Ed Yong writes:

In the 1870s, German physician Robert Koch was trying to curtail an epidemic of anthrax that was sweeping local farm animals. Other scientists had seen a bacterium, Bacillus anthracis, in the victims’ tis­sues. Koch injected this microbe into a mouse – which died. He recovered it from the dead rodent and injected it into another one – which also died. Doggedly, he repeated this grim process for over 20 generations and the same thing happened every time. Koch had unequivocally shown that Bacillus anthracis caused anthrax.

This experiment, and those of contemporaries like Louis Pasteur, confirmed that many diseases are caused by microscopic organisms. Microbes, which had been largely neglected for a couple of centuries, were quickly cast as avatars of death. They were germs, pathogens, bringers of pestilence. Within two decades of Koch’s work on anthrax, he and many others had discovered that bacteria were also associated with leprosy, gonorrhoea, typhoid, tuberculosis, cholera, diphtheria, tetanus, and plague. Microbes became synonymous with squalor and sickness. They became foes for us to annihilate and repel.

Today, we know this view is wrong – as I explain in my new book I Contain Multitudes. Sure, some bacteria can cause disease, but they are in the minority. Most are harmless, and many are even beneficial. We now know that the trillions of microbes that share our bodies – the so-called microbiome – are an essential part of our lives. Far from making us sick, they can protect us from disease; they also help digest our food, train our immune system, and perhaps even influence our behaviour. These discoveries have shifted the narrative. Many people now see microbes as allies to be protected. Magazines regularly warn that antibiotics and sanitisers might be harming our health by destroying our microscopic support system. Slowly, the view that ‘all bacteria must be killed’ is giving ground to ‘bacteria are our friends and want to help us’.

The problem is that the latter view is just as wrong as the former. We cannot simply assume that a particular microbe is ‘good’ just because it lives inside us. There’s really no such thing as a ‘good microbe’ or a ‘bad microbe’. These broad-brush terms belong in children’s stories. They are ill-suited for describing the messy, fractious, contextual relationships of the natural world.

In reality, bacteria exist along a continuum of lifestyles. If they do us harm, we describe them as parasites or pathogens. If they exist neutrally, we call them commensals. If they benefit us, we bill them as mutualists. But these are hardly fixed categories. Some microbes can slide from one end of this parasite-mutualist spectrum to the other, depending on the strain and on the host they find themselves in. For example, the Wolbachia bacteria infect some 40 percent of insects; in some species, these microbes are sexual parasites that kill or manipulate males, whereas in others, they behave as living dietary supplements that provide vitamins missing from the host’s diet.

Other microbes can be pathogen and mutualist at the exact same time. The stomach bacterium Helicobacter pylori is well known as a cause of ulcers and stomach cancer. Less famously, it also protects against oesophageal cancer – and it’s the same strains that account for both these pros and cons. H. pylori is neither a good nor a bad microbe; it’s both. [Continue reading…]