Convergent S318T substitution in ADC enzymes contributes to reduced meropenem susceptibility in epidemic IC2...

Carbapenem resistance in Acinetobacter baumannii is usually mediated by acquired carbapenemases. Here we investigated an alternative mechanism involving variants of the intrinsic Acinetobacter-derived cephalosporinase (ADC).

Eighteen clonally related clinical isolates collected between 2019 and 2024 in a French long-term care facility were analysed together with four epidemiologically related isolates from additional hospitals. Antimicrobial susceptibility testing, whole-genome sequencing, cloning experiments, site-directed mutagenesis and enzyme kinetic analyses were performed to assess the contribution of ADC variants to carbapenem susceptibility.

All isolates were resistant to broad-spectrum cephalosporins, and 72% and 83% were resistant to imipenem and meropenem, respectively. Whole-genome sequencing identified a novel ADC variant, designated ADC-200, differing from ADC-30 by a single amino-acid substitution (S318T) and located downstream of ISAba1. Cloning experiments demonstrated that ADC-200 production increased meropenem MICs compared with reference ADC enzymes. Introduction of the S318T substitution into ADC-50 reproduced this phenotype, indicating that this residue plays a key role in modulating meropenem susceptibility. Kinetic analyses showed increased catalytic efficiency for meropenem hydrolysis in ADC variants harbouring the S318T substitution. Single nucleotide polymorphism-based phylogenetic analysis revealed clonal dissemination of ADC-200-producing isolates belonging to international clone 2 across several institutions.

Our findings demonstrate that naturally occurring ADC variants can contribute to reduced susceptibility to meropenem in epidemic A. baumannii lineages. Although this mechanism alone confers only moderate increases in carbapenem MICs, its occurrence in widespread clones highlights the importance of genomic surveillance to detect emerging resistance determinants beyond classical carbapenemases.