Mixed rumen bacteria grow with similar efficiency on cellulose and glucose

Microbes growing in the rumen represent over half of the protein digested by cattle. Despite this importance, efficiency of microbial growth can vary widely, and the reasons are not fully understood. Here, we investigated if the carbohydrate source (cellulose or glucose) affects growth efficiency. We inoculated a system of 8 fermentors with rumen fluid, fed them glucose or cellulose (30 mmol hexose L^-1·d^-1), and maintained them at a range of dilution rates (2% to 12%·h^-1). We then measured the digestion of carbohydrates and output of microbes and fermentation products. We found that distinct communities of bacteria grew in fermentors fed glucose versus cellulose, with Streptococcus and Kandleria in the former and Fibrobacter, Ruminococcus, and other groups in the latter. Additionally, they formed different fermentation products, with cellulose-fed fermentors producing acetate and propionate and glucose-fed ones also producing butyrate and caproate. A slightly larger mass of microbes grew in fermentors fed glucose versus cellulose, but they also fermented more carbohydrates. As a result, microbes in glucose- and cellulose-fed fermentors grew with similar efficiency. These results suggest that the carbohydrate source may not be an important factor in determining efficiency of microbial growth in the rumen, but it does alter the composition of microbial communities and fermentation products.