Changes in somatic (O) lipopolysaccharide (LPS) antigenic specificities of serostrains were observed after continuous laboratory subculture. Two serostrains ( O34 and O48) lost O specificity and did not react with homologous or any of the available heterologous antisera. The serostrain for serogroup O5, after subculture, yielded a variant that had acquired a new specificity which was detectable with a heterologous antiserum. In a repeat experiment with the original isolate of the O5 strain, a second variant was obtained which had not only acquired the same new determinant but had, unlike the first variant, lost reactivity with the homologous antiserum. Immunoblot experiments with homologous and heterologous antisera indicated that changes in antigenic specificity were associated with the O side chains of the LPS molecules. Results of restriction endonuclease analysis of chromosomal DNA of the variants and their parents revealed minor differences in restriction patterns which suggested that is capable of undergoing genomic re-arrangements that lead to changes in LPS specificity and structure.


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