Actinobaculum schaalii - invasive pathogen or innocent bystander? A retrospective observational study

Area of Science:

• Clinical microbiology
• Infectious diseases
• Bacteriology

Background:

• Actinobaculum schaalii is a Gram-positive, facultative anaerobic coccoid rod, identified as a distinct genus in 1997.
• Its slow growth and tendency to be overgrown by other pathogens often lead to it being missed in routine diagnostics.
• The study investigates the clinical significance of Actinobaculum schaalii identified over an 11-year period.

Purpose of the Study:

• To determine the clinical significance of Actinobaculum schaalii.
• To analyze patient data and isolate characteristics for Actinobaculum schaalii identified in a clinical microbiology laboratory.

Main Methods:

• Retrospective analysis of consecutive Actinobaculum schaalii isolates from a clinical microbiology laboratory database over 11 years.
• Patients with isolates from any body site were analyzed.
• Molecular identification using 16S rRNA gene sequencing confirmed isolates initially suspected based on colony morphology and Gram stain.

Main Results:

• 40 Actinobaculum schaalii isolates were obtained from 27 patients (median age 81 years).
• The majority of patients (92.6%) had underlying diseases, and 44.4% had genitourinary tract pathology.
• Actinobaculum schaalii was isolated from urine (12), blood (21), and deep tissue biopsies (7); 37.5% of specimens were polymicrobial, and 18.6% of urine isolates represented colonization.

Conclusions:

• Detection of Actinobaculum schaalii was associated with infection in 81.5% of patients, primarily sepsis and abscesses.
• Due to its frequent presence in polymicrobial cultures, Actinobaculum schaalii is often misidentified as a contaminant.
• The study concludes that Actinobaculum schaalii in clinical isolates typically signifies infection, not merely colonization or bystander presence.