The induction of cording in an avirulent variant of Mycobacterium tuberculosis.

This variant, designated H37Ra, is morphologically distinguishable from H37Rv. The celLs of H37Rv grow parallel to each other, leading to cord-like formations and flat colonies. The cells of H37Ra, however, are randomly clumped, yielding colonies which are higher and denser than those of H37Rv. No "cord factor" can be extracted from cultures of H37Ra (Bloch, J. Exptl. Med., 91, 197-218, 1950). Young cultures of H37Rv produce more "cord factor" than older cultures of this organism, as asayed on mice (Noll and Bloch, Am. Rev. Tuberc., 67, 16-40, 1953; Bloch, J. Exptl. Med., 92, 507-526, 1950). Chick embryo extracts contain a heat labile, nondialyzable component which stimulates growth of both H37Rv and H37Ra. Furthermore, this extract enhances the cording of H37Rv and causes H37Ra to grow in cords (Bloch, J. Exptl. Med., 88, 355-360, 1948). Detailed investigations in this laboratory have shown that cells of H37Ra grown on solid medium are arranged in cords during the early stages of growth. The cells progressively lose their parallel OBSERVED GROWTH B