[Invasive infections of Bacillus species in Iceland, 2006-2018]

The bacterial genus Bacillus is widely distributed environmentally and is usually considered a low-virulence organism, except for B. anthracis. A blood culture positive for Bacillus is often looked at as contamination. Nevertheless, B. cereus can cause invasive infections in humans and produces harmful toxins. The epidemiology of these infections remains poorly studied.

All possible invasive infections caused by Bacillus during 2006-2018 at Landspitali University Hospital were identified from culture results. Clinical information was used to evaluate if there was a possible infection or confirmed infection. Here, the authors propose and use clinical criteria to categorize each case as contamination, possible infection or confirmed infection. The incidence of possible or confirmed infections was calculated using hospital catchment population data.

Positive cultures of Bacillus sp. from sterile sites during 2006-2018 were identified from 126 patients; blood (116), synovial fluid (8) and cerebrospinal fluid (2). In total, 26 cases were confirmed infection (20.6%), 10 possible infection (7.9%) and 90 contamination (71.4%). The incidence was 1.4 cases/100.000 inhabitants/year. Injection drug use was a risk factor among 11/26 patients with confirmed infection. The most common clinical presentation was sepsis. In this study, Bacillus was resistant to beta-lactam antibiotics in 92% of confirmed infections and 66% of the cases considered contamination (p=0.02).

Positive blood cultures of Bacillus sp. should be taken seriously, especially among patients with injection drug use, malignancy or immunocompromised state. It is important to draw two sets of blood cultures if there is a real suspicion of an infection to establish diagnosis and avoid unnecessary antibiotic therapy.