The cell envelope of Mycobacterium tuberculosis contains glycans and lipids of peculiar structure that play prominent roles in the biology and pathogenesis of tuberculosis. Consequently, the chemical structure and biosynthesis of the cell wall have been intensively investigated in order to identify novel drug targets. Here, we validate that the function of phosphatidyl-myo-inositol mannosyltransferase PimA is vital for M. tuberculosis in vitro and in vivo. PimA initiates the biosynthesis of phosphatidyl-myoinositol mannosides by transferring a mannosyl residue from GDP-Man to phosphatidyl-myo-inositol on the cytoplasmic side of the plasma membrane. To prove the essential nature of pimA in M. tuberculosis, we constructed a pimA conditional mutant by using the TetR-Pip off system and showed that downregulation of PimA expression causes bactericidality in batch cultures. Consistent with the biochemical reaction catalyzed by PimA, this phenotype was associated with markedly reduced levels of phosphatidyl-myo-inositol dimannosides, essential structural components of the mycobacterial cell envelope. In addition, the requirement of PimA for viability was clearly demonstrated during macrophage infection and in two different mouse models of infection, where a dramatic decrease in viable counts was observed upon silencing of the gene. Notably, depletion of PimA resulted in complete clearance of the mouse lungs during both the acute and chronic phases of infection. Altogether, the experimental data highlight the importance of the phosphatidyl-myo-inositol mannoside biosynthetic pathway for M. tuberculosis and confirm that PimA is a novel target for future drug discovery programs.T he lack of proper treatment for serious infectious diseases due to the emergence of multidrug resistance (MDR) underlines the need for the discovery of novel antibiotics (1). This is particularly true for tuberculosis (TB), for which 3.7% of new cases and 20% of previously treated cases are estimated to have MDR-TB. In several countries in Eastern Europe and central Asia, these numbers rise to 9 to 32% and more than 50%, respectively, making the situation particularly worrisome. In addition, in the case of TB, which claimed 1.3 million lives in 2012, the treatment of the leastcomplicated, drug-sensitive cases is lengthy and uncomfortable, and new drugs are urgently needed to shorten the regimen and make it more acceptable to the patients (2).The cell envelope of Mycobacterium tuberculosis represents a highly attractive target for the discovery of novel TB drugs. Supporting this notion, several key medicines used in the current TB therapy are inhibitors of essential enzymes participating in cell envelope biosynthesis. The first-line drug ethambutol has been shown to target at least three unique integral membrane-associated arabinofuranosyltransferases (EmbA, EmbB, and EmbC) involved in the biosynthesis of arabinogalactan (AG) and lipoarabinomannan (LAM) (3, 4). The inhibition of InhA by isoniazid (INH) in M. tuberculosis leads to the specific depl...