Deciphering the emergence of mutations in Pseudomonas aeruginosa after ceftolozane/tazobactam treatment and evaluating therapeutic alternatives.

OBJECTIVES

Ceftolozane/tazobactam is a relevant option for treating multiresistant Pseudomonas aeruginosa, a major public health problem. Over a 14-month period, we studied the frequency of ceftolozane/tazobactam resistance development and mechanisms underlying resistance in P. aeruginosa isolates after ceftolozane/tazobactam treatment as well as alternative therapeutic options.

METHODS

Antimicrobial susceptibility was assessed with the disc diffusion method and minimum inhibitory concentration (MIC) determination. Resistance genes and sequence types (ST) were determined by whole genome sequencing (WGS). The ceftolozane/tazobactam resistance mechanism was explored by cloning ampC into the ampC-deficient strain PAO1 and blaAmpC expression experiments.

RESULTS

Of 50 patients treated, 9 (18%) acquired one or more ceftolozane/tazobactam-resistant isolates during treatment. WGS revealed various mutations in the ampC gene, namely, Thr70Ile, Phe121Leu, Pro154Leu, Gly157Asp, del210-216, Glu221Lys, and the new mutation, Pro217Gln. Cloning experiments revealed that the Pseudomonas-derived cephalosporinase (PDC) polymorphism did not alter mutation effects, mutations did not confer the same resistance level, and those conferring the strongest resistance restored susceptibility to piperacillin, piperacillin/tazobactam and imipenem despite changes in the oprD gene. In clinical isolates, the resistance level conferred by the same mutations differed, indicating the involvement of other mechanisms, particularly the overproduction of AmpC, which is always present due to mutations mainly in AmpD. Finally, imipenem/relebactam, colistin, amikacin and cefiderocol could be good alternatives for treating ceftolozane/tazobactam-resistant strains.

CONCLUTION: Ceftolozane/tazobactam resistance development was associated mainly with chromosomal mutations in the ampC gene but also with mutations in its regulatory genes. Knowledge about these mutations and gene polymorphisms is essential for interpreting resistance phenotypes from genomic data.