Hosts push tuberculosis bacteria into different metabolic states

Mycobacterium tuberculosis (Mtb) survives inside immune cells called macrophages, but scientists have not fully understood how the type of host cell shapes the bacterium’s internal state. In work led by corresponding author Bryan D. Bryson, researchers directly compared the intracellular transcriptome of Mtb when it infects human versus mouse macrophages to see how each host species alters the pathogen’s biology. The team found that the two host species create distinct microenvironments that push Mtb into separable metabolic programs. Across these differences, lipid metabolism and regulatory circuits were especially remodeled. In mouse macrophages, Mtb responses skewed toward iron- and oxidative-stress pathways, while in human macrophages the bacteria adopted programs for fatty acid import. By mapping these broad transcriptional changes, the study set out to link what macrophages supply or withhold to how Mtb adjusts its metabolism, aiming to explain why the bacterium behaves differently depending on the species of its host cell.

To pinpoint functional consequences of the transcriptional differences, the authors used fluorescent fatty acid tracing and direct intracellular measurements. Those experiments revealed a striking species-specific phenotype: Mtb forms intracellular lipid inclusions (ILIs) inside murine macrophages but does not form ILIs inside human macrophages. This species-specific difference was robust: it was independent of culture media, macrophage ontogeny, or host antimicrobial factors such as nitric oxide and itaconate. The team found that access of Mtb to host-derived lipids required the ESX-1 secretion system and was inversely correlated with host triacylglycerol (TAG) synthesis. Further, pharmacological inhibition of TAG formation in human macrophages partially restored Mtb ILI formation, pointing to a controllable metabolic gate that governs lipid flow between host lipid droplets and intracellular Mtb.

These findings provide a cross-species framework for decoding how host cells drive distinct bacterial metabolic states. By identifying a clear barrier in human macrophages that limits Mtb’s access to host lipid stores, the work highlights lipid handling—both bacterial factors like ESX-1 and host processes like TAG synthesis—as central to the intracellular relationship. The species-specific formation of ILIs helps explain why Mtb may look and behave differently in mouse models compared with human cells, and it suggests experimental levers researchers can use to test how lipid access affects persistence and drug susceptibility. Overall, the study points to host lipid metabolism as a key determinant of Mtb physiology and a potential focus for future efforts to better model and ultimately intervene in tuberculosis infection.