Loss of function of LprG-Rv1410c and MmpL11 homologues in Mycobacterium smegmatis leads to altered glycopeptidolipid profile and decreased cellular c-di-GMP levels during biofilm formation

The cell envelope of many bacteria possesses components that play central roles in bacterial pathogenesis. Unique to mycobacteria is the presence of a lipid-rich cell wall which is critical for the virulence of pathogenic mycobacteria such as Mycobacterium tuberculosis (Mtb). The biosynthesis of mycobacterial cell wall lipids has been well characterized, but the mechanisms of lipid transport still remain largely unknown. MmpL (mycobacterial membrane protein large) proteins have been implicated in the biosynthesis and/or transport of mycobacterial cell wall lipids, but due to their cellular location, it is still unclear how cell wall lipids are transported beyond the inner membrane. Here, we further investigate the role of two conserved lipid transport pathways LprG-Rv1410c and MmpL11 in cell envelope biogenesis during biofilm formation in Mycobacterium smegmatis. We found that deletion of the lprG-rv1410c operon homologues MSMEG_3070-3069 and mmpL11 (MSMEG_0241) simultaneously led to similar biofilm defects as observed in the MSMEG_3070-3069 and mmpL11 mutants. Analysis of pellicle biofilms, total lipid extracts, gene expression and cellular c-di-GMP levels revealed that the shared biofilm defect is directly correlated with significant decreases in cellular c-di-GMP levels, but may also be due to changes in the synthesis and/or localization of glycopeptidolipids (GPLs), and changes in the expression of GPL biosynthesis and transport genes. Our findings therefore suggest that while both LprG-Rv1410c and MmpL11 pathways are involved in modulating cellular c-di-GMP levels during biofilm formation, only LprG-Rv1410c are important for regulating GPL synthesis and/or surface localization, and MmpL11 may play a broad role in fine-tuning GPL levels