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SUMMARY: A loss or reduction in the O side-chain material of the cell-wall lipo-polysaccharide is known to abolish or reduce the virulence of Salmonella strains. The present report concerns the effect on virulence of altering the quality of smooth-type O side-chains in a basically virulent Salmonella typhimurium line. The original rfbB locus determining the structure of 4, 12-specific repeating units was replaced either in transduction or in conjugation by the wild-type rfbD locus of group D (O antigens 9, 12), or rfbC of group C (O antigens 6, 7). The LD 50 values of the 4, 5, 12 recombinants or transductants were about 105 and like those of the 4, 5, 12 parent, whereas the LD 50 values of the 9, 12 transductants were about 106, and the LD 50 values of the 6, 7 recombinants and transductants were over 107. The reduced virulence of both 9, 12 and 6, 7 recombinants could be restored to the original level by reintroducing the rfbB locus into these strains through conjugation. It seems, therefore, that different kinds of O side-chains confer different degrees of virulence on S. typhimurium.
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