I Contain Multitudes: The Microbes Within Us and a Grander View of Life

by Ed Yong

Take the case of oxalate. It’s found in beetroot, asparagus, and rhubarb, among other foods. At high concentrations, it stops your body from absorbing calcium, which congeals into a hard lump. That’s one way in which kidney stones form. We can’t digest oxalate; only microbes can do that. One species – a gut bacterium called Oxalobacter formigenes – is so good at it that it uses oxalate as its one and only source of energy. At first glance, this situation looks identical to the Leucaena dilemma. There’s one chemical (oxalate), which causes a defined problem (kidney stones), and can be destroyed by one microbe (Oxalobacter). The solution, surely, is to swallow an Oxalobacter probiotic if you are prone to kidney stones. Unfortunately, such probiotics exist and they aren’t very effective.23 Why? There are two likely answers, and both provide valuable lessons. First, it is not enough to infuse an animal with bacteria and hope for the best. Microbes are living things. They need to eat. Oxalobacter eats nothing but oxalate, and people with kidney stones often go on an oxalate-free diet. They can ingest the bacterium, but it will instantly starve.24 Conversely, farmers are told to feed their herds on Leucaena for at least a week before drenching them in Synergistes. That way, the inoculated bacteria have enough food to digest. Substances that selectively nourish beneficial microbes are called prebiotics – a term that could include oxalate or Leucaena but normally describes plant c...