A total of thirty-two aerobic spore former strains were isolated from intestinal samples of healthy children and analyzed for their hemolytic and antibiotic-resistance activities. Four strains selected as non-hemolytic and sensitive to all antibiotics recommended as relevant by regulatory agencies were short-listed and evaluated for their in silico and in vitro probiotic potentials. The four selected strains were assigned to the Bacillus velenzensis (MV4 and MV11), B. subtilis (MV24) and Priestia megaterium (formerly Bacillus megaterium) (MV30) species. A genomic analysis indicated that MV4, MV11 and MV24 contained a homolog of the gene coding for the fibrinolytic enzyme nattokinase while only MV30 encoded for a glutamic acid decarboxylase essential to synthesize the neurotransmitter GABA. All four strains showed a strong antioxidant activity, formed biofilm and produced/secreted quorum sensing peptides able to induce a cytoprotective stress response in a model of human intestinal (HT29) cells. Altogether, genomic and physiological data indicate that the analyzed strains do not pose safety concerns and have in vitro probiotic potentials.

The development of efficient mucosal vaccines is strongly dependent on the use of appropriate vectors. Various biological systems or synthetic nanoparticles have been proposed to display and deliver antigens to mucosal surfaces. The Bacillus spore, a metabolically quiescent and extremely resistant cell, has also been proposed as a mucosal vaccine delivery system and shown able to conjugate the advantages of live and synthetic systems. Several antigens have been displayed on the spore by either recombinant or non-recombinant approaches, and antigen-specific immune responses have been observed in animals immunized by the oral or nasal route. Here we review the use of the bacterial spore as a mucosal vaccine vehicle focusing on the advantages and drawbacks of using the spore and of the recombinant vs. non-recombinant approach to display antigens on the spore surface. An overview of the immune responses induced by antigen-displaying spores so far tested in animals is presented and discussed.