Bacillus licheniformis in geogenic dust induces inflammation in respiratory epithelium

Exposure to environmental geogenic (or earth-derived) dust can lead to more frequent and severe infections in the human airway. Particulate matter < 10 µm (PM₁₀) is the component of air pollution that is commonly associated with the exacerbation of respiratory diseases. We have previously demonstrated that mice exposed to geogenic dust PM₁₀ experienced an exacerbation of inflammatory responses to influenza A virus. Whether geogenic dust PM₁₀ also exacerbates respiratory bacterial infection is not yet known, nor are the components of the dust that drive these responses.

We treated airway bronchial epithelial cells (NuLi-1) with UV-irradiated geogenic dust PM₁₀ from six remote Western Australian towns. High levels of IL-6 and IL-8 production were observed, as well as persistent microbial growth. 16 S rRNA sequencing of the growth identified the microbe as Bacillus licheniformis, a spore-forming, environmentally abundant bacterium. We next investigated the interaction of ,B. licheniformis with respiratory epithelium in vitro to determine whether this exacerbated infection with a bacterial respiratory pathogen (non-typeable Haemophilus influenzae, NTHi).

Heat treatment (100 °C) of all PM₁₀ samples eliminated B. licheniformis contamination and reduced epithelial inflammatory responses, suggesting that heat-labile and/or microbial factors were involved in the host response to geogenic dust PM10. We then exposed NuLi-1 epithelium to increasing doses of the isolated Bacillus licheniformis (multiplicity of infection of 10:1, 1:1 or 0.1:1 bacteria: cells) for 1, 3, and 24 h. B. licheniformis and NTHi infection (association and invasion) was assessed using a standard gentamicin survival assay, and epithelial release of IL-6 and IL-8 was measured using a bead based immunoassay.

B. licheniformis was cytotoxic to NuLi-1 cells at 24 h. At 3 h post-challenge, B. licheniformis elicited high IL-6 and IL-8 inflammatory responses from NuLi-1 cells compared with cells treated with heat-treated geogenic dust PM₁₀ (p < 0.0001). Whilst treatment of cells with B. licheniformis increased inflammation, this did not make the cells more susceptible to NTHi infection. This study highlights that geogenic dust PM₁₀ can harbour viable bacterial spores that induce inflammation in respiratory epithelium. The impact on respiratory health from inhalation of bacterial spores in PM₁₀ in arid environments may be underestimated. Further investigation into the contribution of B. licheniformis and the wider dust microbiome to respiratory infection is warranted.