Nisin resistance of Streptococcus bovis

The growth of Streptococcus bovis JB1 was initially inhibited by nisin (1 micromolar), and nisin caused a more than 3-log decrease in viability. However, some of the cells survived, and these nisin-resistant cells grew as rapidly as untreated ones. To see if the nisin resistance was merely a selection, nisin-sensitive cells were obtained from agar plates lacking nisin. Results indicated that virtually any nisin-sensitive cell could become nisin-resistant if the ratio of nisin to cells was not too high and the incubation period was long enough. Isolates obtained from the rumen were initially nisin sensitive, but they also developed nisin resistance. Nisin-resistant cultures remained nisin resistant even if nisin was not present, but competition studies indicated that nisin-sensitive cells could eventually displace the resistant ones if nisin was not present. Nisin-sensitive, glucose-energized cells lost virtually all of their intracellular potassium if 1 micromolar nisin was added, but resistant cells retained potassium even after addition of 10 micromolar nisin. Nisin-resistant cells were less hydrophobic and more lysozyme-resistant than nisin-sensitive cells. Because the nisin-resistant cells bound less cytochrome c, it appeared that nisin was being excluded by a net positive (i.e., less negative) charge. Nisin-resistant cells had more lipoteichoic acid than nisin-sensitive cells, and deesterified lipoteichoic acids from nisin-resistant cells migrated more slowly through a polyacrylamide gel than those from nisin-sensitive cells. These results indicated that lipoteichoic acids could be modified to increase the resistance of S. bovis to nisin. S. bovis JB1 cultures were still sensitive to monensin, tetracycline, vancomycin, and bacitracin, but ampicillin resistance was 1,000-fold greater.