Proteolytic activities of the starch-fermenting ruminal bacterium, Streptococcus bovis

The objective of this study was to characterize the extracellular proteolyt ic activity of Streptococcus bovis. Strains KEG, JB1, NCFB 2476, and K11.21 .09.6C produced very similar large molecular weight (160-200 kDa) extracell ular proteases that were specifically inhibited by PMSF a serine protease i nhibitor. Further experiments with S, bovis KEG indicated that cultures gro wn with casein as the sole added N source produced the greatest level of pr oteolytic activity, and the level of proteolytic activity was independent o f growth rate. Clarified ruminal fluid (CRF) decreased proteolytic activity by 54% compared with cultures grown with casein alone, and addition of exo genous peptides and carbohydrates (CHO) to the CRF further reduced the leve l of proteolytic activity by 44% and 52%, respectively. These results sugge sted that the proteolytic activity of S. bovis KEG was modulated by availab le N source and that the proteolytic activity was present for reasons other than providing N for growth. The rule of S. bovis in ruminal proteolysis r equires further definition, but phenotypic similarity among some ruminal st rains would suggest a common niche in ruminal proteolysis. The uniformity o f proteolytic activities could make S, bovis a prime candidate For manipula tion in ruminal proteolysis control strategies.