Department of Microbiology, University of Massachusetts, Amherst, Massachusetts, USA.
Department of Chemistry and Biochemistry, Central Michigan University, Mount Pleasant, Michigan, USA.
mBio. 2021 Jan 19;12(1):e02801-20. doi: 10.1128/mBio.02801-20.
The mycomembrane layer of the mycobacterial cell envelope is a barrier to environmental, immune, and antibiotic insults. There is considerable evidence of mycomembrane plasticity during infection and in response to host-mimicking stresses. Since mycobacteria are resource and energy limited under these conditions, it is likely that remodeling has distinct requirements from those of the well-characterized biosynthetic program that operates during unrestricted growth. Unexpectedly, we found that mycomembrane remodeling in nutrient-starved, nonreplicating mycobacteria includes synthesis in addition to turnover. Mycomembrane synthesis under these conditions occurs along the cell periphery, in contrast to the polar assembly of actively growing cells, and both liberates and relies on the nonmammalian disaccharide trehalose. In the absence of trehalose recycling, trehalose synthesis fuels mycomembrane remodeling. However, mycobacteria experience ATP depletion, enhanced respiration, and redox stress, hallmarks of futile cycling and the collateral dysfunction elicited by some bactericidal antibiotics. Inefficient energy metabolism compromises the survival of trehalose recycling mutants in macrophages. Our data suggest that trehalose recycling alleviates the energetic burden of mycomembrane remodeling under stress. Cell envelope recycling pathways are emerging targets for sensitizing resource-limited bacterial pathogens to host and antibiotic pressure. The glucose-based disaccharide trehalose is a stress protectant and carbon source in many nonmammalian cells. Mycobacteria are relatively unique in that they use trehalose for an additional, extracytoplasmic purpose: to build their outer "myco" membrane. In these organisms, trehalose connects mycomembrane biosynthesis and turnover to central carbon metabolism. Key to this connection is the retrograde transporter LpqY-SugABC. Unexpectedly, we found that nongrowing mycobacteria synthesize mycomembrane under carbon limitation but do not require LpqY-SugABC. In the absence of trehalose recycling, compensatory anabolism allows mycomembrane biosynthesis to continue. However, this workaround comes at a cost, namely, ATP consumption, increased respiration, and oxidative stress. Strikingly, these phenotypes resemble those elicited by futile cycles and some bactericidal antibiotics. We demonstrate that inefficient energy metabolism attenuates trehalose recycling mutant in macrophages. Energy-expensive macromolecule biosynthesis triggered in the absence of recycling may be a new paradigm for boosting host activity against bacterial pathogens.
分枝杆菌细胞包膜的细胞膜层是抵御环境、免疫和抗生素侵害的屏障。有大量证据表明，在感染过程中和应对宿主模拟压力时，细胞膜层具有相当大的可塑性。由于分枝杆菌在这些条件下资源和能量有限，因此重塑过程的要求可能与不受限制生长期间运行的特征明显不同。出乎意料的是，我们发现，在营养饥饿、非复制分枝杆菌中，细胞膜层重塑除了周转外还包括合成。在这些条件下，细胞膜层合成发生在细胞周边，与活跃生长细胞的极性组装相反，并且都释放和依赖于非哺乳动物二糖海藻糖。在没有海藻糖回收的情况下，海藻糖合成为细胞膜层重塑提供燃料。然而，分枝杆菌会经历 ATP 耗尽、增强的呼吸和氧化还原应激，这些都是无效循环的标志，也是一些杀菌抗生素引起的旁系功能障碍的标志。低效的能量代谢会损害海藻糖回收突变体在巨噬细胞中的生存能力。我们的数据表明，海藻糖回收可缓解应激下细胞膜层重塑的能量负担。细胞包膜回收途径正在成为使资源有限的细菌病原体对宿主和抗生素压力敏感的新兴目标。基于葡萄糖的二糖海藻糖是许多非哺乳动物细胞中的应激保护剂和碳源。分枝杆菌相对独特之处在于，它们将海藻糖用于另一个细胞外目的：构建它们的外“菌”膜。在这些生物中，海藻糖将细胞膜层生物合成和周转与中心碳代谢联系起来。这一联系的关键是逆行转运蛋白 LpqY-SugABC。出乎意料的是，我们发现非生长分枝杆菌在碳限制下合成细胞膜层，但不需要 LpqY-SugABC。在没有海藻糖回收的情况下，补偿性合成代谢允许细胞膜层生物合成继续进行。然而，这种解决方法是有代价的，即 ATP 消耗增加、呼吸作用增强和氧化应激。引人注目的是，这些表型类似于无效循环和一些杀菌抗生素引起的表型。我们证明，低效的能量代谢会削弱巨噬细胞中海藻糖回收突变体。在没有回收的情况下触发的能量密集型大分子生物合成可能是提高宿主对细菌病原体活性的新范例。
