LEUCINE AUXOTROPHY RESTRICTS GROWTH OF MYCOBACTERIUM-BOVIS BCG IN MACROPHAGES

The ability of slow-growing mycobacteria to replicate within host mono nuclear phagocytes is thought to be central to the pathogenesis of myc obacterial infection. However, because of the lack of a mycobacterial mutant defective for intracellular replication, it has not been possib le to test this hypothesis directly, Previously, we showed that a BCG leucine auxotroph with a transposon disruption of the leuD gene is una ble to grow in mice, Here we demonstrate that this mutant is also inca pable of replicating within cultured macrophages in vitro. Complementa tion of the leuD mutation with the leuCD genes of Escherichia coli res tored wild-type levels of growth in macrophages, establishing that the defect for intracellular replication was due to leucine auxotrophy pe r se and not to a polar effect of the transposon insertion on an adjac ent gene, These results suggest that the inability of the leucine auxo troph to grow in mice was due to its sequestration, after phagocytosis , in an intracellular compartment from which it could not obtain leuci ne.