INTRODUCTION

This study aimed to investigate the mechanism underlying a peculiar resistance phenotype in , characterized by reduced susceptibility to cefepime compared to ceftazidime.

METHODS

Antimicrobial susceptibility testing, plasmid conjugation experiments, whole-genome sequencing, and bioinformatic analyses were employed to characterize the resistance phenotype and identify genetic determinants.

RESULTS

A total of 20 strains exhibiting peculiar resistance phenotypes were collected and analyzed. Ten distinct sequence types (STs) were identified, including ST25 (4/20), ST967 (3/20), ST65 (2/20), ST133 (2/20), ST48 (2/20), ST353 (1/20), ST628 (1/20), ST753 (1/20), ST792 (1/20), and ST254 (1/20). All strains were resistant to FEP (MIC = 128 µg/mL) but not to CAZ (MIC = 8 µg/mL). This resistance was primarily attributed to the presence of the (14/20) and (3/20). Conjugation experiments demonstrated that 5 out of 14 -positive strains successfully acquired transconjugants, which exhibited the same peculiar resistance phenotype. PCR analysis confirmed that the conjugates contained the IncFII plasmid. To further elucidate the genetic basis of the resistance phenotype, whole-genome long-read sequencing was performed on three -positive strains. The sequencing results confirmed that was located on the IncFII plasmid, and analysis of its genetic environment revealed a frequent association with mobile genetic elements such as IS, IS, and Tn.

DISCUSSION

The primary driver of this phenotype in is the presence of the IncFII plasmid carrying , which contrasts with the resistance mechanisms often reported in exhibiting similar phenotypes. This study emphasizes the critical role of plasmid-mediated resistance in the spread of multidrug resistance in and provides insights into strategies for combating resistance in these pathogens.