The lipid-rich cell wall is a defining feature of Mycobacterium species. Individual cell wall components affect diverse mycobacterial phenotypes including colony morphology, biofilm formation, antibiotic resistance, and virulence. In this study, we describe a transposon insertion mutant of Mycobacterium smegmatis mc 2 155 that exhibits altered colony morphology and defects in biofilm formation. The mutation was localized to the lsr2 gene. First identified as an immunodominant T-cell antigen of Mycobacterium leprae, lsr2 orthologs have been identified in all sequenced mycobacterial genomes, and homologs are found in many actinomycetes. Although its precise function remains unknown, localization experiments indicate that Lsr2 is a cytosolic protein, and cross-linking experiments demonstrate that it exists as a dimer. Characterization of cell wall lipid components reveals that the M. smegmatis lsr2 mutant lacks two previously unidentified apolar lipids. Characterization by mass spectrometry and thin-layer chromatography indicate that these two apolar lipids are novel mycolatecontaining compounds, called mycolyl-diacylglycerols (MDAGs), in which a mycolic acid (␣-or ␣-mycolate) molecule is esterified to a glycerol. Upon complementation with an intact lsr2 gene, the mutant reverts to the parental phenotypes and MDAG production is restored. This study demonstrates that due to its impact on the biosynthesis of the hydrophobic MDAGs, Lsr2 plays an important role in the colony morphology and biofilm formation of M. smegmatis.The cell wall is a defining feature of mycobacteria. This complex, lipid-rich, hydrophobic structure is responsible for the acid-fast staining properties, distinctive colony morphology, and innate antibiotic resistance of Mycobacterium species (12,25,28). Among pathogens, including Mycobacterium tuberculosis, the causative agent of tuberculosis, cell wall components contribute to virulence, persistence within macrophages, and modulation of the host immune response (16, 45).The cell wall forms an asymmetric lipid bilayer (25,28). The inner leaflet is composed of mycolic acids that are covalently bound to arabinogalactan, which is further linked to peptidoglycan via a phosphodiester bridge (12). The outer leaflet contains a variety of lipid components (26,28). In total, lipids comprise 60% (wt/wt) of the cell wall (12, 25). In addition to mycolic acids, various types of complex lipids are present in the cell wall. These include lipoglycans (e.g., lipoarabinomannan [LAM]), trehalose-containing glycolipids, phthiocerol dimycocerosates, phenolic glycolipids, and glycopeptidolipids (GPLs) (12, 25). The distribution of these lipids varies among mycobacterial species (12). Triacylglycerols (TAGs) are also present in the mycobacterial cell wall (35) and are thought to fill the gap between the meromycolate arm and the shorter ␣-chain of mycolic acids (28). Different lipids appear to have different roles. For example, LAM from M. tuberculosis, but not the structurally distinct LAM of nonpathogenic mycobacteria,...

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This study describes the isolation and characterization of a unique class of TolC mutants that, under steady-state growth conditions, secreted normal levels of largely inactive alpha-hemolysin. Unlike the reduced activity in the culture supernatants, the cell-associated hemolytic activity in these mutants was identical to that in the parental strain, thus reflecting a normal intracellular toxin activation event. Treatment of the secreted toxin with guanidine hydrochloride significantly restored cytolytic activity, suggesting that the diminished activity may have been due to the aggregation or misfolding of the toxin molecules. Consistent with this notion, sedimentation and filtration analyses showed that alpha-hemolysin secreted from the mutant strain has a mass greater than that secreted from the parental strain. Experiments designed to monitor the time course of alpha-hemolysin release showed delayed appearance of toxin in the culture supernatant of the mutant strain, thus indicating a possible defect in alpha-hemolysin translocation or release. Eight different TolC substitutions displaying this toxin secretion defect were scattered throughout the protein, of which six localized in the periplasmically exposed ␣-helical domain, while the remaining two mapped within the outer membraneembedded ␤-barrel domain of TolC. A plausible model for the secretion of inactive alpha-hemolysin in these TolC mutants is discussed in the context of the recently determined three-dimensional structure of TolC.