SUMMARY: When the antibiotic avoparcin was fed to sheep, changes were seen in the composition of the cellulolytic bacterial flora of the rumen. Avoparcin significantly reduced the numbers of ruminococci, but did not affect the total numbers of bacteria present, or the titre of filter paper degraders. Isolation and characterization of cellulolytic bacteria from sheep before and after avoparcin feeding began suggested that avoparcin induced a shift in the balance of the cellulolytic bacterial population from ruminococci to . Comparison of the cellulolytic activity of isolates from sheep showed that released label from tritiated cellulose more rapidly and extensively than did , and that was particularly active in the solubilization of highly ordered forms of cellulose, including dewaxed cotton fibres and Avicel. The pattern of establishment of antibiotic-resistant bacteria in the rumen of sheep after addition of avoparcin to the diet varied considerably one animal to another.


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