Toxicity study of SARS-CoV-2 Spike protein expressed on Bacillus subtilis spores

The SARS-CoV-2 spike protein is the primary antigen used in most COVID-19 vaccines. To improve protection, especially at mucosal surfaces, a robust and safe delivery system capable of inducing both systemic and mucosal immunity is needed. Bacillus subtilis spores offer a promising platform due to their stability, safety, and capacity for protein display. A study to evaluate the potential acute oral toxicity of recombinant B. subtilis spores expressing SARS-CoV-2 spike proteins was undertaken.

B. subtilis spore was used to express the SARS-CoV-2 S1 domain spike proteins during the bacterial sporulation. No in vitro cytotoxicity was observed when the spores were used to treat cells. BALB/c mice were given a single oral dose (5 × 10¹¹ CFU/ml) over three consecutive days and monitored for 14 days to assess acute toxicity. BALB/c mice fed with up to 3 × 10¹¹ CFU/mouse of the spores showed no adverse effects. A gross necropsy and histology of the treated mice's vital organs, kidneys, liver, heart, lungs, and spleen revealed no pathological alterations, and the biochemical parameters remained within normal ranges.

These findings suggest that the SARS-CoV-2 S1 domain spike proteins expressed on B. subtilis spores is well tolerated in mice when administered orally at a high dose. Findings from this study, hence, support further investigation of the recombinant bacterial spores expressing SARS-CoV-2 S1 domain spike proteins as an oral vaccine candidate against SARS-CoV-2 infection.