Salmonella typhimurium cob mutants are not hyper-virulent.

It was previously reported that Salmonella typhimurium LT2 cob mutants defective in the biosynthesis of vitamin B12 (cobalamin) are more virulent than the wild type in mice. Here we show that the strains used previously are non-isogenic and that the proposed increase in virulence of the cob mutant strain results from an uncharacterized mutation in the "wild type" which attenuates virulence, most likely by decreasing expression of the spv genes on the virulence plasmid. As a result the cob mutant will appear as hyper-virulent. Examination of the virulence of reconstructed wild-type and cob mutant strains showed that their growth rates were similar in mice, and we conclude that vitamin B12 does not affect the virulence of S. typhimurium LT2.