Growth of recombinant Mycobacterium tuberculosis H37Ra in mouse macrophages
Mycobacterium tuberculosis H37Rv and H37Ra were derived from the same parental strain but differ strikingly in their virulence for experimental animals. Transfer of genetic material between these closely related strains resulted in the isolation of a number of recombinant H37Ra clones bearing the in vivo growth‐promoting ivg locus of H37Rv. The recombinant strain was phagocytosed by murine peritoneal macrophages infected in vivo or in vitro and their intracellular growth rates were compared with the vector control. The intracellular growth of the recombinant was significantly faster than the vector control, but substantially slower than the wild‐type H37Rv control, regardless of the method used to infect the macrophages. The slower intracellular growth observed for the recombinant strains was not due to a genetically induced metabolic defect, since they grew in synthetic liquid medium at rates equal to those observed for both H37Rv and H37Ra. Peritoneal macrophage monolayers provide a rapid and convenient assay by which to screen H37Ra recombinants for the presence of putative virulence genes.