Studies on the purification of the leucocidin of Fusobacterium necrophorum and its neutralization by specific antisera.

Leucocidin from several strains of Fusobacterium necrophorum was partially purified by gel filtration on Fractogel HW55 (F), the majority of the activity being present in the 50 ml of filtrate collected after 1.1 void volumes had passed through the column (termed Fraction 1, or #1). The material also contained lipopolysaccharide in 12.5% SDS-PAGE gels run under reducing conditions, but the protein did not migrate into 7.5% PAGE gels run under non-reducing conditions. Rabbit and bovine antisera to the leucocidin possessed antibodies against antigens in concentrated, washed culture supernates from toxigenic F. necrophorum and neutralized the leucocidal activity of such supernates. Absorption of the antisera with homologous, washed F. necrophorum cells reduced ELISA antibody titres by greater than 50%, but decreased neutralization titres by 15%. Absorbed rabbit IgG anti-#1 precipitated a single rocket in crossed immunoelectrophoresis and identified two proteins, of molecular weights (M.W.) 14 000 and 13 000, and 1 protein of M.W. 13 500 in immunoblots from toxigenic and non-toxigenic strains, respectively. An additional protein of M.E. 103 000 was present after SDS-PAGE separation of supernates from toxigenic but not non-toxigenic F. necrophorum and was not present in whole cell components. It was considered that the leucocidin may be present in a dimeric form in culture supernates from toxigenic strains. Antisera to leucocidins from several strains of F. necrophorum exhibited variable neutralization titres against leucocidins from heterologous bacteria.