Bruce A.D. Stocker

Bruce A.D. Stocker, Professor Emeritus Active, Microbiology and Immunology, Beckman Center, Stanford University School of Medicine, Stanford, CA 94305-5124 Tel: (650) 723-2006, Fax: (650) 725-6757

We construct nutritionally-exacting derivatives of pathogenic bacterial strains for use as live vaccines. Our method is to block a biosynthetic pathway, such that the bacteria cannot grow unless provided with a metabolite not available in mammalian tissues, specifically p-aminobenzoate (pAB). Blocking aromatic biosynthesis by mutation of gene aroA causes virtually complete loss of virulence, so that the injection of even live 106 aroA into the mouse causes no ill-effects whereas the LD50 of the virulent Salmonella typhimurium parent strain is less than 20 bacteria. Strains of S. typhimurium and S. dublin with deletions at aroA have proven to be safe and effective live vaccines in mice, calves, and sheep and strains of S. typhi with aro deletions tested in volunteers are promising candidate typhoid vaccines. A strain of Shigella flexneri (a common cause of dysentery) with a deletion in gene aroD fed, as live vaccine, to monkeys protected them against later challenge with a virulent strain and when fed to volunteers did not cause disease in any of them. We are also concerned with the genetic determination of two Salmonella surface components, the O antigen (LPS) and the flagella. Live-vaccine strains of Salmonella can "present" antigens of other pathogens, so that the vaccine induces a protective response to the passenger antigen. Mice fed an aroA strain of S. dublin carrying a cloned gene for part of the M .surface protein of Streptococcus pyogenes type 5 survived a challenge dose of streptococci which killed nonvaccinated mice. We have also inserted synthetic oligonucleotides specifying antigenic determinants of foreign proteins into a cloned Salmonella flagellin gene, so that the foreign epitope forms part of the surface of the flagella. Live-vaccine Salmonella with flagella composed of flagellin (flagellar protein) made to include a short amino acid sequence of subunit B of cholera toxin when given to mice caused production of antibody to cholera toxin. This method makes it possible to construct a vaccine to protect against infections (bacterial, viral, parasitic or other) for which no method of immunization is at present available.

Johnson, B.N., Weintraub, A., Lindberg, A.A., and Stocker, B.A.D. (1992) Structural and immunochemical studies of the lipopolysaccharides of Salmonella strains with both antigen 04 and antigen 09. J. Bacteriol. 174:1916-22.
Karnell, A., Stocker, B.A., Katakura, S., Reinhold, F.P., Lindberg, A.A. (1992) Live oral auxotrophic Shigella flexneri SF124 vaccine with a deleted aroD gene: characterization and monkey protection studies. Vaccine 10:389-94.
Newton, S.M.C., Kotb, M., Poirier, T.P., Stocker, B.A.D., and Beachey, E.H. (1991) Expression and immunogenicity of a streptococcal M protein inserted in Salmonella flagellin. Infect. Immun. 59:2158-65.
Stocker, B.A.D. (1990) Aromatic-dependent Salmonella as live vaccine presenters of foreign epitopes as inserts in flagellin. Res. Microbiol. 141:787-96.
Newton, S.M.C., Jacob, C.O., and Stocker, B.A.D. (1989) Immune response to cholera toxin epitope inserted in Salmonella flagellin. Science 244:70-72.
Levine, M.M., Herrington, D., Murphy, J.R., Morris, G.J., Losonsky, G., Tall, B., Lindberg, A.A., Svenson, S., Bagar, S., Edwards, M.F., and Stocker, B.A.D. (1987) Safety, infectivity, immunogenicity, and in vivo stability of two attenuated auxotrophic mutant strains of Salmonella typhi, 541Ty and 543Ty, as live oral vaccines in humans. J. Clin. Invest. 79:888-902.

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