Differentiation of Bacteroides nodosus biotypes and colony variants in relation to their virulence and immunoprotective properties in sheep.

To obtain a wider definition of variation in the virulence of Bacteroides nodosus and in the protective potency of B. nodosus vaccines, we made a comparison of the in vitro characteristics of isolates from clinical infections of sheep and cattle and of certain colony variants observed previously. Three basic colony types were distinguished: papillate or beaded (B)-type colonies were produced by fresh isolates from advanced ovine foot rot; mucoid (M)-type colonies were formed by isolates from noninvasive B. nodosus infections of the interdigital skin of sheep and cattle, and also by cultures of some primary B-type colonies passaged nonselectively in vitro; and circular (C)-type colonies were formed by B. nodosus that eventually predominated in repeatedly passaged liquid subcultures. Each type could be maintained by selective agar subculturing; one strain was thus passaged 40 times as the B-type colony, at which point the organisms induced severe foot rot in experimentally infected sheep. Cultures of M-type colonies were uniformly less pathogenic and those of C-type colony organisms were avirulent. In vitro changes from prototype B-type colonies to M- and C-variants were nonreversible in these experiments, were accelerated in liquid cultures, and wee accompanied by a diminution in elastase activity of the organisms and in their immunoprotective properties against homologous challenge. Strains differed in their stability to these changes; therefore the choice of an appropriate strain and colony type may be an important consideration in studies of B. nodosus virulence and in the development of effective vaccines.