A widespread dissemination of Bacillus licheniformis in a tertiary hospital: an outbreak or pseudo-outbreak?

Objective: Bacillus licheniformis is a Gram-positive bacterium commonly found in soil and water. There have been very few reports on the pathogenicity of B. licheniformis in humans. In this prospective study, the emergence of cases affected by B. licheniformis during a period of 38 days was reported together with investigations into the sources of spread to hospitalized patients in a tertiary hospital.

Methods: Blood cultures of 45 patients grew Bacillus spp. in October and November, 2021. To identify the source and prevent further dissemination of the pathogen, all commonly used materials were examined. Samples obtained from alcohol/water solutions yielded positive results for Bacillus spp., which pointed to the main distilled water tank of the hospital, subsequently found to be the main source. All isolates were sent for molecular analysis by arbitrarily-primed polymerase chain reaction (AP-PCR).

Results: Molecular analysis with AP-PCR of 29 positive cultures showed a closely related clone of B. licheniformis in 25 specimens, including 23 blood samples and two distilled water samples. Considering the rarity of true infections with B. licheniformis and the mild clinical picture of the affected patients, the dissemination was considered to be a pseudo-outbreak.

Conclusions: Prompt detection and elimination of any pathogenic spread and differentiation of a pseudo-outbreak from a true outbreak are of utmost importance in preventing unnecessary antibiotic prescriptions, diagnostic procedures, and interventions.