BlaGES-6 producing Pseudomonas aeruginosa ST235 is involved in resistance to different β-lactams.

Multidrug resistance in Pseudomonas aeruginosa, particularly resistance to carbapenem, represents a major challenge for public health. This study investigated resistance mechanisms in three P. aeruginosa isolates: HU63 (blaGES-6 carbapenemase-positive), HU141 (carbapenem-resistant without carbapenemase), and PAO1 (control). Genomic analysis revealed distinct sequence types (ST235 for HU63, ST253 for HU141) and chromosomal integration of resistance genes. HU63 harbored diverse resistance mechanisms, including β-lactamases (blaGES-6, blaPDC-35, blaOXA-488) and efflux pumps. Minimum inhibitory concentration assays demonstrated HU63's resistance to all β-lactams tested (meropenem, imipenem-cilastatin, ceftazidime, piperacillin-tazobactam), while HU141 remained susceptible except to cefoxitin and cloxacillin. Time-kill assays revealed tolerance phenotypes, with HU63 showing regrowth after 8-24 h despite initial reductions in bacterial density. Gene expression varied significantlydepending on the antibiotic and the isolate. The HU63 isolate (GES-6 positive) stands out for its marked induction of blaGES-6 in all the antibiotics tested, contributing to its resistance to carbapenems and broad-spectrum cephalosporins. These expression profiles corroborate the classic molecular mechanisms of resistance: regulation of entry pores (oprD), activation of efflux pumps (mexA) and production of β-lactamases (blaGES-6, ampC) adapted to each situation. These findings underscore the multifactorial nature of resistance in Carbapenem-resistant Pseudomonas aeruginosa (CRPA), combining enzymatic inactivation, efflux, and genetic adaptability. The study emphasizes the urgent need for genomic surveillance to track high-risk clones and develop therapies targeting tolerance mechanisms alongside traditional resistance.