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Recently, people have become more aware of the microbial world: the growth of antibiotic resistance, which threatens to roll back almost a century of human progress against disease; the use of genes taken from bacteria in creating genetically modified organisms; and research about the importance of bacteria to digestion and health.

It was against this background that Ed Yong wrote I Contain Multitudes, a book on microbiomes—the collection of microbes that live on or inside other organisms—and how they interact with their hosts.

The excellent thing is that it excitedly covers recent research, and yet, in every chapter, Yong is able to pull back and drop in words of caution on staying away from simplistic narratives about bad versus good bacteria, or bad bacteria infecting good hosts.What then are the narratives in the book if not simple ones of good versus evil?

As Yong takes pains to explain, few metaphors can capture the complexity of what is going on. But at the risk of incurring his displeasure, I will suggest that the one that comes the closest is the body as a city. It is a city with different neighbourhoods—gut, genitalia, etc.—in which a diverse and still uncounted number of species of microbes act as good residents, or commuters, or squatters. And sometimes, they act as urban planners, or politicians, or even gang members fighting each other for turf.

A microbe is not necessarily “good" or “bad". It acts in its self-interest, and the consequences of this on other microbes or on the host itself can be beneficial, or detrimental, or can have any combination of effects. The body is not just a body. It is a landscape, a setting for a magical spontaneous order to rival any complex economy.

The same microbe which steals its host’s food may also protect that host from more vicious parasites. A microbe which minds its own business in one host may turn out to be a powerful defence against viral infections when transplanted to another. Even that is an oversimplification, because it is rarely that one can speak of the effects of a single type of microbe. Instead, it is the collection of several species of bacteria in a particular environment, working together or against each other, that leads to an effect on their hosts.

This theme of bacteria working for themselves, but conferring extraordinary advantages or disadvantages on their hosts, pervades I Contain Multitudes. Yong has chapters which describe how bacteria can turn their hosts’ genetic programming on or off. The outcomes of this range from humans being less susceptible to allergies, to an ocean flatworm being able to regenerate its entire body from any chopped up segment. Other chapters deal with the use of bacteria to slow down the potential extinction of frogs across the US. Most excitingly for Indian readers, there is a section on how to infect Aedes mosquitoes with Wolbachia bacteria, and make it impossible for them to transmit dengue or chikungunya.

In an early Calvin And Hobbes strip, Calvin, attacked by existential dread, wonders why man was put on this world. Hobbes replies, beaming, “Tiger food." Seventeen years after Bill Watterson proposed that our highest purpose is to be food for big cats, Yong provides another option for finding self-fulfilment—we are also shelter for microbes.

Man’s search for meaning usually involves the discovery of universal truths, the uplift of other humans, or the creation of great art. We are brought up to find meaning in our identity or our actions. And to realize that our most significant accomplishment is to passively and unconsciously host trillions of organisms cuts us down to size. The realization that so much of what we consider intrinsic to ourselves—digestion, mood, or fitness—is controlled by these organisms is even more humbling. And yet, I Contain Multitudes provides us meaning, and perhaps even exaltation, by showing us that even if we are diminished in our actions, we are part of a larger and wonderful story. Our bodies are the Hastinapurs in which a biological Mahabharat plays out, free of morals, free of heroes or villains, but still grand and inspiring.

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