Adjectives and Verbs, not Nouns

We know microbes cause diseases. We also know that parts of the microbiome (the trillions of bacteria that live within us) helps us digest food. We’ve even coined terms like ‘pathogens’ (bad), ‘commensal’ (neutral), and ‘mutualists’ (beneficial) to describe microbes. But as Ed Yong writes:

“But these are hardly fixed categories. Some microbes can slide from one end of this parasite-mutualist spectrum to the other.”

 

While seemingly contradictory, some microbes can be “pathogen and mutualist at the exact same time”! Yong cites h.pylori as an example: it “protects against oesophageal cancer” while also being a “cause of ulcers and stomach cancer”. In addition, context matters. A microbe may be a ‘mutualist’ if it stays in the gut, but behave like a ‘pathogen’ if it enters the bloodstream.

 

All of which is why Yong says:

“All of this means that labels like mutualist, commensal, pathogen or parasite don’t work as definitive badges of identity. These terms are more like states of being, like hungry or awake or alive, or behaviours like cooperating or fighting. They are adjectives and verbs rather than nouns. They describe how two partners relate to one another at a given time and place.”

 

Plus, of course, there’s evolutionary pressure always at work. If a microbe in a ‘mutualist’ relation can “cheat” or “betray” and gain more than it gives, well, that’s what it’s going to do:

“These principles are easy to forget. We like our black-and-white narratives, with clear heroes and villains.”

Our bodies, of course, don’t believe in this false good/bad dichotomy. That’s why it has evolved its solutions:

“We have evolved many solutions to the ever-present conflicts that exist with our microbes, and many ways of enforcing our contracts with them. We can restrict them to certain parts of our bodies by creating physical corrals or creating chemical no-go zones. We can go for the carrot, by nourishing the species we want using dedicated foods. We can beat them with the stick, by using our immune system to keep them in place.”

 

Coral biologist Forest Rohwer summarized it all perfectly:

“It’s not a nice relationship. It’s just biology.”