Aztreonam-avibactam may not replace ceftazidime/avibactam: the case of KPC-21 carbapenemase and penicillin-binding protein 3 with four extra amino acids

Aztreonam/avibactam is a promising antimicrobial combination with additional coverage for metallo-β-lactamases compared with ceftazidime/avibactam. A carbapenem-resistant blaKPC-2-carrying Escherichia coli clinical isolate had four extra amino acids in penicillin-binding protein 3 (PBP3), which has been known to mediate reduced susceptibility to aztreonam/avibactam. This prompted us to investigate whether the strain could develop resistance to aztreonam/avibactam after exposure to the combination. A mutant with high-level resistance to aztreonam/avibactam [minimum inhibitory concentration (MIC), 512/4 mg/L] was obtained after 5-day exposure to 0.5 × MIC but it remained susceptible to ceftazidime/avibactam (MIC, 4/4 mg/L). The mutant had a single nucleotide polymorphism (SNP) in blaKPC-2 to encode KPC-21 with a Trp105Arg amino acid substitution. By cloning into E. coli BL21, blaKPC-21 could mediate reduced susceptibility to aztreonam/avibactam (MIC, from ≤0.03/4 to 1/4 mg/L), which was still below the breakpoint to define resistance. In contrast, when blaKPC-21 was cloned in E. coli 035125ΔpCMY42 with four extra amino acids in PBP3, which was generated in our previous work, the strain exhibited high-level resistance to aztreonam/avibactam (MIC, 256/4 mg/L). The above findings highlight that although aztreonam/avibactam has a broader spectrum than ceftazidime/avibactam, strains may develop resistance to the former combination but remain susceptible to the latter. The discrepancy is due to mutation of KPC-2 to KPC-21 in combination with the insertion of four extra amino acids in PBP3.