For most of the history of leptospiral research, scientists have grown these fastidious organisms in Ellinghausen-McCullough-Johnson-Harris medium, commonly abbreviated as EMJH. That formulation, while effective for keeping cultures alive in the laboratory, bears little resemblance to the chemical landscape a pathogen actually encounters inside a mammalian host. Nutrient concentrations in EMJH differ dramatically from those in blood plasma, which means that metabolic behaviors observed in conventional cultures may not reflect what happens during a genuine infection. The research team recognized that this mismatch could be masking important aspects of leptospiral biology, and they set out to build a more faithful experimental system.

Their solution was a culture medium they call supplemented Human Plasma-Like Medium, or sHPLM. As the name suggests, the formulation was designed to mimic the composition of human plasma far more closely than traditional media do, with physiologically relevant concentrations of amino acids, vitamins, salts, and other metabolites. The researchers established and validated the medium specifically for Leptospira culture, confirming that the organism could grow in it reliably. By moving the bacteria into an environment that better approximates the host, the team hoped to reveal metabolic dependencies that standard conditions would never expose. The gamble paid off almost immediately.

To interrogate what the bacteria were actually consuming, the group pioneered a set of metabolomics approaches tailored to leptospiral cultures. They developed a liquid chromatography coupled with mass spectrometry workflow capable of performing two complementary tasks: profiling the metabolites present in the medium itself, and tracing the fate of specific labeled nutrients as the bacteria metabolized them. Stable isotope tracing is a powerful technique in which a nutrient is supplied in a chemically identical but isotopically tagged form, allowing researchers to follow exactly where its atoms end up inside the cell. Applying this workflow to L. interrogans growing in sHPLM gave the team an unprecedented, real-time picture of the pathogen’s feeding habits.

The central finding was striking. Among the many nitrogen-containing compounds available in the plasma-like medium, glutamine stood out as a major nitrogen source for the bacteria. This was unexpected because ammonium has long been considered the sole nitrogen source for Leptospira species. Nitrogen is an indispensable element for every living cell, forming the backbone of amino acids, nucleotides, and countless other building blocks, so identifying an alternative and physiologically abundant nitrogen supply fundamentally changes how scientists should think about the pathogen’s nutritional strategy inside the host. Glutamine, one of the most plentiful amino acids in human blood, appears to be far more than passive background chemistry for these organisms.

To confirm that the observation was not an artifact of a single strain or condition, the researchers ran growth experiments across three pathogenic Leptospira strains as well as the nonpathogenic strain L. biflexa Patoc-1. When glutamine-derived nitrogen was the only nitrogen available, all of the tested strains proliferated robustly. That breadth matters. It indicates that the capacity to assimilate glutamine nitrogen is not a quirk of one laboratory isolate but a shared trait across the genus, present in both disease-causing and benign relatives. From an evolutionary standpoint, this suggests that glutamine utilization is a deeply rooted metabolic capability rather than a recent adaptation confined to virulent lineages.

The findings also carried immediate therapeutic implications. The team tested JHU-083, a small-molecule inhibitor designed to interfere with glutamine utilization, and found that it effectively impaired the proliferation of L. interrogans cultures. Because severe leptospirosis currently lacks consistently effective treatments, and because the bacterium’s unusual biology makes it resistant to many conventional antibiotic strategies, a metabolic inhibitor that targets nitrogen assimilation represents an intriguing conceptual blueprint. The result does not mean JHU-083 itself is ready for clinical use, but it demonstrates that blocking glutamine metabolism can starve the organism of a resource it clearly depends on under host-like conditions.

Perhaps the most surprising result came when the researchers added glutamine to the non-physiological EMJH medium. Even in that artificial environment, the amino acid rapidly induced a short-term boost in proliferation and increased biofilm formation in L. interrogans. Biofilms, the structured bacterial communities that adhere to surfaces and often display heightened tolerance to antimicrobial agents, are increasingly recognized as important in persistent infections. The fact that a single nutrient could trigger both faster growth and biofilm development hints that glutamine is not merely food but also a signal, a molecule the bacterium uses to sense its surroundings and reprogram its behavior accordingly.

