Aromatic-dependent "Salmonella sp." as live vaccine in mice and calves

A block in the common aromatic biosynthesis (aro) pathway makes E. coli, Salmonella, etc., exacting for aromatic metabolites; two of these, paraaminobenzoic acid and dihydroxybenzoic acid, are not mammalian metabolites. Bacteria needing substances not available in host tissues should be unable to grow there and so be non-virulent. Transposon-generated deletion and deletion-inversion mutations at aroA caused loss of mouse virulence in Salmonella typhimurium; e.g., LD50, intraperitoneal route, increased from less than 10 to greater than 10(6), and LD50 by feeding increased from ca. 10(5) to greater than 10(8). Furthermore, a single injection of 10(5) live aro- bacteria protected mice of a Salmonella-susceptible line, BALB/c, against challenge, 4 weeks later, with 5 X 10(5) virulent S. typhimurium (i.e., greater than 10(4) LD50); a single dose of the live-vaccine strain given i.p. also protected against challenge one month later by feeding 2 X 10(7) of a virulent strain. Mice vaccinated by feeding 2 X 10(8) live aro- bacteria were unaffected by feeding, one month later, 2 X 10(7) of a virulent S. typhimurium strain ( = ca 100 LD50). Intravenous injection of microparticulate silica did not reduce the mouse i.p. LD50 of an aro- strain, and 3 i.p. injections of cyclophosphamide, 4 mgm, caused only a small reduction. Thus the reduced virulence of the aro- strain does much depend on the integrity of host cellular defense mechanisms. Non-reverting aro- derivatives made from proven calf-virulent S. typhimurium and S. dublin strains are being tested at the School of Veterinary Medicine, University of California at Davis. None of 26 calves given aro- live-vaccine bacteria intramuscularly (usually 10(9)) and none of 8 calves fed ca. 10(11) live aro- bacteria, died or became seriously ill. Vaccinated and control calves were challenged by feeding 10(11) bacteria of a virulent strain, S. typhimurium or S. dublin. This challenge caused death of 14 of 16 non-vaccinated calves. The results, to date, in vaccinated animals indicate that one aro- strain of each of the two Salmonella species is effective if given as a live vaccine in two intramuscular doses.