The generation of smooth aromatic-dependent serotype Bovismorbificans (Group C2, O6, 8) from a smooth wild-type parent strain by transduction with phage P1, and conjugation with serotype Typhimurium carrying F'-8 is described. The smooth aromatic-dependent serotype Bovismorbificans was non-lethal for mice at an oral challenge dose of 2 x 10 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 with that of its virulent parent organisms. Mice immunised with live, smooth aromatic-dependent serotype Bovismorbificans by either the oral or intraperitoneal (i.p.) route were protected against oral challenge with virulent serotype Bovismorbificans; the degree of protection was significantly better (p<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 serotype Bovismorbificans i.p. developed significant (p<0.01–0.05) delayed-type hypersensitivity.


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