RNA sequencing reinforced that interpretation. After exposing the bacteria to glutamine, the team observed transcriptional trends pointing toward increased biosynthetic activity, consistent with the organism ramping up its internal manufacturing to support the accelerated growth and biofilm phenotypes. In other words, the presence of glutamine appears to shift the pathogen’s gene expression in ways that prepare it for expansion. If similar signaling occurs during infection, glutamine availability in the host could influence not only how fast L. interrogans multiplies but also how it organizes itself and resists clearance, factors that directly affect disease severity and transmission.

Taken together, the study delivers a two-part message. First, glutamine provides a second nitrogen source for biosynthesis in multiple Leptospira strains, overturning the textbook view that ammonium is the only option. Second, the amino acid may act as a metabolite signal that alters L. interrogans physiology in ways that could shape infection. Both conclusions underscore the methodological lesson embedded in the work: studying pathogens under physiological conditions is not a luxury but a necessity, because nutrient environments that look adequate in the laboratory can conceal the very dependencies that matter most in the host. Nitrogen assimilation now stands as a candidate target for new therapies, and the sHPLM platform combined with isotope tracing offers a template for uncovering similar metabolic blind spots in other understudied pathogens.

Subject of Research: Glutamine utilization as a nitrogen source in the pathogenic bacterium Leptospira interrogans

Article Title: Glutamine from plasma-like medium fuels nitrogen metabolism and influences proliferation in Leptospira interrogans

Article References: Ward, M. H., Scherer, N., Shriver, L. P., & Patti, G. J. (2026). Glutamine from plasma-like medium fuels nitrogen metabolism and influences proliferation in Leptospira interrogans. PLOS Pathogens, 22(10), e1014534. https://doi.org/10.1371/journal.ppat.1014534

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014534

Keywords: Leptospira interrogans, leptospirosis, glutamine, nitrogen metabolism, metabolomics, stable isotope tracing, Human Plasma-Like Medium, biofilm, JHU-083, PLOS Pathogens, bacterial physiology, antimicrobial targets

The review, led by researchers at Inner Mongolia Agricultural University, organizes decades of evidence into a Mechanism-Technology-Application framework designed to separate what has actually been demonstrated from what has merely been inferred. The authors are blunt about the distinction. Cross-feeding, as they define it, is any interaction in which a resource produced, released, or made accessible by one microorganism is acquired or further transformed by another. It is not inherently cooperative, not always beneficial to the host, and not a universal organizing principle of the gut ecosystem. The same resource transfer that sustains a health-associated butyrate producer can, in a different context, feed a dangerous pathogen.

The clearest examples come from sequential degradation, sometimes called a trophic relay. Complex dietary polysaccharides and host-derived mucus glycans are too large for most gut bacteria to touch. Specialist degraders such as Bacteroides species, Akkermansia muciniphila, and Bifidobacterium bifidum cleave these polymers into sugars and oligosaccharides that downstream consumers can use. In one well-characterized relay, Bacteroides ovatus releases breakdown products during inulin degradation that support B. vulgatus, B. thetaiotaomicron, and Parabacteroides goldsteinii. In another, Akkermansia muciniphila liberates mucin-derived sugars and acetate that fuel the butyrate producer Anaerostipes caccae, linking mucus turnover to the production of a short-chain fatty acid that nourishes the colonic lining and modulates immune signaling through receptors such as GPR43.

But relays have a dark side. In antibiotic-treated mice, the same liberation of host sugars becomes a lifeline for pathogens: sialic acid and fucose released from mucus by the depleted community boost the competitive fitness of Salmonella enterica serovar Typhimurium, while sialic acid catabolism supports the post-antibiotic expansion of Clostridioides difficile. Amino acid exchange tells a similar story. In antibiotic-perturbed mice, enterococci supply leucine and ornithine that enhance C. difficile fitness, and their arginine catabolism simultaneously generates metabolic signals associated with increased virulence. Metagenomic surveys suggest that more than 40 percent of gut microbes lack complete pathways for making branched-chain amino acids, implying that such dependencies are widespread, though predicted auxotrophy alone does not prove that transfer actually occurs.

Diffusible metabolites add another layer of coupling. Hydrogen produced by fermentative bacteria such as Roseburia intestinalis can be consumed by acetogens like Ruminococcus hydrogenotrophicus or by the methanogen Methanobrevibacter smithii, keeping fermentation thermodynamically favorable while shifting the end products toward acetate or methane. Elevated methane has been associated with delayed transit and constipation-predominant irritable bowel syndrome, although the review stresses that these associations do not establish hydrogen cross-feeding as a direct cause of disease. Perhaps the most elegant demonstrated exchange is bidirectional: Bacteroides xylanisolvens supplies folate to Clostridium butyricum, which in turn provides p-aminobenzoic acid that B. xylanisolvens needs to synthesize folate, a reciprocal loop shown under controlled culture conditions but not yet traced in a living gut.

