Ciprofloxacin-susceptible but levofloxacin-resistant Pseudomonas aeruginosa clinical strains with Vitek ® 2: which mechanism involved and consequences in case of fluoroquinolone treatment?

Pseudomonas aeruginosa clinical strains isolated harbored sometimes an atypical phenotype using the automated Vitek2 : ciprofloxacin-susceptibility but levofloxacin-resistance according to 2019 CA-SFM criteria. The aims of this study are to investigate the resistance mechanism(s) involved and to identify the consequences on fluoroquinolone treatment. Strain resistance profile, patient's data were recovered and reviewed from the database. Minimum inhibitory concentrations of levofloxacin, ciprofloxacin, moxifloxacin and delafloxacin were determined by using a concentration gradient strip. gyrA, gyrB, parC, parE and mexR genes were PCR amplified and sequenced. A PFGE analysis was performed for strains, recovered in a short period of time from the same patient. 46 strains were studied. A couple of seldom mutations were detected in gyrA, gyrB, parC and parE genes. Phenotypically, most of the strains (91%) were resistant to ticarcillin/ clavulanic acid combination and aztreonam suggesting a MexAB-OprM efflux-pump overexpression. mexR sequencing demonstrated either a deletion, a mutation or a premature stop codon appearance leading to amino acid substitution for 75% of the strains. Interestingly, four patients presented successively a fully fluoroquinolone susceptible isolate, thereafter a ciprofloxacin-susceptible but levofloxacin-resistant isolate (discordant phenotype) and finally a fluoroquinolone-resistant isolate. Molecular typing of these strains highlighted a strong relatedness between those isolates. The phenotype detected by the automate Vitek2 is linked to a likely efflux-pump overexpression mechanism and not fluoroquinolone-target mutation. Regarding this discordant phenotype, an alert should be provided to clinicians concerning the high risk of selecting a fluoroquinolone-resistant mutant.