Dilution rates influence ammonia-assimilating enzyme activities and cell parameters of Selenomonas ruminantium strain D in continuous (glucose-limited) culture.

Aims: The objective of this study was to examine the effect of dilution rates (Ds, varying from 0.05 to 0.42 h(-1)) in glucose-limited continuous culture on cell yield, cell composition, fermentation pattern and ammonia assimilation enzymes of Selenomonas ruminantium strain D.

Methods And Results: All glucose-limited continuous culture experiments were conducted under anaerobic conditions. Except for protein, all cell constituents including carbohydrates, RNA and DNA yielded significant cubic responses to Ds with the highest values at Ds of either 0.10 or 0.20 h(-1). At Ds higher than 0.2 h(-1), fermentation acid pattern shifted primarily from propionate and acetate to lactate production. Succinate also accumulated at the higher Ds (0.30 and 0.42 h(-1)). Glucose was most efficiently utilized by S. ruminantium D at 0.20 h(-1) after which decreases in glucose and ATP yields were observed. Under energy limiting conditions, glutamine synthetase (GS) and glutamate dehydrogenase (GDH) appeared to be the major enzymes involved in nitrogen assimilation suggesting that other potential ammonia incorporating enzymes were of little importance in ammonia assimilation in S. ruminantium D. GS exhibited lower activities than GDH at all Ds, which indicates that the bacterial growth rate is not a primary regulator of their activities.

Conclusions: Studied dilution rates influenced cell composition, fermentation pattern and nitrogen assimilation of S. ruminantium strain D grown in glucose-limited continuous culture.

Significance And Impact Of The Study: Selenomonas ruminantium D is an ecologically and evolutionary important bacterium in ruminants and is present under most rumen dietary conditions. Characterizing the growth physiology and ammonia assimilation enzymes of S. ruminantium D during glucose limitation at Ds, which simulate the liquid turnover rates in rumen, will provide a better understanding of how this micro-organism responds to differing growth conditions.