Cell cycle differences in host cells shape TB bacteria behavior

Scientists have long known that individual cells can behave differently even within the same tissue, but it has been less clear how that host variability affects bacteria that live inside cells. A new study led by Varadharajan Sundaramurthy used single cell approaches to probe how the state of the host cell influences Mycobacterium tuberculosis that live within it. Rather than treating all infected cells as the same, the researchers tracked bacterial redox state inside individual host cells using redox-sensitive Mycobacterium tuberculosis reporters. By comparing bacteria in host cells at different points in the cell cycle, they uncovered a link between the host’s interphase stage and how the host cell takes up material from its surroundings. In plain terms, the phase of the host cell’s cycle changes how active the cell is at endocytosis — the process of bringing things inside — and that shifting activity creates different microenvironments for the bacteria. The work reframes host cell cycle as an important, intrinsic factor shaping the behavior of an intracellular pathogen.

The team combined single cell approaches with genetically encoded redox-sensitive Mycobacterium tuberculosis reporters to measure bacterial physiological states inside individual host cells. They found that endocytic capacity — how readily a host cell imports material — varies with interphase stage. Bacilli residing in host cells in G2, which showed higher endocytic activity, adopted more oxidized redox states. In contrast, bacilli inside G1 host cells, which had lower endocytic activity, remained in a more reduced state. These opposing intrabacterial redox states produced clear phenotypic diversity within a single infected population. To test cause and effect, the researchers experimentally manipulated host cell cycle stage and observed corresponding changes: reprogramming the host cell cycle altered endocytic capacity and shifted intrabacterial redox, establishing a causal link between the host’s cell-cycle state and bacterial phenotype. Importantly, the same pattern of variability remained detectable in post-differentiated macrophages, showing the effect is not limited to actively dividing cells.

The findings have several important implications. Demonstrating that the cell cycle regulates endocytic capacity is itself a fundamental cell-biological discovery with broad consequences for how we think about cell function. For infection biology, the study identifies interphase-regulated endocytosis as a host-intrinsic mechanism that creates distinct intracellular niches and thereby shapes Mycobacterium tuberculosis phenotypes. Because these differences persist in post-differentiated macrophages, the results suggest that a cell’s proliferative history can leave a lasting imprint on innate immune cells and on the way they handle intracellular bacteria. By revealing a direct route from host cell state to bacterial physiology, the work points to new host-directed avenues to influence infection trajectories and bacterial persistence without relying solely on targeting the microbe. Overall, the study highlights why single-cell resolution matters when studying infectious disease: diversity among host cells can create diversity among pathogens, with likely consequences for treatment and long-term outcomes.