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

Abstract

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.

Keywords: Citrobacter spp.; Switzerland; antimicrobial resistance; bloodstream infections; epidemiological surveillance; incidence; urinary tract infections.

Figure 1

Map of Switzerland indicating the 55 included acute care hospitals that reported at least one Citrobacter spp. to ANRESIS each year from 1 January 2010 to 31 December 2022. Green circles represent university hospitals; orange circles represent non-university hospitals.

Figure 2

(A–D): Incidence of Citrobacter bacteremia per 100,000 patient-days across 55 ANRESIS acute care hospitals in Switzerland, 2010–2022, N = 1590, stratified by linguistic region (A), Citrobacter species (B), sex (C), and age group (D). (E–H): Proportion of Citrobacter spp. from urinary tract samples per 1000 positive urinary tract samples sent to ANRESIS across 55 acute care hospitals in Switzerland, 2010–2022, N = 19,699, stratified by linguistic region (E), Citrobacter species (F), sex (G) and age group (H).

Figure 3

Susceptibility of Citrobacter freundii and non-freundii isolates to cefepime (A,B), third-generation cephalosporins (C,D), carbapenems (E,F), and fluoroquinolones (G,H) among patients admitted to intensive care units from 55 ANRESIS acute care hospitals in Switzerland, 2010–2022. Isolates were considered as ‘s’ (susceptible or susceptible, increased exposure), r’ (resistant), or ‘Unknown’.

Figure 4

Susceptibility of Citrobacter freundii and non-freundii isolates to cefepime (A,B), third-generation cephalosporins (C,D), carbapenems (E,F), and fluoroquinolones (G,H) among patients in non-ICU inpatient wards across 55 ANRESIS acute care hospitals in Switzerland, 2010–2022. Isolates were considered as ‘s’ (susceptible or susceptible, increased exposure), r’ (resistant), or ‘Unknown’.

Figure 5

Susceptibility of Citrobacter freundii and non-freundii isolates to cefepime (A,B), third-generation cephalosporins (C,D), carbapenems (E,F), and fluoroquinolones (G,H) among outpatient departments across 55 ANRESIS acute care hospitals in Switzerland, 2010–2022. Isolates were considered as ‘s’ (susceptible or susceptible, increased exposure), r’ (resistant), or ‘Unknown’.