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ABSTRACT Pseudomonas mosselii is an environmental bacterium with increasing reports of pathogenic potential. Although carbapenems are considered highly effective against this species, emerging resistance presents a potential therapeutic concern. In this study, P. mosselii isolates exhibiting reduced susceptibility to meropenem (MEM) while remaining susceptible to imipenem (IPM) were identified from fecal samples of patients collected in 2023 at a tertiary hospital in Lanzhou, Gansu Province, China. Antimicrobial susceptibility testing confirmed the discrepant carbapenem susceptibility profiles. Whole-genome sequencing and comparative genomic analyses were performed to elucidate the underlying mechanisms. Among 63 isolates, 15 exhibited reduced susceptibility to MEM and were selected for further analysis. Genomic analysis revealed that the classical carbapenem-resistance determinant oprD remained intact, whereas mutations were observed in efflux pump-associated genes ( oprM , tolC , and ttgC ). In addition, virulence gene profiling demonstrated the conserved presence of determinants associated with alginate biosynthesis ( algD , algU , mucD ), motility ( fleQ , fliA ), type IV pili ( pilH , pilJ ), type VI secretion system ( hcp1 , vipA/vipB , clpV1 ), and siderophore production ( pvdH , pvdM , pvdS ), indicating a shared virulence potential among the isolates. These findings suggest that efflux pump alterations rather than oprD inactivation may contribute to the unusual phenotype characterized by reduced meropenem susceptibility and preserved imipenem susceptibility in P. mosselii . This study provides the first genomic insight into this unique carbapenem susceptibility pattern and broadens understanding of carbapenem resistance diversity in Pseudomonas species. IMPORTANCE This study reports an unusual carbapenem susceptibility pattern in P. mosselii clinical isolates, characterized by reduced susceptibility to meropenem while remaining susceptible to imipenem and explores its potential underlying mechanisms. The findings reveal that efflux pump variations, rather than defects in oprD , may drive this unusual phenotype. As P. mosselii is typically susceptible to carbapenems, the emergence of isolates with reduced meropenem susceptibility highlights an overlooked antimicrobial resistance risk. Understanding this atypical mechanism provides valuable insights into the evolutionary plasticity of Pseudomonas and may guide future surveillance and treatment strategies against carbapenem resistance.
Kaiqi et al. (2026) studied this question.