Micronutrients reveal an even more competitive dimension. Bacteroides thetaiotaomicron makes no siderophores of its own yet steals iron from enterobactin and salmochelin secreted by Enterobacteriaceae using its XusABC system, a capacity that improves its colonization fitness during inflammation. Remarkably, the XusB protein captures enterobactin and shields it from the host’s iron-sequestering lipocalin-2, but Salmonella can then reacquire the protected complex. A single iron-trafficking route can therefore bolster a commensal, blunt host nutritional immunity, and still hand iron to a pathogen. Vitamin provisioning shows similar ambiguity: auxotrophic strains in humanized germ-free mice maintain stable abundances despite dietary swings, consistent with community-level sharing, yet the actual donors and transfer routes remain unidentified.

Given this complexity, the review proposes a tiered evidence framework that mirrors a Design-Build-Test-Learn engineering cycle. Multi-omics and computational platforms such as COMETS, MICOM, and the Gut Cross-feeding Predictor generate candidate donor-resource-recipient hypotheses, but correlation networks cannot establish causality. Causal attribution demands co-culture experiments, targeted metabolite perturbation, and genetic disruption of production or uptake pathways, exemplified by a study in which paired mutants of Bifidobacterium breve and Limosilactobacillus reuteri confirmed that a nutritional interaction depended on the intermediate 1,2-propanediol. Stable-isotope tracing then resolves metabolite fate directly: labeled acetate has been shown to be incorporated into butyrate by Faecalibacterium prausnitzii and Roseburia strains, and labeled lactate and acetate traced the carbon route from Bifidobacterium adolescentis into butyrate synthesis by Eubacterium hallii via acetyl-CoA.

Even validated pairwise exchanges, however, may not survive contact with a real community. In a twelve-member synthetic gut community, pairwise interactions explained only part of the observed dynamics, and initial abundances and negative interactions imposed strong historical contingency. Functional redundancy complicates matters further: depleting specific Bacteroides members triggers compensatory expansion of relatives that partially restores glycan degradation. Host-associated models add spatial architecture, dietary fluctuation, and colonization history, all of which can attenuate or redirect interactions that looked stable in a flask. The authors argue that no single method suffices; only convergent evidence across candidate identification, causal validation, and ecological robustness testing can support claims of functional cross-feeding.

The translational implications are sobering but instructive. Cross-feeding-informed synbiotics would require a primary responder to release a defined intermediate that a secondary consumer then transforms, with a combination-specific benefit demonstrated against substrate-only and microbe-only controls, a standard that current human evidence does not meet. Among live biotherapeutic products, the dual-strain pairing of B. xylanisolvens and C. butyricum improved metabolic and barrier parameters in high-fat-diet mice, but the folate exchange was never traced in vivo. The seventeen-strain consortium GUT-103 attenuated colitis in gnotobiotic mice without resolving any donor-metabolite-recipient chain, and a thirty-seven-strain synthetic microbiota suppressed C. difficile through competitive proline depletion rather than resource transfer at all. Engineered examples cut both ways: redirecting polysaccharide-utilization loci in Bacteroides uniformis altered glycan flow to downstream consumers, a relatively direct case of engineered cross-feeding, while the clinical candidate NOV-001 achieved porphyran-dependent, largely reversible engraftment in a Phase 1/2a trial yet showed variable colonization, genetic instability, and no significant benefit over placebo for urinary oxalate. The Phase 2b ECO-RESET trial of SER-287 similarly failed its primary endpoint despite confirmed engraftment, demonstrating that colonization is not efficacy.

