The Majority of Bacillus subtilis Strains Isolated From Blood Cultures Were Derived From Traditional Japanese Fermented Soybeans Natto: A Single-center Retrospective Study.

Background: Natto consumption has been associated with improved cardiovascular and survival outcomes. Bacillus subtilis variant natto (B. subtilis var. natto) is essential for soybean fermentation in natto production. Recently, several cases of B. subtilis var. natto bacteremia have been reported, although the contribution of B. subtilis var. natto remains unclear. This study assessed the prevalence and clinical impact of B. subtilis var. natto in B. subtilis bacteremia.

Methods: This retrospective cohort study analyzed medical records of patients with positive B. subtilis blood cultures at Tenri Hospital from 1 April 2016 to 31 March 2023. True bacteremia required portal-of-entry or, for intra-abdominal/abscess, 2-physician adjudication; if no portal, 2 or more same-day blood culture-positive sets. Genetic testing confirmed B. subtilis var. natto through specific mutations in the bioF and bioW genes.

Results: Of 4634 positive blood cultures, 70 (1.5%) were identified as B. subtilis, with 69 (99%) classified as B. subtilis var. natto. Of these, 25 cases (36%) were confirmed as true bacteremia. The primary diagnoses included intra-abdominal infections, pneumonia, and urinary tract infection. The median patient age was 79 years, with 9 being women. The median Pitt bacteremia score was 0 (interquartile range: 0-1). Seven patients (28%) required urgent surgery or endoscopic procedures, while 4 (16%) died within 30 days.

Conclusions: Most B. subtilis blood strains isolates were B. subtilis var. natto. This bacteremia carried significant severity, with a 16% 30-day mortality rate and 28% requiring urgent interventions. Clinicians should not dismiss B. subtilis var. natto, a probiotic strain, as harmless.