The antibacterial activity and sensitivity of Streptococcus bovis strains isolated from the rumen of cattle
Approximately 50% of Streptococcus bovis isolates (n=90) from cattle fed hay- or grain-based diets inhibited the growth of S. bovis JB1. The bacteriocin-like inhibitory substance (BLIS) of the most active strain (HC5) was sensitive to Pronase E, and 16S rDNA analyses indicated that HC5 was closely related to S. bovis strains. When repetitive DNA (BOX) sequences were amplified by PCR, blis+ strains could be organized into 16 BOX-PCR groups, but the similarity indexes were as low as 40%. PCR analyses indicated that none of the blis+ strains had the gene for bovicin 255, a bacteriocin produced by Streptococcus gallolyticus LRC0255, and many of the blis+ isolates were sensitive to bovicin 255. Isolates that survived bovicin 255 and were transferred a second time became less sensitive (≤2 log reduction in viability). The isolates were more sensitive to the BLIS of S. bovis HC5 than bovicin 255, the initial average decrease in viability was approximately 3 logs greater (4.8±2.7 versus 1.8±2.1, respectively, P<0.001), and the HC5-sensitive strains did not adapt. Sixteen of the blis+ isolates were highly resistant to S. bovis HC5 (≤1 log reduction in viability), and BOX-PCR indicated that only five of them had the same BOX pattern as S. bovis HC5. The remaining blis+, HC5-resistant isolates (n=11) had distinctly different BOX patterns and were not always highly resistant to bovicin 255. Because blis+ and blis− strains could be isolated from the same animal, it appears that bacteriocin production is not the only factor affecting S. bovis competition in the rumen.