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

Purpose: 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.

Methods: 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.

Results: 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.

Conclusion: 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.