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

by Ed Yong

Most microbes are not pathogens. They do not make us sick. There are fewer than 100 species of bacteria that cause infectious diseases in humans;

Speaking of palms, your right hand shares just a sixth of its microbial species with your left hand.

mammals, including pangolins, armadillos, anteaters, aardvarks, and aardwolves (a type of hyena), all have similar gut microbes, even though they have been evolving independently for around 100 million years.

The immune system’s main function is to manage our relationships with our resident microbes. It’s more about balance and good management than defence and destruction.

To allow our first microbes to colonise our newborn bodies, a special class of immune cells suppresses the rest of the body’s defensive ensemble, which is why babies are vulnerable to infections for their first six months of life.

Milk is one of the most astounding ways in which mammals control their microbes.

As it digests HMOs, B. infantis releases short-chain fatty acids (SCFAs) that feed an infant’s gut cells – so while mothers nourish this microbe, the microbe in turn nourishes the baby. Through direct contact, B. infantis also encourages gut cells to make adhesive proteins that seal the gaps between them, and anti-inflammatory molecules that calibrate the immune system. These changes only happen when B. infantis grows on HMOs; if it gets lactose instead, it survives but doesn’t engage in any repartee with the baby’s cells.

mice are not people,

We have taken cleanliness to mean a world without microbes, without realising the consequences of such a world.

Even the most concentrated probiotics contain just a few hundred billion bacteria per sachet. That sounds like a lot but the gut already holds at least a hundredfold more. Gulping down a yoghurt is like ingesting scarcity. Rarity, too: the bacteria in these products are not important members of the adult gut.

According to their verdicts, probiotics can shorten bouts of infectious diarrhoea, and reduce the risk of diarrhoea brought about by antibiotic treatments. They can also save lives from necrotising enterocolitis – a horrible gut disease that affects premature infants. And there ends the list.

Every person aerosolises around 37 million bacteria per hour.