ORIGINAL ARTICLE Dilution rates influence ammonia-assimilating enzyme activities and cell parameters of Selenomonas ruminantium strain D in continuous (glucose-limited) culture J.A. Patterson 1 , V.I. Chalova 2 , R.B. Hespell 3  and S.C. Ricke 2 1 Department of Animal Sciences, Purdue University, West Lafayette, IN, USA 2 Center for Food Safety-IFSE and Departments of Food Science and Poultry Science, University of Arkansas, Fayetteville, AR, USA 3 USDA-ARS, NCAUR, Peoria, IL, USA Introduction Selenomonas ruminantium is unique in that although being considered Gram-negative bacteria, enzymatic and 16S ribosomal comparative studies have aligned it more closely with Gram-positive bacteria, namely Clostridia (Stackebrandt et al. 1985; Takatsuka and Kamio 2004). In pure culture, the rumen bacterium S. ruminantium D grows well in simple inorganic mineral medium and sugars with a high level production of lactate and other organic acids and can produce substantial amounts of lactate (Ricke and Schaefer 1996). However, in the rumen, S. ruminantium is one of the more metabolically diverse bacteria possessing the ability to use a wide variety Keywords ammonia assimilation, continuous culture, Selenomonas ruminantium D. Correspondence Steven C. Ricke, Department of Food Science, University of Arkansas, 2650 North Young Ave., Fayetteville, AR 72704, USA. E-mail: sricke@uark.edu  Author deceased. 2008 ⁄ 1712: received 6 October 2008, revised 26 June 2009 and accepted 27 June 2009 doi:10.1111/j.1365-2672.2009.04475.x Abstract Aims: The objective of this study was to examine the effect of dilution rates (Ds, varying from 0Æ05 to 0Æ42 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 0Æ10 or 0Æ20 h )1 . At Ds higher than 0Æ2h )1 , fermentation acid pattern shifted primarily from propionate and ace- tate to lactate production. Succinate also accumulated at the higher Ds (0Æ30 and 0Æ42 h )1 ). Glucose was most efficiently utilized by S. ruminantium D at 0Æ20 h )1 after which decreases in glucose and ATP yields were observed. Under energy limiting conditions, glutamine synthetase (GS) and glutamate dehydro- genase (GDH) appeared to be the major enzymes involved in nitrogen assimi- lation 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 limita- tion 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. Journal of Applied Microbiology ISSN 1364-5072 ª 2009 The Authors Journal compilation ª 2009 The Society for Applied Microbiology, Journal of Applied Microbiology 108 (2010) 357–365 357