The Possibility of Ceftolozane/Tazobactam-Resistant Pseudomonas aeruginosa Emergence After Two Days of Antibiotic Therapy: A Case Report

Although antibiotic use is known to induce antimicrobial resistance, the duration of exposure necessary for resistance development remains uncertain. In this case, a patient was initially treated with tazobactam/piperacillin (TAZ/PIPC) for bacterial pneumonia. When the treatment proved ineffective, the regimen was switched to tazobactam/ceftolozane (TAZ/CTLZ) after confirming that the <i>Pseudomonas aeruginosa</i> isolated at admission was susceptible to TAZ/CTLZ. Although the patient's symptoms initially improved, pneumonia exacerbation occurred 10 days after the initiation of TAZ/CTLZ. Drug susceptibility testing in <i>P. aeruginosa</i> isolated on the second day of TAZ/CTLZ treatment revealed resistance to the antibiotics. Genetic analysis using the polymerase chain reaction (PCR)-based open reading frame typing method demonstrated that the <i>P. aeruginosa</i> strains isolated before and after TAZ/CTLZ treatment were genetically identical. This case highlights the possibility of TAZ/CTLZ-resistant <i>P. aeruginosa</i> emerging after only two days of antibiotic exposure.