Although the bovine tuberculosis (TB) agent, Mycobacterium bovis, may infect humans and cause disease, long-term epidemiological data indicate that humans represent a spill-over host in which infection with M. bovis is not self-maintaining. Indeed, human-tohuman transmission of M. bovis strains and other members of the animal lineage of the tubercle bacilli is very rare. Here, we report on three mutations affecting the two-component virulence regulation system PhoP/PhoR (PhoPR) in M. bovis and in the closely linked Mycobacterium africanum lineage 6 (L6) that likely account for this discrepancy. Genetic transfer of these mutations into the human TB agent, Mycobacterium tuberculosis, resulted in downregulation of the PhoP regulon, with loss of biologically active lipids, reduced secretion of the 6-kDa early antigenic target (ESAT-6), and lower virulence. Remarkably, the deleterious effects of the phoPR mutations were partly compensated by a deletion, specific to the animal-adapted and M. africanum L6 lineages, that restores ESAT-6 secretion by a PhoPR-independent mechanism. Similarly, we also observed that insertion of an IS6110 element upstream of the phoPR locus may completely revert the phoPR-bovis-associated fitness loss, which is the case for an exceptional M. bovis human outbreak strain from Spain. Our findings ultimately explain the long-term epidemiological data, suggesting that M. bovis and related phoPR-mutated strains pose a lower risk for progression to overt human TB, with major impact on the evolutionary history of TB.evolution | phylogeny | adaptation | zoonosis T uberculosis (TB) is caused by bacilli from the genetically compact Mycobacterium tuberculosis complex (MTBC), which gathers eight defined phylogenetic lineages in addition to the more distantly related Mycobacterium canettii group (1-3): M. tuberculosis sensu-stricto from lineages L1-L4 and L7 form a large group of human-adapted strains responsible for the vast majority of global human TB cases, whereas Mycobacterium africanum lineages (L5, L6), which are restricted to humans from West Africa, are phylogenetically linked with the eighth lineage, comprising the various animal-adapted strains, with Mycobacterium bovis as the most downstream member in the phylogeny (Fig. 1A) (4, 5). Animal strains exhibit a wide host range that includes livestock animals in close contact with humans. Episodes of bovine TB in cattle herds have been reported in 128 of 155 countries during the period 2005-2008 (6). Although the bulk of these episodes is mainly found in developing countries (6), bovine TB remains a major problem even in some industrialized countries, best exemplified by the United Kingdom, which has experienced an important resurgence of bovine TB since the 1980s (7). Because M. bovis and other closely related animal-adapted strains are also capable of causing TB in humans, this situation raises concerns regarding the zoonotic risk. Indeed, human TB cases resulting from M. bovis are estimated to be around 2% worldwide (8), with higher incidence (...

SummarySeveral specific lipids of the cell envelope are implicated in the pathogenesis of M. tuberculosis (Mtb), including phthiocerol dimycocerosates (DIM) that have clearly been identified as virulence factors. Others, such as trehalose-derived lipids, sulfolipids (SL), diacyltrehaloses (DAT) and polyacyltrehaloses (PAT), are believed to be essential for Mtb virulence, but the details of their role remain unclear. We therefore investigated the respective contribution of DIM, DAT/PAT and SL to tuberculosis by studying a collection of mutants, each with impaired production of one or several lipids. We confirmed that among those with a single lipid deficiency, only strains lacking DIM were affected in their replication in lungs and spleen of mice in comparison to the WT Mtb strain. We found also that the additional loss of DAT/PAT, and to a lesser extent of SL, increased the attenuated phenotype of the DIM-less mutant. Importantly, the loss of DAT/PAT and SL in a DIM-less background also affected Mtb growth in human monocyte-derived macrophages (hMDMs). Fluorescence microscopy revealed that mutants lacking DIM or DAT/PAT were localized in an acid compartment and that bafilomycin A1, an inhibitor of phagosome acidification, rescued the growth defect of these mutants. These findings provide evidence for DIM being dominant virulence factors that mask the functions of lipids of other families, notably DAT/PAT and to a lesser extent of SL, which we showed for the first time to contribute to Mtb virulence.