<title>ABSTRACT</title>
Swine erysipelas is caused by the Gram-positive pathogen <named-content content-type="genus-species">Erysipelothrix rhusiopathiae</named-content>. The swine erysipelas live vaccine in Japan, the <italic>E. rhusiopathiae</italic> Koganei 65-0.15 strain (Koganei), has been reported to cause arthritis and endocarditis. To develop a vaccine with increased safety, we used a virulent Fujisawa strain to construct transposon mutants for a total of 651 genes, which covered 38% of the coding sequence of the genome. We screened the mutants for attenuation by inoculating mice with 10⁸ CFU of each mutant and subsequently assessed protective capability by challenging the surviving mice with 10³ CFU (10² times the 50% lethal dose) of the Fujisawa strain. Of the 23 attenuated mutants obtained, 6 mutants were selected and evaluated for protective capability in pigs by comparison to that of the Koganei strain. A mutant in the ERH_0432 (<italic>tagF</italic>) gene encoding a putative CDP-glycerol glycerophosphotransferase was found to be highly attenuated and to induce humoral and cell-mediated immune responses in conventional pigs. An in-frame deletion mutant of the gene, the Δ432 mutant, was constructed, and attenuation was further confirmed in germfree piglets; three of four piglets subcutaneously inoculated with 10⁹ CFU of the Δ432 mutant showed no apparent clinical symptoms, whereas all four of the Koganei-inoculated piglets died 3 days after inoculation. It was confirmed that conventional pigs inoculated orally or subcutaneously with the Δ432 strain were almost completely protected against lethal challenge infection. Thus, the <italic>tagF</italic> homolog mutant of <italic>E. rhusiopathiae</italic> represents a safe vaccine candidate that can be administered via the oral and subcutaneous routes.