Genetic manipulation of Salmonella serotype Bovismorbificans to aromatic-dependence and evaluation of its vaccine potential in mice

The generation of smooth aromatic-dependent Salmonella serotype Bovismorbificans (Group C2, O6, 8) from a smooth wild-type parent strain by transduction with phage P1, and conjugation with Salmonella serotype Typhimurium carrying F’-8gal is described. The smooth aromatic-dependent S. serotype Bovismorbificans was non-lethal for mice at an oral challenge dose of 2 x 10(9) cfu (equivalent to 200 LD50 of the parent, wild-type strain). The safety of the auxotrophic mutant was further substantiated by comparing its multiplication kinetics in vivo with that of its virulent parent organisms. Mice immunised with live, smooth aromatic-dependent S. serotype Bovismorbificans by either the oral or intraperitoneal (i.p.) route were protected against oral challenge with virulent S. serotype Bovismorbificans; the degree of protection was significantly better (p less than 0.05) at a challenge dose of 100 or 200 LD50 in mice receiving two rather than one vaccination. In contrast, mice immunised with three doses of the formalin-killed virulent, parent organisms by the i.p. route were not protected, in spite of high antibody titres. Only those mice immunised with the live, smooth aromatic-dependent S. serotype Bovismorbificans i.p. developed significant (p less than 0.01-0.05) delayed-type hypersensitivity.