Degradation of protein by mixed cultures of rumen bacteria: identification of Streptococcus bovis as an actively proteolytic rumen bacterium.

Mixed cultures of rumen bacteria were inoculated into anaerobic buffer solutions containing mixed carbohydrates, casein and ammonia, and rates of bacterial growth, protein degradation, ammonia formation or utilization and lactate production were determined. Bacterial growth rate was varied by the provision of excess carbohydrate (one large dose at the onset of the incubation) or limited carbohydrate (small doses every hour or every 2 hr). When carbohydrate was limited, growth rate was slow, the extent of protein degradation was small and lactate did not accumulate in the fermentation vessels. Lactate production and protein degradation were also negligible during the initial phases of the high carbohydrate, fast growth rate incubations, but large increases in each were seen after 3 hours. Microscopic examination of the fast growth incubations revealed large numbers of small ovoid cells similar to Streptococcus bovis, while tha slow growth incubations exhibited a variety of morophological types and very few small ovoid cells. Because the lactic acid and morphological data suggested that proliferation of S. bovis might be responsible for rapid proteolysis, effects of gram-positive antibiotics were examined. When compared against a fast growth control, both thiopeptin (5 ppm) and monensin (5ppm) were found to decrease protein degradation, but the inhibition by thiopeptin (50%) was greater than that by monensin (13%). The ratios of protein degraded to bacteria protein synthesized were .659, .362 and .628 for the control, thiopeptin and monensin treatments, respectively. Actively proteolytic strains of S. bovis were isolated from fast growth incubations, and subsequent experiments showed that the ratio of protein degraded to bacterial protein synthesized was approximately 1.50. Collectively, the data indicate that S. bovis is a very proteolytic rumen bacterium.