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

We conducted a retrospective analysis of <i>Citrobacter</i> spp. surveillance data from acute care hospitals that contributed <i>Citrobacter</i> spp. data to the national surveillance system ANRESIS from January 2010 to December 2022. The incidence of <i>Citrobacter</i> spp. bloodstream infections (BSIs) in Switzerland was calculated, as well as the proportion of <i>Citrobacter</i> spp. isolates from urinary tract samples. We also evaluated the susceptibility of <i>Citrobacter</i> spp. isolates to clinically important antibiotics. From 2010 to 2022, there were 33,958 <i>Citrobacter</i> 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 <i>Citrobacter</i> 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 <i>Citrobacter</i> 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 <i>C. freundii</i> isolates (26.8-44.0%), compared with non-<i>freundii</i> isolates (1.7-6.9%). <i>Citrobacter</i> spp. is gaining clinical importance in Switzerland; further studies are needed to better understand the underlying mechanisms.