Purpose:To identify ceftolozane/tazobactam (C/T)-resistant Pseudomonas aeruginosa isolates in Korea and to investigate the resistance mechanism.

Methods:We performed antibiotic susceptibility testing for 42 carbapenem-resistant, non-carbapenemase-producing P. aeruginosa isolates from Korea. Multilocus sequence typing was performed for all isolates. Whole-genome sequencing was performed for four carbapenem-resistant P. aeruginosa isolates, including two C/T-resistant isolates and two C/T-susceptible isolates.

Results:We identified two C/T-resistant P. aeruginosa isolates (P70 and P112) among 42 carbapenem-resistant, non-carbapenemase-producing P. aeruginosa isolates. We found multiple amino acid substitutions in AmpC, AmpR, efflux pump systems, and a porin protein in C/T-resistant P. aeruginosa isolates. These were not reported previously in C/T-resistant P. aeruginosa isolates.

Conclusion:C/T-resistant P. aeruginosa isolates with multiple amino acid alterations were identified, implying that resistance to this novel antibiotics should be constantly monitored.