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Spirochetes have periplasmic flagella composed of a core surrounded by a sheath. The pathogenLeptospira interrogans has fourflaB (proposed core subunit) and twoflaA (proposed sheath subunit) genes. TheflaA genes are organized in a locus withflaA2 immediately upstream offlaA1 . In this study,flaA1 andflaA2 mutants were constructed by transposon mutagenesis. Both mutants still produced periplasmic flagella. TheflaA1 mutant did not produce FlaA1 but continued to produce FlaA2 and retained normal morphology and virulence in a hamster model of infection but had reduced motility. TheflaA2 mutant did not produce either the FlaA1 or the FlaA2 protein. Cells of theflaA2 mutant lacked the distinctive hook-shaped ends associated withL. interrogans and lacked translational motility in liquid and semisolid media. These observations were confirmed with a second, independentflaA2 mutant. TheflaA2 mutant failed to cause disease in animal models of acute infection. Despite lacking FlaA proteins, the flagella of theflaA2 mutant were of the same thickness as wild-type flagella, as measured by electron microscopy, and exhibited a normal flagellum sheath, indicating that FlaA proteins are not essential for the synthesis of the flagellum sheath, as observed for other spirochetes. This study shows that FlaA subunits contribute to leptospiral translational motility, cellular shape, and virulence.

FlaA Proteins in Leptospira interrogans Are Essential for Motility and Virulence but Are Not Required for Formation of the Flagellum Sheath