Abstract
BACKGROUND: The combination aztreonam/avibactam is currently under Phase 3 trials for the treatment of serious infections caused by Gram-negative bacteria including those with MBLs. OBJECTIVES: To investigate the resistance mechanisms in Enterobacterales exhibiting aztreonam/avibactam MICs of ≥4 mg/L. METHODS: Among 8787 Enterobacterales, 17 (0.2%) isolates exhibited an aztreonam/avibactam MIC of ≥4 mg/L. Isolates were sequenced and screened for β-lactamases. Sequences of porins, penicillin-binding protein 3 (PBP3) and expression levels of AmpC and AcrA were evaluated. RESULTS: Eleven (11/4154 isolates; 0.26%) Escherichia coli, three (3/1981; 0.15%) Klebsiella pneumoniae and three (3/628; 0.5%) Enterobacter cloacae were identified. All E. coli showed either an 'YRIK' or 'YRIN' insertion in PBP3. In general, these isolates carried blaCMY and/or blaCTX-M variants, except for one isolate from Korea that also produced NDM-5 and one isolate from Turkey that produced OXA-48. Two DHA-1-producing K. pneumoniae overexpressed acrA and had a premature stop codon in either OmpK35 or OmpK36, whereas a third K. pneumoniae carried blaPER-2 and had a premature stop codon in OmpK35. All three E. cloacae expressed AmpC at levels ≥570-fold, but sequence analysis did not reveal known amino acid alterations associated with decreased avibactam binding or increased hydrolysis of β-lactams. Minor amino acid polymorphisms within OmpC, OmpF and PBP3 were noted among the E. cloacae. CONCLUSIONS: A small number of isolates (0.2%) met the inclusion criteria. E. coli showed altered PBP3 as the most relevant resistance mechanism, whereas K. pneumoniae had multiple resistance mechanisms. Further investigations are needed to clarify resistance in E. cloacae.