The laboratory worm Caenorhabditis elegans feeds on Escherichia coli its whole life, but in the wild it eats rotting plants, which are home to all sorts of bacteria. One of them, Lactococcus lactis JCM 5805, extends the worm's life by four days, while another, Lactiplantibacillus plantarum MAFF 401534, gives no such effect. Why? It turned out that the helpful bacterium does part of the work of maintaining the worm's body itself. It delivers building materials for the outer skeleton, like a contractor who doesn't just advise you on how to reinforce the foundation but also brings the bricks and cement.
Scientists compared worms fed different bacteria. It turned out that only live Lc5805 cells triggered the accumulation of citrulline in the worm's tissues. Citrulline is an amino acid, a building block for collagen, the main protein of connective tissue. The worm uses collagen for its cuticle, the tough outer skeleton that protects it from damage. At the same time, the classic markers of stress resistance and longevity (sod-3, gst-4) were not switched on. That means the bacterium wasn't forcing the worm to spend energy on defense; it was simply handing it ready-made materials.
The mechanism was dubbed "metabolic outsourcing": the host hires bacterial contractors to repair its cuticle. This explains why probiotic effects often fail to reproduce: what matters is not just the bacterial species but its metabolic activity in a live state. Perhaps in the future we'll be able to engineer bacteria that deliver the right substances to the human gut, to support connective tissue health and slow aging.
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.