Phenotypic and genomic characterization of <i>Roseomonas mucosa</i> , an opportunistic pathogen with discrepancies among antimicrobial susceptibility testing methods

<i>Roseomonas mucosa</i> is an opportunistic bacterium found in clinical and environmental samples that primarily affects immunocompromised patients. Treatment is challenging owing to the lack of standardized susceptibility testing methods, breakpoints, and variable antimicrobial resistance profiles published. This study evaluated different antimicrobial susceptibility testing approaches and searched for new insights into resistance mechanisms, especially against third-generation cephalosporins. Antimicrobial susceptibility profiles of a panel of 17 <i>R</i>. <i>mucosa</i> strains were analyzed using disk diffusion method (DDM), broth microdilution (BMD) method, and MIC gradient strips. Discrepancies between susceptibility methods were further explored using beta-lactamase inhibitors, whole-genome sequencing, and transcriptomic analyses. Antimicrobial susceptibility testing revealed high susceptibility to aminoglycosides, fluoroquinolones, and carbapenems, while resistance to many beta-lactams was detected. Significant discrepancies were observed between the DDM and BMD methods, particularly with respect to the use of third- and fourth-generation cephalosporins and aztreonam. Genomic analysis identified two putative class-A and one class-C beta-lactamases within all strains. Transcription of one class A beta-lactamase, controlled by an <i>lysR</i> regulator, was significantly induced by cephalosporins and explained the phenotype observed. This study provides the antimicrobial susceptibility profiles against a large panel of antibiotics from 17 <i>R</i>. <i>mucosa</i> strains. It also explained the deep discrepancies between phenotypic approaches regarding cephalosporins.