Trehalose is uniquely needed to build the mycobacterial membrane

Mycobacteria, the group that includes the tuberculosis bacterium, are protected by an unusually impermeable outer membrane. That membrane is built largely from mycolic acids (MAs), long fatty molecules that make the cell surface unusually tough and resistant to many antibiotics. Scientists have long thought that MAs are moved across the bacterial inner membrane attached to a sugar carrier, most commonly believed to be trehalose monomycolates (TMMs). But two basic puzzles remained unanswered: is trehalose the only sugar carrier, and why is trehalose itself essential for growth? To answer those questions, Shu‐Sin Chng and colleagues used a trehalose auxotrophic Mycobacterium smegmatis strain — a bacterium genetically engineered so it cannot make trehalose on its own — to watch what happens to MA production when trehalose is missing. By studying the steps that build and move mycolic acids within the cell, they were able to test whether other carriers could substitute for trehalose and to probe the biochemical consequences of trehalose depletion.

Using the trehalose auxotrophic Mycobacterium smegmatis system, the researchers examined the biochemical steps that create TMMs and other forms of mycolates. They found that in the absence of trehalose, not only are TMMs absent but mycolic acids generally are not made. The team traced this failure to a product inhibition mechanism: un-reduced MA precursors accumulate on Pks13, the enzyme that ligates mycolic acids to the sugar head group. Crucially, these un-reduced mycolates could only be released from Pks13 by trehalose, revealing exquisite Pks13 specificity for trehalose. Once released, the mycolates could be reduced by CmrA in vitro. The authors also showed that only trehalose and its analogs can reactivate MA biosynthesis in cells. Finally, by replacing trehalose with a 6-deoxy analog in cells, they demonstrated that the cord factor trehalose dimycolate is dispensable for M. smegmatis growth in vitro, separating the roles of trehalose in MA biosynthesis from its role in forming surface glycolipids.

These findings clarify the order of critical steps in mycolic acid biosynthesis and explain why trehalose is indispensable to mycobacterial growth. The work shows that Pks13 will not release MA precursors unless trehalose is available, so trehalose is not merely a convenient carrier but an essential unlocking key for MA production. That exquisite specificity helps explain why mycobacteria must maintain trehalose and why loss of trehalose production halts membrane assembly. By demonstrating that trehalose dimycolate is not required for growth in M. smegmatis under laboratory conditions, the study separates the structural requirement for trehalose in MA assembly from other trehalose-dependent surface functions. Altogether, the study by Shu‐Sin Chng and colleagues paints a clearer picture of outer membrane biogenesis in mycobacteria and points to biochemical steps — the interaction between trehalose and Pks13 and the subsequent reduction by CmrA — that could be explored in future efforts to weaken these stubborn pathogens.