The researchers began with a phenomenon they call acid growth arrest. When M. tuberculosis is cultured in a defined minimal medium at acidic pH and given certain single carbon sources, including glycerol, propionate, and lactate, it simply stops growing. This is not death, but a deep physiological pause, and understanding what triggers it could reveal vulnerabilities that new drugs might exploit. Lactate is a particularly interesting trigger because it is abundant in the granulomas and necrotic lesions where the bacillus spends much of its life, as host immune cells churn out the compound through glycolytic metabolism. The team set out to dissect, at the level of individual genes, why lactate at acidic pH brings the bacterium to a standstill.

Their approach was a classic genetic screen with a modern twist. Using transposon mutagenesis, they generated a library of thousands of mutants, each disrupted in a random gene, and then searched for the rare mutants that could keep growing on lactate at acidic pH, conditions under which the wild-type bacterium halts. Five such escape-artist mutants carried insertions in phoT or pstC2, two genes encoding components of an ATP-binding cassette transporter dedicated to importing phosphate. That result was a genuine surprise: the genetic key to escaping acid growth arrest on lactate turned out to be a broken phosphate uptake system.

To confirm that this was not some indirect artifact, the investigators took the opposite tack. Rather than breaking the transporter, they simply starved the cultures of phosphate. Depleting phosphate from the medium restored growth of normal M. tuberculosis on lactate at acidic pH, demonstrating that the growth arrest was phosphate-dependent. In other words, it was not lactate alone, and it was not acidity alone, that froze the bacterium in place. It was the combination of the two, with phosphate acting as the enabling partner in a metabolic trap.

What, then, is the actual physiological damage? The team turned to pH-sensitive fluorescent reporters to peer inside the bacterial cell. When wild-type M. tuberculosis was exposed to lactate and phosphate together at acidic pH, its cytoplasm acidified to below pH 6.7, a level that would perturb countless enzymes and metabolic reactions. Crucially, the phoT mutant, the mutant that could not import phosphate efficiently, maintained a cytoplasmic pH above 7.2 under the same conditions and kept growing. The interpretation is elegant: phosphate uptake at acidic pH effectively drags acidity into the cell, overwhelming the bacterium’s cytoplasmic pH homeostasis machinery, while blocking that uptake keeps the interior comfortably neutral.

The story deepened when the researchers examined the proton motive force, or PMF, the electrochemical gradient across the bacterial membrane that powers ATP synthesis and much of cellular transport. Lactate slightly decreased membrane potential in wild-type M. tuberculosis in a dose-dependent manner, while the phoT mutant maintained a higher membrane potential than its wild-type counterpart. Transcriptional profiling reinforced this picture: lactate exposure produced a signature the authors describe as a PMF stress response, including the upregulation of electron transport chain genes, as if the cell were scrambling to rebuild its energized membrane. Together, these data associate the combined lactate- and phosphate-driven cytoplasmic acidification with proton motive force stress, linking an energy crisis to the growth arrest.

The study also uncovered a regulatory layer that governs how the bacterium copes with phosphate scarcity. When the phoT mutant grew on lactate at acidic pH, it ramped up the senX3/regX3 regulon, a two-component signaling system that M. tuberculosis uses to sense and respond to low phosphate. To test whether this response actually mattered for survival, the team constructed a regX3 mutant. The result was decisive: growth on lactate under low-phosphate conditions required regX3. Without the regulator, the bacterium could not take advantage of the phosphate-limited escape route that the transporter mutants had stumbled into.

Based on these findings, the authors propose a two-part model. First, acidic pH, lactate, and phosphate together modulate cytoplasmic pH homeostasis, and their combination is associated with proton motive force stress and acid growth arrest. Second, when phosphate is scarce, the SenX3-RegX3 system springs into action, and the authors suggest it may alter cell-envelope physiology through ESX-5- and PE/PPE-dependent pathways, thereby promoting growth on lactate at acidic pH. The ESX-5 secretion system and its PE/PPE protein substrates are major determinants of the mycobacterial cell surface, hinting that the bacterium may remodel its outer layers as part of its adaptation to phosphate stress in acidic niches.

The implications for tuberculosis research are considerable. Host tissues where M. tuberculosis persists are thought to be rich in lactate, acidic, and variable in phosphate availability, which means the conditions defined in this minimal-medium system may mirror real aspects of infection. If cytoplasmic acidification driven by lactate and phosphate contributes to the dormant, drug-tolerant state that makes tuberculosis so difficult to cure, then pharmacologically amplifying that acidification, or blocking the SenX3-RegX3-mediated adaptation that lets the bacterium escape it, could shorten therapy. Conversely, the finding that phosphate import is central to the trap raises questions about how nutrient availability within granulomas shapes bacterial physiology and drug susceptibility.

Like all good reductionist work, this study also carries caveats that define the next frontier. The experiments were performed in defined minimal medium, and the authors frame their conclusions carefully, describing associations between cytoplasmic acidification, PMF stress, and growth arrest rather than a fully proven causal chain. Testing whether the same lactate-phosphate interplay operates during infection, and whether manipulating phosphate uptake or RegX3 activity changes the course of disease in animal models, will be the critical follow-up. Even so, the work transforms a vague notion, that acid stresses tuberculosis, into a mechanistic, testable framework built on named transporters, a named regulator, and a measurable biophysical insult. For a pathogen that infects a quarter of the world’s population, every such handle on its survival machinery is a potential foothold for the next generation of therapies.

Subject of Research: Phosphate- and lactate-dependent cytoplasmic acidification causing growth arrest of Mycobacterium tuberculosis at acidic pH

Article Title: Combined lactate- and phosphate-dependent cytoplasmic acidification is associated with Mycobacterium tuberculosis growth arrest at acidic pH

Article References: Kibiloski, A. P., Dechow, S. J., Abdalla, B. J., Murdoch, H. M., Tischler, A. D., & Abramovitch, R. B. (2026). Combined lactate- and phosphate-dependent cytoplasmic acidification is associated with Mycobacterium tuberculosis growth arrest at acidic pH. PLOS Genetics, 22(10), e1012331. https://doi.org/10.1371/journal.pgen.1012331

Image Credits: AI Generated

DOI: 10.1371/journal.pgen.1012331

Keywords: Mycobacterium tuberculosis, acid growth arrest, cytoplasmic pH, phosphate transporter, lactate metabolism, proton motive force, SenX3-RegX3, phoT, pstC2, ESX-5 secretion system, PLOS Genetics, tuberculosis persistence