrivibrio sp. C3 > Megasphaera (Peptostreptococcus) elsdenii B159 > Selenomonas ruminantium GA192. Washed cell suspensions of Butyrivibrio fibrisolvens D1, Megasphaera elsdenii B159, and Selenomonas ruminantium GA192, but of none of the other strains were capable of degrading acetohydroxamic acid. In the presence of 5 × 10−3 M acetohydroxamic acid, the extent of glucose, starch, and xylan fermentation by the strains was modified. The amounts and molar proportions of volatile fatty acids produced from these substrates were also changed. In Bacteroides ruminicola 23, the presence of acetohydroxamic acid resulted in an 89% reduction in glucose fermentation and a shift in volatile fatty acid production from propionate to acetate. Butyrivibrio fibrisolvens D1 and Butyrivibrio sp. C3 fermented more glucose, but less xylan, when acetohydroxamic acid was present; with glucose as substrate, volatile fatty acid production shifted markedly from n-butyrate to propionate. The results support the conclusion drawn from previous studies with the mixed rumen microbiota, that the potential effect of the compound upon rumen microbial activities in vivo is not limited to inhibition of urease activity.
