Bacillus subtilis as a tool for vaccine development: from antigen factories to delivery vectors

Abstract

Bacillus subtilis and some of its close relatives have a long history of industrial and biotechnological applications. Search for antigen expression systems based on recombinant B. subtilis strains sounds attractive both by the extensive genetic knowledge and the lack of an outer membrane, which simplify the secretion and purification of heterologous proteins. More recently, genetically modified B. subtilis spores have been described as indestructible delivery vehicles for vaccine antigens. Nonetheless both production and delivery of antigens by B. subtilis strains face some inherent obstacles, as unstable gene expression and reduced immunogenicity that, otherwise, can be overcome by already available gene technology approaches. In the present review we present the status of B. subtilis-based vaccine research, either as protein factories or delivery vectors, and discuss some alternatives for a better use of genetically modified strains.

Assembly of Minicellulosomes on the Surface of Bacillus subtilis

“…Sortases are widely distributed in Gram-positive bacteria and catalyze a transpeptidation reaction that joins proteins bearing a highly conserved Leu-Pro-X-Thr-Gly (LPXTG, where X is any amino acid) sorting signal to the cross-bridge peptide of lipid II, a cell wall precursor that is subsequently incorporated into the peptidoglycan. In this study we demonstrate that it is possible to use sortase enzymes to attach a minicellulosome to the surface of B. subtilis, a rod-shaped Gram-positive bacterium used in a wide range of industrial processes, including the production of antibiotics, vaccines, and pharmaceutically relevant proteins (13,22,58,60,67). Although the native organism shows minimal cellulolytic activity, B. subtilis strains displaying cellulases and a multienzyme minicellulosome degrade HCl-treated amorphous cellulose.…”

Oral Spore Vaccine Based on Live Attenuated Nontoxinogenic Bacillus anthracis Expressing Recombinant Mutant Protective Antigen

“…Various approaches or a combination of approaches can be examined to overcome the observed partial humoral response with the MASC-13 oral spore vaccine; these approaches include (i) generation of a strain expressing higher levels of PA through utilization of promoters that are much more potent than ␣-amylase (23), (ii) coadministration with mucosal adjuvants (5,25), (iii) engineering the spores with M cells targeting molecules that potentially improve the uptake of the spores by M cells (4,28), and (iv) development of strains which are able to persist longer in phagocytic cells (18).…”

Gut Adhesive Bacillus subtilis Spores as a Platform for Mucosal Delivery of Antigens

“…Bacteria in the former group of are capable of inducing strong and long-lasting immune responses to passenger antigens but present serious safety concerns, whereas bacteria in the latter group are safer but typically induce much lower immune responses to the passenger antigens (1-3). As a safe nonpathogenic live bacterial vector, Bacillus subtilis, a spore-forming soil Gram-positive bacterial species, has been engineered to express antigens as either vegetative cells or spores (4,5). As antigen carriers, B. subtilis spores have several attractive features, including a safe record of human and animal use as both probiotic and food additives, remarkable heat resistance, and rather easy genetic and bacteriological manipulation.…”