The review’s conclusion is neither dismissive nor utopian. Cross-feeding offers a rigorous, mechanistically grounded framework for prioritizing dietary substrates, strain combinations, and engineered resource-allocation strategies, and emerging tools, from spatially resolved transcriptomics to gut-on-chip platforms and digital-twin modeling, should progressively narrow the gap between inference and demonstration. But a reproducible, closed engineering cycle for gut microbial communities has not been achieved across heterogeneous human hosts, and the immediate priority is prospective validation of the full causal chain linking substrate availability, microbial activity, metabolite transfer, ecological persistence, safety, and clinically meaningful benefit. Cross-feeding, the authors emphasize, must be evaluated within the competitive, host-regulated ecology of the gut rather than as an isolated, programmable switch. The microbial economy of the intestine is real, increasingly legible, and, in carefully defined cases, already manipulable. Turning that legibility into reliable medicine is the task the field now faces.

Subject of Research: Metabolic cross-feeding interactions in the gut microbiome and their application to live biotherapeutic development

Article Title: Programming Gut Microbiome Function Through Cross‐Feeding: From Ecological Mechanisms to Live Biotherapeutics

Article References: Sun, C., Ma, T., Jin, H., Li, Z., Cheng, Y., Li, J., Kwok, L.-Y., & Sun, Z. (2026). Programming Gut Microbiome Function Through Cross‐Feeding: From Ecological Mechanisms to Live Biotherapeutics. Advanced Science, 13(55), Article e77267. https://doi.org/10.1002/advs.77267

Image Credits: AI Generated

DOI: 10.1002/advs.77267

Keywords: gut microbiome, cross-feeding, microbial metabolism, live biotherapeutics, synbiotics, butyrate, short-chain fatty acids, synthetic communities, stable-isotope tracing, microbiome therapeutics, colonization resistance, precision nutrition

The study, led by Namid Krüger of the University of Kaiserslautern-Landau together with colleagues at the University of Münster and the University of Hamburg, tackled a long-standing gap in wetland biogeochemistry. In well-aerated upland soils, scientists have documented for decades that living roots can stimulate the decomposition of soil organic matter, a phenomenon known as the rhizosphere priming effect. Roots leak sugars, organic acids, and other easily metabolized compounds into the soil, and microbes, energized by this fresh food, ramp up their consumption of older, more recalcitrant carbon stocks. But wetlands complicate this picture. In waterlogged peat, the soil matrix is not only carbon-rich but also oxygen-depleted, and wetland plants such as sedges do something unusual: they transport oxygen from their shoots down through internal air channels and release it into the rhizosphere. This means wetland roots simultaneously deliver both an energy source and the oxidant needed to break it down, a combination that could either amplify or suppress priming in ways no one had systematically tested.

To disentangle these effects, the researchers designed an elegant microcosm experiment in which two sedge species with contrasting belowground growth forms were grown on waterlogged peat containing uniformly carbon-13-labeled litter. The species, cotton-grasses of the genus Eriophorum, represent two distinct root strategies: Eriophorum angustifolium spreads through creeping rhizomes, producing a diffuse, exploratory root system, while Eriophorum vaginatum forms dense tussocks with a more compact root architecture. By labeling the litter with a rare stable isotope of carbon, the team could trace precisely how much carbon dioxide released from the microcosms came from the decomposing litter versus from other sources, using a two-end-member mixing model to partition the respiration fluxes.

The results were striking. Cumulative litter-derived carbon dioxide respiration was 87 percent higher under the rhizomatous E. angustifolium and 106 percent higher under the tussock-forming E. vaginatum than in unplanted control treatments. In other words, simply having sedge roots in the system nearly doubled the respiratory loss of carbon from the litter, regardless of which root strategy the plant employed. Yet the physical disappearance of litter told a more modest story: litterbag mass loss increased by only 10 percent under E. angustifolium and 19 percent under E. vaginatum. This mismatch between gas flux and mass loss hints that roots may be mobilizing litter carbon into microbial biomass and dissolved forms that are respired but not immediately lost from the solid litter matrix, a decoupling that could matter greatly for long-term carbon accounting.

Perhaps the most dramatic finding was the temporal trajectory of the priming effect itself. In the earliest phase of the experiment, priming effects exceeded +200 percent, meaning the planted microcosms were respiring litter carbon at more than three times the rate of the unplanted controls. Within three months, however, this fervent activity reversed, with priming effects falling below −20 percent. The researchers interpret this swing as a signature of early-stage litter decomposition dynamics. Fresh litter is initially rich in labile, water-soluble compounds that microbes devour readily once oxygen and root-derived substrates become available. As the easily decomposable fraction is exhausted, the remaining litter becomes dominated by structurally reinforced compounds such as lignin and cellulose, and the priming effect flips as root-associated microbial communities shift their enzymatic focus or as root-derived carbon begins to compete with litter carbon for microbial attention.

