A Bit of History: Microbes and Humans

Microbes from hell ... Everyone has heard of microbes, and almost everyone has a vague idea of what the term represents: very small creatures that can do a lot of harm. If they're in hell, why not let them stay there? To get to the heart of the subject, we'll have to move beyond these prejudices, get to know microscopic life better, and look a little more closely at what the word "microbe" really means.

Did You Say "Microbe"?

For a long time, our knowledge of the living world was limited to macroscopic organisms — that is, those visible to the naked eye. The living world was generally divided into two groups: animals and plants. It was only in the seventeenth century, with the invention of the microscope, that the Dutch clothing manufacturer and microscope maker Antonie van Leeuwenhoek (1632–1723) discovered the existence of the then unimaginable world of microscopic living creatures. Two centuries later, Charles Sedillot, a contemporary and colleague of Louis Pasteur, would dub them "microbes," from the Greek bios for "life" and mikros for "small" (figure 1.1a).

Their sizes proved to vary considerably, with the largest among them measuring a few tenths of a millimeter in length and the smallest being no bigger than a micron (less than a thousandth of a millimeter). Very soon it appeared that these minuscule beings, invisible to the naked eye (or even the most powerful magnifying glass), were present everywhere: in water, in the air, in the ground, and even in the human body. In the nineteenth century, people were frightened to realize that these microbes, far from representing just a simple curiosity of nature that could be ignored, were the cause of many of the scourges that had stricken humanity for centuries: plague, typhus, tuberculosis, syphilis, and so on. Today we know that most microbes are innocent, and some are even our benefactors — in particular, those that live in our intestines and protect us from attacks by their pathogenic siblings (other microbes that are responsible for illnesses) and those that help us digest food. In the past few years, after sequencing the human genome, scientists have focused on the sequencing of the genomes of all microbes — the "microbiome." In fact, we contain ten times more microbial cells than human cells; these microbes belong to a considerable number — several thousand — of different species. Scientists have recently discovered that these microbes might even manipulate the brain: for example, by producing molecules that make us want to eat what they need. If we get to know them better and learn to manipulate them in turn, we should be able to derive enormous benefits in the realm of human health. Nevertheless, the term "microbe" in the collective subconscious retains a rather fearsome connotation. (I hope that after reading this book you will have changed your mind a bit.)

In general, biologists no longer use the word "microbe" (perhaps due to its rather pejorative nature, or because it doesn't sound very scientific). Instead, they use the word "microorganisms" to describe the living beings that can be observed only under a microscope. For a long time, scientists were unable to situate these organisms in relation to other representatives of the living world. They even tried to force them into the old traditional framework by seeking characteristics in their appearances that might connect them to animals or plants. If a microbe was able to move on its own, thanks to a flagellum, it was a minianimal, and it became a subject of study for zoologists. If, on the contrary, it remained immobile most of the time and was made up of a cell surrounded by a thick wall, it was a miniplant, and was of concern to botanists. It was from that time that biologists, in order to name microbes, began to use binomial nomenclature (in which every living being is designated by a genus name and a species name), invented in the seventeenth century by the great Swedish naturalist Carl von Linné (Linnaeus) for animals and plants. For example, the bacterium responsible for plague was named Yersinia pestis, a name that recalls the illness for which it is responsible and honors the one who discovered it: Pasteur's young colleague Alexander Yersin. The name of the bacterium used as a model by many biologists, Escherichia coli (commonly called E. coli), recalls its preferred habitat, our colon, and honors another microbiologist, Theodor Escherich, who was working in Vienna at the beginning of the twentieth century.

A decisive tool in the study of microbes, the petri dish (figure 1.1d), was invented in the 1880s in Germany by the team of Pasteur's German rival, Robert Koch, who discovered Mycobacterium tuberculosis, responsible for tuberculosis. The story goes that it was while observing colonies of bacteria and mushrooms on the surface of slices of potatoes, the leftovers of a meal eaten quickly between two experiments, that the German scientists understood that each colony (circular spots of varying size, of different colors, whose edges were more or less sharp) corresponded to a clone of billions of microbes, all of which came from a single cell, bacterium or mushroom, that had settled on the surface of the potato. Julius Richard Petri, one of Koch's assistants, had the idea of filling a circular glass dish with a jellylike substance, agar, containing a culture medium to obtain bacterial colonies isolated on the surface of the agar (figure 1.1d). He had only to dilute the bacterial cultures sufficiently to place a few bacteria on the petri dish. This method, still used today, makes it very easy to isolate pure cultures of bacteria, which can then be characterized; this enables species to be defined. The glass of Petri's dishes has simply been replaced with plastic; agar, initially used by the wife of another of Koch's assistants to cover the surface of her jam jars to keep them sterile, is also used today.

Microbes Are Cells

During the nineteenth century, scientists gradually came to understand that all living beings, which at the time they classified in the category of either animals or plants, were formed by the assembly of a very great number of cells (a hundred thousand billion for a human being), with each cell forming the morphological unit — the elementary brick, in a sense — of the living parts of organisms. Observed under an optical microscope, the cells appeared as masses of a gelatinous substance (cytoplasm) surrounded by a thin membrane and containing a spherical central region enclosing nucleic acids (observed by using certain dyes): a sort of nucleus. Animals, plants, most algae, and mushrooms were thus formed by a great variety of organs and tissue, with each tissue composed of myriad cells. It was then noted that most microbes were formed by a single cell. Alongside the division of the living world into...