LITMIR.BIZ

1 Travel Companions

      We humans have managed to escape from our predators. In the security of urban settings, we don’t have to worry about being devoured by lions, tigers or other carnivores that are stronger and faster than we are. As a result, we’ve successfully occupied all the ecosystems and multiply like no other animal has ever done before. We therefore tend to think we are invulnerable, at least when it comes to competing with other inhabitants of the Earth for our everyday survival.

This idea couldn’t be more wrong. We are immersed in a constant struggle against an adversary so powerful it can eliminate the human race in a matter of months. Indeed, it’s been on the point of doing so more than once. I refer to microorganisms, our invisible enemies, the millions of microscopic beings sharing our habitat, the bacteria, viruses and many other minuscule life forms with which we have a very special love–hate relationship. Thanks to them, we’re alive. Because of them, some 14 million people die every year. Why are we still vulnerable to such infinitesimal organisms? This chapter will introduce the main kinds of microbes and describe how they interact with humans.

They were here first

      Microbes are the oldest inhabitants of this planet. They’ve been around for between 3,000 and 4,000 million years, but humans didn’t discover them until a little more than a century ago when science was sufficiently advanced to let us see them close up with the aid of a microscope. During the first 2,000 million years, microbes, and specifically bacteria, had the Earth to themselves. They were the first to appear and will doubtless be the last to leave because they are not only the most diverse form of life but are also the one that most easily adapts to any conditions. If a catastrophe wiped out most life on Earth, they would probably be the only survivors.

      There are more than a million kinds of microorganisms, most of them inoffensive. The main ones are bacteria and viruses, but to these must be added certain fungi, algae and amoebas. In both number and weight, microorganisms are predominant among living beings. If we could put all the microbes on one side of the scale and all the animals on the other, the microbes would weigh twenty-five times more. The fastest growing microbes duplicate every thirteen minutes, and the slowest every fourteen days. At this rate, if a single bacterium had all the possible nutrients and the right conditions, it could generate a colony that would weigh as much as the whole Earth in three days.

      We shouldn’t forget that it’s thanks to microbes that this planet is habitable. Some 2,700 million years ago, there appeared a certain kind of bacterium that was able to use sunlight to transform water and CO₂ into energy. A secondary effect of this process, which is called photosynthesis, is generation of oxygen. Like toxic exhaust fumes given off by a car engine, the oxygen kept accumulating in huge quantities and ‘contaminating’ the Earth’s atmosphere. This inadvertent pollution ended up being providential for us as it permitted the appearance of a new class of beings – humans among them – that needed oxygen for their basic functions. To this very day, bacteria are important for keeping the planet in balance. Without them, life on Earth would be wiped out.

Space: the last frontier

      Bacteria could also survive in outer space. Some very tiny species have been found (among them Herminiimonas glaciei, which was discovered in 2009). They can endure for more than 120,000 years under layers of ice three kilometres thick, practically without oxygen and nutrients. These conditions are very similar to those that might be found outside our planet.

      We should be grateful for other things, too. Bacteria are also our ancestors. As I said, life on Earth was at first limited to minuscule single cell organisms. They gradually began to come together in groups of cells that, acting in concert, went on to specialize in different functions, now in the form of what are known as multicellular organisms. This is the path that led to the fabulous biological diversity we have today. As proof of our humble origins, there are still structures in human cells that come directly from those original bacteria. And they are essential for human life.

Peaceful passengers

      We shouldn’t necessarily see microorganisms as a threat. On the contrary, coexistence with many of them is highly beneficial for humans and determines proper functioning of the organism. The human body, one of the most complex multicellular organisms in existence, consists of approximately 100 billion cells. But this needs to be clarified: I mean 100 billion human cells. If we are to be exact, we also need to count all the microorganisms that inhabit us. Initially, it was calculated that they might be ten times more numerous than our own cells, but more recent data suggest that a closer estimate would be one microbe for each human cell. In any case, we can venture that the human body is colonized by millions of microorganisms, of some 400 different species, which normally don’t cause any illness. Put together they would weigh a kilogram. These data are mind-boggling, enough to make us wonder what a human being really is. A mixture of highly specialized cells and microbes that live in harmony? From this standpoint, we are perhaps nothing more than a walking ecosystem in which a series of microorganisms peaceably survive.

      The microbes that are always with us are not only freeloaders but ‘stowaways’ that are very important for our metabolism. Humans, like all other animals, depend on them to survive. From them, we obtain vitamins, nutrients and protection against infections caused by their more toxic kin. Cows, for example, couldn’t ingest grass without the help of the bacteria they have in their digestive tract, and neither could termites benefit from the cellulose in wood. There are plants we use as food, peas and beans for example, that need bacteria in order to fix the essential nitrogen from the sun.

      Further proof of their importance is that it is thought that, when

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