To visualize what was happening at the scale of individual roots, the team deployed two sophisticated imaging techniques. Zymography, a method that maps enzyme activity across the rhizosphere by pressing a fluorescently coated membrane against the soil, revealed that β-glucosidase, an enzyme that cleaves simple sugars from cellulose, was elevated around mature roots in both species. Acid phosphatase, which liberates phosphorus from organic molecules, showed an even broader pattern, elevated across the entire root system compared with background soil levels. This widespread enzymatic activation suggests that sedges do not merely fertilize the soil immediately adjacent to their root tips but create an extended zone of intensified organic matter processing that envelops the whole root architecture.

Complementing the enzyme maps, planar optode imaging allowed the researchers to watch oxygen and carbon dioxide dynamics unfold in near real time across transparent observation windows in the microcosms. This technology confirmed that the roots were actively releasing oxygen into the otherwise anoxic peat, creating ephemeral oxic hotspots where aerobic decomposition could proceed at rates impossible in the surrounding waterlogged matrix. The combination of oxygen leakage and enzyme stimulation provides a coherent mechanistic explanation for the observed priming: roots deliver the oxidant that unlocks aerobic metabolism in an anaerobic world, while their exudates and the oxygen itself fuel microbial communities that then attack the surrounding litter with renewed vigor.

What makes the study particularly consequential is the finding that both sedge species, despite their morphologically distinct root systems, produced qualitatively similar priming responses. The rhizomatous strategy, which explores soil volume widely, and the tussock-forming strategy, which concentrates roots in a dense cylinder, both enhanced litter-derived respiration by roughly the same magnitude. This suggests that the key mechanism, root oxygen loss coupled with rhizosphere enzyme activity, is a general property of these wetland sedges rather than a quirk of one growth form. For ecosystem modelers, this is both good news and a caution: it implies that priming in sedge-dominated peatlands may be reasonably represented without resolving fine-scale root architecture, but it also means that any factor increasing sedge root activity, from warming to nutrient deposition, could uniformly accelerate carbon cycling across diverse peatland types.

The broader implications reach into some of the most urgent questions in climate science. Peatlands have acted as net carbon sinks for millennia precisely because waterlogging suppresses decomposition. If living roots can double the respiratory loss of litter carbon even under anoxic conditions, then the vegetation composition of a peatland becomes a critical variable in its carbon budget. Sedges are often dominant in natural bogs and fens, and they are also among the first plants to colonize restored peatlands after rewetting. Understanding how their root activity modulates decomposition could therefore refine predictions of whether restored wetlands will regain their carbon sequestration function quickly or slowly. Moreover, as permafrost peatlands thaw and water tables fluctuate under climate change, the interplay between root-derived oxygen and priming effects may determine whether these landscapes release their ancient carbon stores as carbon dioxide or continue to bury new inputs faster than old ones decompose.

The research, supported by the German Research Foundation through its Emmy Noether programme, represents a methodological milestone as much as a conceptual one. By combining isotope tracing, enzyme imaging, and optode visualization within a single experimental framework, the team demonstrated that rhizosphere processes in wetlands can be measured with a resolution previously reserved for upland systems. The work opens the door to studying priming across a wider range of wetland plants, water regimes, and litter types, and it underscores a deceptively simple message: in the world’s waterlogged carbon vaults, the roots of living plants are quietly turning the locks. Whether that door opens wider as the climate warms is now one of the most pressing questions facing peatland science.

Subject of Research: Rhizosphere priming of litter decomposition by sedge roots in anoxic peat

Article Title: Root trait controls on rhizosphere priming effects during early-stage litter decomposition in anoxic peat

Article References: Krüger, N., Dorodnikov, M., Thomsen, S., Knorr, K.-H., & Mueller, P. (2026). Root trait controls on rhizosphere priming effects during early-stage litter decomposition in anoxic peat. Biogeochemistry. https://doi.org/10.1007/s10533-026-01373-5

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01373-5

Keywords: peatlands, rhizosphere priming, litter decomposition, sedges, Eriophorum, root traits, stable isotope tracing, zymography, planar optode imaging, wetland biogeochemistry, carbon cycle, root oxygen loss