Laboratory worms Caenorhabditis elegans feed on Escherichia coli OP50 their whole lives, but in nature their diet consists of rotting plants rich in bacilli and lactic acid bacteria. It was previously shown that some probiotic strains can noticeably extend the lifespan of these nematodes, yet the molecular mechanisms remained a 'black box.' Of particular interest was the fact that the worms lack key urea cycle enzymes and nitric oxide synthase, so they cannot produce arginine and nitric oxide on their own. Perhaps bacteria fill these gaps by supplying the missing metabolites. Understanding such interactions matters not only for the biology of aging but also for developing probiotic strategies in humans.
The researchers compared three diets for the nematodes: standard OP50, Lactococcus lactis JCM 5805 (Lc5805), which extended lifespan, and Lactiplantibacillus plantarum MAFF 401534 (Lp1534), which produced no such effect. The worms were fed live or heat-killed bacteria, and then changes in gene expression (RNA sequencing) and metabolite composition (capillary electrophoresis with mass spectrometry) were measured at 10, 60, and 300 minutes. To understand the origin of the accumulated citrulline, the authors quantified arginine, citrulline, and ornithine in the bacterial cells themselves and in the worms fed different strains. Additionally, using quantitative PCR, they assessed the activity of the bacterial gene arcA, which encodes arginine deiminase, directly in the nematode intestine.
The main result: only live Lc5805 cells caused citrulline to accumulate in the worm's tissues, reaching 100 nmol/g within just 10 minutes of feeding, whereas in worms on OP50 or Lp1534 the level stayed below 10 nmol/g. Heat treatment of Lc5805 completely abolished both citrulline accumulation and lifespan extension. Analysis of the bacterial cells showed that citrulline is present only in Lc5805, and the arcA gene, responsible for converting arginine to citrulline, is actively expressed inside the worm's intestine. Transcriptome analysis revealed that worms that ate Lc5805 specifically activated 74 genes linked to cuticle formation, including 58 collagen genes. At the same time, classic markers of stress resistance and longevity (sod-3, gst-4) were not turned on, pointing to an unusual mechanism: the bacterium itself supplies the building blocks for collagen, sparing the host the need to launch energy-expensive defense programs.
The work introduces the concept of 'metabolic outsourcing' into the scientific literature: the host doesn't just receive nutrients but directly uses bacterial enzymes and intermediate metabolites to maintain its own structures. This shifts the focus away from the traditional view of probiotics as immune modulators or vitamin suppliers. For gerontology, a new class of interventions is opening up, targeting not the organism's own longevity genes but its microbial partners. Moreover, the results explain why probiotic effects often fail to reproduce: what matters is not only the bacterial species but also its metabolic activity in a live state.
The immediate next step is to test whether a similar mechanism works in mammals, particularly in mice with deficiencies in urea cycle enzymes or with age-related decline in collagen synthesis. It would also be interesting to find out which specific receptors or transporters in the worm's intestine recognize bacterial enzymes and metabolites. Perhaps synthetic bacterial consortia optimized to deliver specific amino acids or enzymes to the gut could be created. The long-term prospect is the development of 'living therapeutics' to maintain connective tissue health and slow aging in humans.
The results will influence microbiome research, gerontology, and nutrition science by offering a new framework for understanding symbiotic relationships. They are also important for developing probiotics with proven efficacy, since they highlight the need to preserve the viability and metabolic activity of bacteria.
It is necessary to identify the receptors and signaling pathways in the C. elegans intestine that mediate the uptake of bacterial metabolites and enzymes. It is also worth checking whether other Lactococcus species with active arginine deiminase can reproduce the Lc5805 effect, and to study how such 'outsourcing' affects reproductive health and stress resistance.
The work directly addresses an unresolved problem in the biology of aging: why some probiotics extend lifespan and others do not, and what role the microbiome plays in maintaining proteostasis and the structure of the extracellular matrix. It also touches on the question of how organisms with lost metabolic pathways compensate for these losses through symbionts, which is relevant for understanding the evolution of metabolic networks.
Citrulline got its name from the Latin citrullus, meaning watermelon, from which it was first isolated in 1914. In the paper, the citrulline concentration in worms that ate Lc5805 reached 100 nmol/g, comparable to the content of this amino acid in watermelon flesh.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.