Bacillus subtilis var. natto bacteremia: A case series and review

Natto, a traditional Japanese fermented soybean food, contains Bacillus subtilis var. natto, which is generally regarded as safe but can rarely cause bloodstream infections in susceptible individuals. We report three cases of B. subtilis var. natto bacteremia confirmed by biotin operon analysis: a 63-year-old man receiving maintenance hemodialysis, a 35-year-old woman at 36 weeks of gestation, and a 62-year-old man whose bacteremia led to the diagnosis of colorectal cancer. All patients reported habitual natto consumption and had underlying conditions associated with impaired intestinal barrier function or altered immune status. Blood cultures yielded Bacillus subtilis, which was initially regarded as a contaminant. Targeted antimicrobial therapy resulted in resolution of bacteremia in all patients; the pregnant patient underwent emergency cesarean delivery, and the third patient subsequently underwent surgical resection for colorectal cancer. When considered together with previously published cases, our findings suggest that B. subtilis var. natto bacteremia primarily occurs in patients with impaired host defenses or intestinal barrier dysfunction. Biotin operon analysis identified characteristic genetic features of B. subtilis var. natto, consistent with a likely dietary origin associated with natto consumption. Because Bacillus species isolated from blood cultures are often dismissed as contaminants, B. subtilis var. natto bacteremia is likely underrecognized. Clinicians should consider this diagnosis in patients with relevant risk factors and obtain a detailed dietary history when Bacillus species are isolated from blood cultures.