The history of live bacterial vaccines.

Recent developments have made it possible to construct non-reverting live bacterial vaccine candidates with defined deletions of two or more genes. Such vaccines have proven safe and immunogenic in human volunteers. Since the virulent parent strains are only pathogenic to man (S. typhi, S. flexneri, and V. cholerae), they pose no threat to the environment. Besides holding promise as efficacious vaccines for protection against typhoid fever, bacillary dysentery and cholera, the attenuated strains are well suited as vectors for delivery of heterologous antigenic epitopes from micro-organisms such as Helicobacter pylori, Neisseira gonorrhoeae, rotavirus, HIV and many others. Instead of using a virulent parent bacterium as the starting organism for making a vector, attempts have recently been made to employ non-pathogenic bacteria of the normal human flora, such as Streptococcus gordonii for delivery of foreign antigens. At present, the feasibility of this approach for human beings remains to be proven.