The association of plasmids with virulence characters and O-antigen expression was studied in two virulent and seven avirulent mutant strains of type 1. Deletion of a 12-Mda segment from a 140-Mda plasmid in two smooth avirulent mutants made the derivatives avirulent in the Sereny test and noninvasive in HeLa cells. The mutants were unable to bind Congo red, and did not express the virulence marker antigen. Mutants completely lacking the 140-Mda plasmid also showed similar avirulent characters. However, rough mutants retained the ability to bind Congo red. Our results indicate that the essential gene(s) for virulence may be located on the 140-Mda plasmid, a small deletion from which may lead to avirulence. This deletion did not affect the protein antigen expression nor change their antigenicity. Analysis of lipopolysaccharide (LPS) patterns showed that strains containing the 6-Mda plasmid produced the complete LPS and were smooth, whereas strains containing either the 140- or a 4- or 2-Mda plasmid, in the absence of the 6-Mda plasmid, produced smaller amounts of O antigen and were rough. Western-blot analysis and crossed immuno-electrophoresis gave similar results. The 140-, 4-, or 2-Mda plasmid, in the absence of the 6-Mda plasmid, may code for non-specific galactosyl transferase-like activity which can add, non-specifically and at a reduced level, the galactose residue (the first sugar in the O antigen repeat unit) to the LPS core. This permits the completion of the O-antigen side chain in the absence of the gene (present in the 6-Mda plasmid) which encodes the specific galactosyl transferase involved in O-antigen biosynthesis. Thus, our findings indicate that the 6-Mda plasmid of type 1 has an important role in the synthesis of the O-antigen side chains of LPS. However, the 140-, 4- and 2-Mda plasmids may also be indirectly responsible for these properties, but this may be less pronounced in the absence of the 6-Mda plasmid.


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