Antimicrobial activities of amphiphilic cationic polymers and their efficacy of combination with novobiocin

Today, drug-resistant bacteria represent a significant problem worldwide. In fact, bacteria are becoming resistant even to newly developed antibiotics. Therefore, there is an urgent need to develop antibiotics to which bacteria cannot become resistant. In this study, antimicrobial polymers to which bacteria cannot develop resistance were prepared from 6-aminohexyl methacrylamide and N-isopropyl acrylamide. The polymers with molecular weights of the order of 105 showed little antimicrobial activity against Staphylococcus aureus and Escherichia coli as well as low toxicity. On the other hand, polymers with lower molecular weights (of the order of 104) did show antimicrobial activity against S. aureus and E. coli. These polymers were combined with novobiocin to investigate the combined usage effects against E. coli. The combined usage of novobiocin and the low-molecular-weight polymers reduced the minimum inhibitory concentration, which was less than 0.0625 μg/mL against E. coli. This result indicates that the combination is useful for increasing the efficacy of antibiotics and broadening their antimicrobial spectrum. Furthermore, the results showed the possibility that the antimicrobial polymers serve not only as antibiotics to which bacteria have not developed resistance but also as adjuvants for other antibiotics.