They are so small, we rarely give them a thought and yet they are out there in numbers too enormous for us to comprehend. In fact, they are estimated to constitute no less than half of the total mass of all life on Earth. So, when you next look out over the landscape covered by plants and teeming with insects, birds and animals, think of the roughly equivalent mass of life that you don’t see. This invisible other half of life is the microbial world, made up of a great variety of microorganisms, so called because they typically measure only 0.5 to 5 micrometres across (that is about one-thousandth to one-hundredth the width of the 0.5 millimeter sized dot atop the letter i).  Although they are nearly everywhere, to see them requires a microscope (although some exceptionally large species are visible to our naked eye, such as Thiomargarita namibiensis, which lives by accumulating spheres of elemental sulphur in its elongate cells in shelf sediments offshore Namibia in southwest Africa).

For what they lack in size, microorganisms make up for by their ubiquitous multitudes: 10 to 1000 billion in every teaspoon of soil, a billion or so in every 20 drops (one millilitre) of water, and each of us hosts on the order of 40,000 billion microbes thriving inside our body and out. Microbes have evolved to eke out a living in nearly every habitat on Earth, even those we don’t normally associate with the living world: kilometres below the surface in deep rock fractures, and deep below the surface of massive ice sheets. Microbes can even survive in the vacuum of space, and include those left behind on the Moon by astronauts (most likely in a dormant, resting state). We tend to consider microbes as dangerous and something to kill off indiscriminately with antibiotics and disinfectants. Although some are pathogens, most microbes are benign and many others beneficial, if not essential to our well-being. We are only beginning to appreciate the roles played by the highly diverse microbes that thrive in our gut, our mouths and in every nook and cranny of our skin.