Mechanism-Aware View of Carbapenem Resistance and Diagnostics

Carbapenem resistance in Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii is presented in a recent carbapenem resistance diagnostics review as a multilayered system rather than a single mechanism.

The authors synthesize enzymatic carbapenemases alongside permeability and efflux adaptations, and they add genome-dynamic “dosage” effects that can shift resistance intensity over time. Within that framework, the review links mechanistic diversity to the possibility of unstable resistance phenotypes that can complicate routine susceptibility testing. Overall, it describes how mechanism, detection, and surveillance considerations intersect in contemporary carbapenem resistance.

Enzymatic resistance is commonly organized by Ambler class, including class A serine carbapenemases such as KPC, class B metallo-β-lactamases (MBLs) including NDM, VIM, and IMP, class C enzymes, and class D OXA-48-like enzymes. The review notes this taxonomy as clinically relevant because inhibitor activity differs by catalytic type: serine-carbapenemase inhibition patterns are distinct from MBL settings where licensed inhibitors do not apply. In that context, the authors map newer agents and combinations to mechanism categories, describing ceftazidime–avibactam, meropenem–vaborbactam, imipenem–relebactam, aztreonam–avibactam, cefiderocol, and sulbactam–durlobactam in relation to the enzyme classes they are designed to address. The review frames Ambler classification as clinically relevant because it helps indicate when a β-lactamase inhibitor (BLI) is likely to restore antibiotic activity.

Allelic diversity is treated as an additional clinically visible layer, with the review detailing KPC Ω-loop substitutions such as D179Y and D179N as variants reported to emerge under ceftazidime–avibactam pressure. The authors describe a “seesaw” observation in which some isolates showing ceftazidime–avibactam resistance display comparatively restored carbapenem susceptibility, attributing the pattern to summarized structural and clinical observations around altered inhibitor interaction and enzyme function. The review also links uncommon variants and evolving alleles to the possibility of diagnostic edge cases where detection is less straightforward than for canonical carbapenemase types. In this account, allele-level changes can shift both the resistance profile and the likelihood of being cleanly captured by routine detection approaches.

Non-enzymatic mechanisms are presented as organism-specific contributors that can modulate carbapenem MICs and yield agent-dependent patterns even when a carbapenemase is present. For Enterobacterales, the review highlights porin alterations, including OmpK35 and OmpK36 changes; for P. aeruginosa, it emphasizes OprD loss with RND efflux upregulation; and for A. baumannii, it describes outer-membrane remodeling that can include CarO changes alongside ISAba1-linked expression effects in OXA contexts. Against that backdrop, the review summarizes diagnostic performance notes spanning phenotypic activity assays and rapid antigen formats, including sensitivity limitations of Carba NP with low-expression OXA-48-like producers, as well as performance patterns reported for mCIM/eCIM (high sensitivity in Enterobacterales with more variable results in P. aeruginosa) and lateral-flow immunoassays that generally show high agreement for common carbapenemases but occasional false negatives for uncommon variants. The throughline is that assay readouts are portrayed as contingent on both mechanism and expression level.

Genome dynamics are framed as a further layer that can complicate single timepoint genotype–phenotype alignment, with the review describing IS26-mediated tandem amplification and plasmid multimerization as reversible routes to increased β-lactamase copy number. The authors link these dosage shifts to heteroresistance and inoculum effects, emphasizing that resistance can fluctuate as copy number expands under selective pressure and contracts when pressure is removed. To capture these dynamics, the review discusses quantitative genomics approaches such as WGS read-depth methods and digital droplet PCR, and it also discusses the rationale for repeated susceptibility assessment during therapy, particularly during prolonged courses where resistance phenotypes may be unstable. The dissemination framing is explicitly plasmid-aware, naming backbones and mobile elements such as IncX3, IncL, Tn4401, and IS26 in connection with how carbapenemase determinants move and persist across settings. The final picture is of resistance portrayed as shifting, mobile, and sometimes reversible within the same lineage over short timescales.

Key Takeaways:

• The review depicts carbapenem resistance as the interaction of enzymatic carbapenemases, non-enzymatic permeability/efflux changes, and genome-dynamic dosage phenomena.
• Diagnostic sensitivity is described as mechanism- and variant-dependent, with enzyme class, expression level, and uncommon alleles influencing assay readouts.
• Plasmid context and reversible copy-number changes are presented as factors that can complicate genotype–phenotype interpretation and surveillance narratives.