Decline of antimicrobial resistance in Pseudomonas aeruginosa bacteraemia following the COVID-19 pandemic: a longitudinal observational study.

Background: Understanding the effects of changes brought by the COVID-19 pandemic on antimicrobial resistance in P. aeruginosa (PA) is essential to inform clinical management.

Methods: This single-centre retrospective cohort study included adult inpatients with PA bacteraemia at the University Hospital Zurich between January 2014 and December 2023. The primary outcome was the association between the start of the COVID-19 pandemic and PA with multidrug resistance (MDR), defined as resistance to ≥3 of 5 antibiotic classes. We used logistic regression to adjust for age, sex and ICU treatment. Secondary outcomes included changes in resistance patterns, patient demographics and antimicrobial consumption.

Results: A total of 493 instances of PA bacteraemia in 333 patients were observed during the study period. The proportion of MDRPA declined from 21% (62/291) pre-pandemic to 9% (19/202) post-pandemic (adjusted OR 0.38, 95% CI 0.18-0.79, p = 0.01). The occurrence of MDRPA during hospitalization following an initial instance of non-MDRPA bacteraemia was rare and unlikely to happen earlier than after 2 weeks. After the start of the pandemic, we observed no MDRPA cases involving cardiovascular or pulmonary diseases and marked reductions in patients with burn injuries or organ transplants. Furthermore, ciprofloxacin and tobramycin use significantly decreased after the start of the pandemic. Overall in-hospital mortality among patients with MDRPA bacteraemia remained high (28%), with no substantial differences between time periods.

Conclusions: We observed a decline in MDRPA occurrence after the start of the COVID-19 pandemic, possibly driven by intensified infection control measures, shifts in antimicrobial use and changes in patient populations.