Digestion of barley, maize, and wheat by selected species of ruminal bacteria

Differences in the digestion of barley, maize, and wheat by three major ruminal starch-digesting bacterial species, Streptococcus bovis 26, Ruminobacter amylophilus 50, and Butyrivibrio fibrisolvens A38, were characterized. The rate of starch digestion in all cereal species was greater for S. bovis 26 than for R. amylophilus 50 or B. fibrisolvens A38. Starch digestion by S. bovis 26 was greater in wheat than in barley or maize, whereas starch digestion by R. amylophilus 50 was greater in barley than in maize or wheat. B. fibrisolvens A38 digested the starch in barley and maize to a similar extent but was virtually unable to digest the starch in wheat. The higher ammonia concentration in cultures of B. fibrisolvens A38 when grown on wheat than when grown on barley or maize suggests that B. fibrisolvens A38 utilized wheat protein rather than starch. Scanning electron microscopy revealed that B. fibrisolvens A38 initially colonized cell wall material, while S. bovis 26 randomly colonized the endosperm and R. amylophilus 50 preferentially colonized starch granules. There was subsequent colonization but only superficial digestion of wheat starch granules by B. fibrisolvens A38. Variation in the association between starch and protein within the endosperm of cereal grains contributes to the differential effectiveness with which amylolytic species can utilize cereal starch.