Cranberry-derived proanthocyanidins potentiate beta-lactam antibiotics against resistant bacteria.
The emergence and spread of extended-spectrum beta-lactamases (ESBLs), metallo-beta-lactamases (MBLs), or variant low-affinity penicillin-binding proteins (PBPs) pose a major threat to our ability to treat bacterial infection using beta-lactam antibiotics. Although combinations of beta-lactamase inhibitors with beta-lactam agents have been clinically successful, there are no MBL inhibitors in current therapeutic use. Furthermore, recent clinical use of new-generation cephalosporins targeting PBP2a, an altered PBP, has led to the emergence of resistance to these antimicrobial agents. Previous work shows that natural polyphenols such as cranberry-extracted proanthocyanidins (cPAC) can potentiate non-beta-lactam antibiotics against Gram-negative bacteria. This study extends beyond previous work by investigating the in vitro effect of cPAC in overcoming ESBL-, MBL-, and PBP2a-mediated beta-lactam resistance. The results show that cPAC exhibit variable potentiation of different beta-lactams against beta-lactam-resistant Enterobacteriaceae clinical isolates as well as ESBL- and MBL-producing E. coli. We also discovered that cPAC have broad-spectrum inhibitory properties in vitro on the activity of different classes of beta-lactamases, including CTX-M3 ESBL and IMP-1 MBL. Furthermore, we observe that cPAC selectively potentiate oxacillin and carbenicillin against methicillin-resistant but not methicillin-sensitive staphylococci, suggesting that cPAC also interfere with PBP2a-mediated resistance. This study motivates the need for future work to identify the most bioactive compounds in cPAC and to evaluate their antibiotic-potentiating efficacy in vivo.