Incidence and Resistance Patterns of Citrobacter spp. in Switzerland: A Nationwide, Retrospective Surveillance Study (2010-2022).

We conducted a retrospective analysis of Citrobacter spp. surveillance data from acute care hospitals that contributed Citrobacter spp. data to the national surveillance system ANRESIS from January 2010 to December 2022. The incidence of Citrobacter spp. bloodstream infections (BSIs) in Switzerland was calculated, as well as the proportion of Citrobacter spp. isolates from urinary tract samples. We also evaluated the susceptibility of Citrobacter spp. isolates to clinically important antibiotics. From 2010 to 2022, there were 33,958 Citrobacter spp. from patients across 55 acute care hospitals continuously participating in ANRESIS included in this analysis. We observed an annual increase in the number of Citrobacter spp. BSIs, from 2.5 to 4.2 cases per 100,000 patient days (IRR: 1.04, 95% CI: 0.96-1.12). We found a higher incidence among male versus female patients (IRR: 2.47, 95% CI: 1.28-4.74) and in those aged ≥65 years, as compared with younger patients (IRR: 2.26, 95% CI: 1.18-4.32). The proportion of Citrobacter spp. among positive urinary tract samples also increased (from 18.6 to 24.7 per 1000 samples). Among ICU patients, there was a considerable proportion of resistance to third-generation cephalosporins among C. freundii isolates (26.8-44.0%), compared with non-freundii isolates (1.7-6.9%). Citrobacter spp. is gaining clinical importance in Switzerland; further studies are needed to better understand the underlying mechanisms.