Copper sensing helps tuberculosis adapt and persist inside cells

Mycobacterium tuberculosis must survive in a hostile environment inside host cells, but researchers are still piecing together which host cues the bacterium senses and how it adapts. Olivier Neyrolles and his team set out to define how M. tuberculosis responds to physiologically relevant copper levels encountered during macrophage infection. Copper is increasingly recognized as a host-derived cue during infections, but the magnitude, location inside cells, and consequences for bacterial physiology were incompletely understood. To fill those gaps, the investigators combined a suite of complementary approaches to follow copper at high resolution and to measure bacterial responses. They used high-resolution imaging to locate copper within infected cells, NanoSIMS to map elemental distributions inside individual bacilli, transcriptomic profiling to see which genes are turned on or off, intracellular reporter assays to monitor gene expression inside host cells, isotope tracing to follow metabolic changes, and mouse infection models to test the importance of the findings in vivo. By working across these methods, the study aimed to connect the physical presence of copper in bacteria with specific transcriptional and metabolic adaptations that help the pathogen persist in the host.

The team found that copper does reach intracellular bacilli and accumulates in discrete phosphorus-rich foci, as revealed by NanoSIMS analysis. Exposing bacteria to physiological copper concentrations in vitro triggered a focused transcriptional response dominated by the copper-inducible CsoR and RicR regulons. One of the most strongly induced loci was the RicR-regulated gene cysK2, which encodes the S -sulfocysteine synthase CysK2. In infected macrophages, expression of cysK2 was modulated by extracellular copper availability, by host copper transport pathways, and by hypoxia. The researchers also tested a genetic mutant: an H37Rv cysK2 -deficient mutant showed reduced amino acid biosynthesis in response to copper exposure in vitro. When the mutant was examined in vivo using mouse infection models, it was impaired in long-term persistence in mice and displayed a higher oxidation status. Together, these methods and results identify CysK2 as a copper-responsive metabolic effector linked to sulfur metabolism, transcriptional regulation by CsoR and RicR, and measurable fitness defects when absent.

These findings reposition copper in the biology of M. tuberculosis: rather than acting only as a toxic antimicrobial weapon, host-derived copper also functions as an environmental signal that remodels bacterial physiology. By identifying S -sulfocysteine synthase CysK2 as a key component of the copper response, the study links copper sensing to sulfur metabolism and redox homeostasis and shows how that link supports intracellular adaptation and long-term persistence. The work explains how the RicR-regulated cysK2 locus can be rapidly induced by physiologically relevant copper levels and by conditions inside macrophages such as hypoxia and altered host copper transport. Because the H37Rv cysK2 -deficient mutant shows both metabolic changes and a persistence defect in mice, CysK2 emerges as a concrete molecular mediator of the copper-triggered program that helps M. tuberculosis survive the phagosomal environment. Overall, the study uncovers a new mechanism by which the pathogen maintains intracellular survival and highlights metabolic adaptation as central to bacterial fitness during infection.