Genomic analysis of carbapenem-resistant Pseudomonas aeruginosa ST143 clone showing susceptibility to broad-spectrum cephalosporins

Objectives: Using whole-genome sequencing (WGS), we aimed to characterise a Pseudomonas aeruginosa ST143 clinical strain (Pb9) that presented resistance to meropenem and imipenem and susceptibility to piperacillin/tazobactam and broad-spectrum cephalosporins. Methods: The antimicrobial susceptibility profile was confirmed by broth microdilution. WGS was performed using an Illumina MiSeq platform to identify possible genetic determinants of β-lactam resistance. Transcription levels of chromosomally encoded efflux systems and oprD were evaluated by RT-qPCR. Results: WGS analysis showed that no acquired carbapenemase-encoding gene was found in isolate Pb9, although mutations in the chromosomally encoded β-lactamase genes bla_OXA-488, bla_PIB-1 and bla_PDC-5 were observed. In addition, we detected a premature stop codon in the major porin-encoding gene oprD coupled with hyperexpression of MexAB–OprM and MexEF–OprN. Conclusion: Our results suggest that the β-lactam resistance phenotype presented by strain Pb9 might be related to an association of OprD loss with hyperexpression of the efflux pump systems MexAB–OprM and MexEF–OprN. However, the contribution of OXA-488, PDC-5 and PIB-1 to this phenotype remains unclear and warrants further investigation.