Critical Roles for Lipomannan and Lipoarabinomannan in Cell Wall Integrity of Mycobacteria and Pathogenesis of Tuberculosis

Lipomannan (LM) and lipoarabinomannan (LAM) are mycobacterial glycolipids containing a long mannose polymer. While they are implicated in immune modulations, the significance of LM and LAM as structural components of the mycobacterial cell wall remains unknown. We have previously reported that a branch-forming mannosyltransferase plays a critical role in controlling the sizes of LM and LAM and that deletion or overexpression of this enzyme results in gross changes in LM/LAM structures. Here, we show that such changes in LM/LAM structures have a significant impact on the cell wall integrity of mycobacteria. In Mycobacterium smegmatis, structural defects in LM and LAM resulted in loss of acid-fast staining, increased sensitivity to β-lactam antibiotics, and faster killing by THP-1 macrophages. Furthermore, equivalent Mycobacterium tuberculosis mutants became more sensitive to β-lactams, and one mutant showed attenuated virulence in mice. Our results revealed previously unknown structural roles for LM and LAM and further demonstrated that they are important for the pathogenesis of tuberculosis.