A bacteriocin-mediated antagonism by ruminal lactobacilli against Streptococcus bovis

Cattle that were fed an all forage diet had higher numbers of S. bovis than lactobacilli (3×107 versus 4×103). Gradual adaptation of the cattle to 80% cereal grain caused only a modest decline in ruminal pH (always≥5.6), but there was a dramatic decrease in S. bovis and an increase in lactobacilli. The lactobacilli were more resistant to low pH than the S. bovis isolates, but pH alone could not explain the antagonism between ruminal S. bovis and lactobacilli. The ruminal lactobacilli were identified as Lactobacillus fermentum, a bacterium that produces a bacteriocin. Agar overlays, zones of clearing and batch culture growth experiments supported the hypothesis that L. fermentum was producing a bacteriocin that inhibited the growth of S. bovis in the rumen.