Characterization of Metallo β-Lactamase Producing Enterobacterales Isolates with Susceptibility to the Aztreonam/Avibactam Combination

Abstract

Background/Objectives: Metallo-β-lactamases (MBLs) in Enterobacterales and other Gram-negative organisms pose significant public health threats due to their association with multidrug resistance (MDR). Although aztreonam (AZT) can target MBL-producing organisms, its efficacy is compromised in organisms expressing additional β-lactamases that inactivate it. Combining AZT with the β-lactamase inhibitor avibactam (AVI) may restore its activity against MBL-producing isolates. Methods: AZT-AVI, along with other clinically relevant antimicrobials, was tested against thirteen MBL-producing clinical isolates of Enterobacterales (nine Klebsiella pneumoniae, three Enterobacter cloacae, and one Providencia stuartii) using whole-genome sequencing (WGS) for genetic characterization. Results: AZT-AVI demonstrated full susceptibility across all isolates, whereas aztreonam alone was ineffective. The newer β-lactam/β-lactamase inhibitor combinations imipenem/relebactam and meropenem/vaborbactam were inactive in 100% and 92.3% of isolates, respectively. WGS-based analysis revealed multiple resistance mechanisms consistent with MDR phenotypes, including high-risk K. pneumoniae clones (ST147 and ST11). Conclusions: AZT-AVI is effective against MDR MBL-producing Enterobacterales, highlighting its therapeutic potential for challenging infections. While WGS does not replace phenotypic testing, it provides valuable insights for antimicrobial stewardship and the monitoring of resistance gene dissemination.

Keywords: Enterobacterales; antimicrobial resistance; metallo-β-lactamase; whole genome sequencing; β-lactam/β-lactamase inhibitor combination.

Figure 1

Mapping of antimicrobial resistance genes/determinants across the bacterial isolates (n = 13) included in the study. All isolates exhibited multiple antimicrobial resistance mechanisms. Colored blocks indicate the presence of a gene/determinant associated with resistance to specific classes of antimicrobial agents. Asterisks denote mutated genes. Detected point mutations include those associated with resistance to carbapenems (ompK36_G133D), colistin (pmrB_R256G), and fluoroquinolones (gyrA_S83F, gyrA_S83I, gyrA_S83Y, gyrA_D87A, gyrA_D87N, and parC_S80I). TMP/SMX, trimethoprim/sulfamethoxazole.

Figure 2

Core genome phylogeny of nine K. pneumoniae isolates (Kp1 to Kp9) sequenced in this study. (a) The MLST-based minimum spanning tree shows the relatedness among the isolates, represented by colored circles at the tree nodes. Numbers indicate allele differences between nodes. (b) The SNP-based neighbor-joining tree shows the relatedness among the isolates, represented by colored circles (colors correspond to the different STs identified) at the terminal nodes of the tree. The tree is organized according to the presence (colored) or absence (gray) of matrix-distributed genomic features, such as plasmids and mobile genetic elements. These include insertion sequences, integrative conjugative elements, and transposons. Kp, Klebsiella pneumoniae; MLST, multilocus sequence typing; SNP, single nucleotide polymorphism; ST, sequence type.