The biological significance of fimbrial phase variation in Salmonella serotypes is currently unknown. Exposure to long polar (LP) fimbriae of Salmonella enterica serotype Typhimurium results in selection against lpf phase ON cells of serotype Enteritidis during a subsequent challenge, suggesting that fimbrial phase variation may be a mechanism to evade cross-immunity between Salmonella serotypes. This notion was tested by assessing the effect of an immune response against serotype Typhimurium LP fimbriae on colonization of mice with a serotype Enteritidis mutant in which the lpf promoter region was replaced with the Escherichia coli lac promoter. During a challenge with a serotype Enteritidis mutant carrying the lac promoter in front of the Ipf operon, significantly lower numbers were recovered from organs and feces of mice previously immunized with an lpf phase ON culture of serotype Typhimurium than from mice not previously exposed to LP fimbriae. Immunization with the lpf phase ON culture of serotype Typhimurium elicited antibodies that cross-reacted with a purified gluthathione-S-transferase-LpfA fusion protein of serotype Enteritidis. These data suggested that cross-immunity against LP fimbrial proteins cannot be evaded if phase variation on the transcriptional level is prevented by expressing the lpf operon from the lac promoter. These data hence support the idea that phase variation of LP fimbriae is a mechanism to evade cross-immunity between serotypes Enteritidis and Typhimurium.
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