COMPARABLE GROWTH OF VIRULENT AND AVIRULENT MYCOBACTERIUM-TUBERCULOSIS IN HUMAN MACROPHAGES IN-VITRO

The relative virulence and avirulence of Mycobacterium tuberculosis st rains H37Rv and H37Ra were previously defined using animal infection m odels. To investigate host species' specificity of mycobacterial virul ence, growth of the 2 M. tuberculosis strains in human monocyte-derive d macrophages in vitro was studied. Mycobacterial growth was evaluated by acid-fast staining, electron microscopy, and colony-forming units (cfu) assay. As expected, the 2 strains demonstrated significantly dif ferent growth rates in mouse macrophages in vitro (53 h for H37Rv, 370 h for H37Ra). In marked contrast, in human macrophages the average di vision times of the strains were nearly equal (80 h for H37Rv and 76 h for H37Ra by cfu measurement, and 96 h for H37Rv and 104 h for H37Ra by acid-fast staining). These findings indicate that observations of m ycobacterial virulence in murine systems may not necessarily translate to the human system, in which different mechanisms to control mycobac terial growth may be expressed.