Cortes, Melissa

Abstract Background: Clostridium ramosum ( C. ramosum ) is an anaerobic gram-positive bacillus of the human intestine and a rare cause of bacteremia. This retrospective cohort study evaluated clinical characteristics and outcomes of a multicenter cohort of adults with C. ramosum bacteremia. Methods: This multicenter retrospective cohort included adults (≥ 18 years) with true C. ramosum bacteremia at Mayo Clinic hospitals in Arizona, Florida, Minnesota, and Wisconsin from September 1, 2012, through December 31, 2025. Data were collected through search for C. ramosum in microbiological culture records. Clinical characteristics assessed included immune status, source of bacteremia, presence of sepsis, neutropenic fever, monomicrobial vs. polymicrobial bacteremia, antimicrobial resistance patterns, antimicrobial therapy, length of stay and 30-day all cause mortality. Exploratory odds ratios (ORs) with 95% confidence intervals (CIs) were reported. Results: Among 90 patients with a positive C. ramosum blood culture, 17 were classified as contaminants and 73 comprised the study cohort. Median age was 66.0 years (IQR, 56.0–75.0); 44 (60.3%) were male and 33 (45.2%) were immunocompromised (including 26 with active malignancy, while the remaining 7 were received biologic immunosuppression). The most common infection sources were gastrointestinal (32/73, 43.8%) and central line-related (8/73, 11%). At presentation, 50.7% of patients met criteria for sepsis and 15/73 (20.5%), were in septic shock. Of 73 isolates tested, 31/73 (44.3%) were clindamycin resistant and 9/73 (12.9%) tested intermediate. Anaerobic-active antimicrobial therapy was received by 53/73 (72.6%). Thirty-day mortality was 11/73 (15.1%). Mortality was 30.0% (6/20) with therapy without reliable anaerobic activity and 9.4% (5/53) with anaerobic-active antimicrobial therapy (OR, 4.02; 95% CI, 0.88–19.46; P = 0.060). The median hospital LOS was 8.0 days. Among 30-day survivors, anaerobic active antimicrobial therapy was associated with a shorter median LOS compared to therapy without reliable anaerobic activity (8.5 vs. 12.0 days). Immunocompromised patients had higher odds of monomicrobial C. ramosum bacteremia than immunocompetent patients (OR 3.16, 95% CI 1.19–8.39; p = 0.03). Patients with polymicrobial bacteremia had higher odds of sepsis (OR, 3.89; 95% CI, 1.43–10.55; P = 0.008) and septic shock (OR, 5.66; 95% CI, 1.17–27.36; P = 0.020). Patients with malignancy or neutropenia demonstrated significantly lower odds of presenting in septic shock (p = 0.018; p = 0.029 retrospectively). Microbiological clearance was achieved in 91.3% (42/46) of evaluable cases. Conclusions: This largest reported cohort defines the clinical and microbiological profile of C. ramosum bacteremia. Frequent clindamycin nonsusceptibility is clinically actionable. Several clinical associations found on univariable analysis should be further investigated in larger studies.