Recombinant Gram-positive bacteria as vehicles of vaccine antigens

Gram-positive bacteria which are being developed as vaccine delivery systems include Lactococcus lactis, Lactobacillus spp., Streptococcus gordonii, nonpathogenic species of Staphylococcus, and Listeria. The use of these microorganisms relies on the fact that they are safe vehicles that can efficiently target vaccine antigens to the immune system. Two main approaches have been followed, according to the capacity of the bacterial vehicle to colonise the mucosal surfaces. Colonisers such as S. gordonii and some Lactobacilli can stimulate immune system by being transiently implanted among the normal flora of the host, while other lactobacilli, lactococci and staphylococci persist very shortly in the host and therefore their ability to generate an immune response must rely on high-level expression of the recombinant antigen. The immunogenicity of recombinant antigens by mucosal delivery has been demonstrated using colonising (S. gordonii) and noncolonising (Lc. lactis) bacterial vehicles. There is also great potential in the cellular responses induced by recombinant Listeria.