The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

nothing of the sort has ever been seen in bacteria. They proliferate by dividing, and then can do so vigorously and indefinitely without ever betraying that need for the refreshment that sexual union affords.

Considering the great number of different kinds of bacteria that are known, the forms that they may assume are very limited. Their names are based in part on the shapes of their bodies. A rod-shaped bacterium, for example, is called a bacillus (or stick), a spherical bacterium is a coccus (or berry), and a curved or wriggling bacterium is a spirillum. Some bacteria have long, lashing whips—either one, or a tuft at one end of the body, or a number bristling all over the surface of the creature. And they may stick together in bunches or clusters or chains whose shape is characteristic of the species. But, nevertheless, their range of form is very limited. Often quite distinct species of bacteria look so much alike as to be distinguishable only by the effects that they produce. There are, for example, three kinds of bacteria almost indistinguishable in appearance, with globular bodies that form little chains; the first causes the souring of milk, the second flourishes in our blood and causes a rapidly fatal infection, while the third is a perfectly harmless creature nearly always present in the healthy human mouth.

It is because of their physiological diversity that the bacteria are remarkable. There are bacteria which show the profoundest departures from the normal chemical method by which animals and plants get their energy. Their engines run on unusual fuel. Some get energy by oxidizing simple inorganic compounds—for instance, by turning the inflammable, foul-smelling gas sulphuretted hydrogen into sulphuric acid. Others have to use sodium thiosulphate to make sulphuric acid. Others, again, can energize themselves by burning ammonia. And by means of these devices the creatures can build up the complicated organic substances of which their bodies, like any other living bodies, consist, from such simple ingredients as water and carbon dioxide. As a further curiosity, there are bacteria which can live like yeasts, without oxygen, and there are even bacteria which shun oxygen and are poisoned by it. But most bacteria are more conventional than this ; they burn fuel and need oxygen and nourish themselves like animals or fungi on the living or dead bodies of other creatures.

We noted in an earlier section how difficult it is to classify micro-organisms into cate-

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THE SCIENCE OF LIFE

CHAPTER 6

gories. The separation of bacteria from animals and plants is another example of this difficulty; here, again, there are border forms. There are, for example, the so-called filamentous bacteria which are very like the humbler blue-green alge, except that they are not blue-green. And there are the spirochetes, a group which includes the parasites responsible for syphilis, that are in many respects intermediate between bacteria and the flagellate protozoa. It is still uncertain whether they are most nearly related to the former or to the latter. Moreover, in some ways, bacteria are very like yeasts. The bacteria, then, merge into the simpler animals and plants just as the latter two groups merge into each other. Usually they are regarded as being less animal-like than vegetable-like (they are described, for example, in standard textbooks of botany but not of zoology), but this again is a nice question ; there are bacteria that wriggle very much like animals and nourish themselves on organic food not at all after the typical plant manner. Indeed, the line between animal and plant is such a vague one that the question has little meaning. At this level. of being, the distinction of plant and animal, like the distinction of sex, has practically disappeared.

It would be difficult to overstate the practical importance of these creaturesinvisibly small, but swarming in incalculable multitudes. There is hardly a human activity that is not affected in some way by bacteria, acting either as friends or as enemies. There is hardly a nook or cranny on the surface of the earth that they do not inhabit. They are found in the air, in the soil, in the waters. The bacteria in the air are chiefly carried on particles of dust ; the air of an assembly hall in Paris was found to contain nearly ten thousand bacteria and two thousand five hundred mould-spores per cubic yard. The decay of dead things is in large measure due to bacteria. They penetrate into the bodies of living creatures and there behave in various ways, some being harmful to their hosts and some, as we shall learn, doing good. ‘They teem in the soil, and especially in the upper few inches of it ; the smell of a newlytilled field is due to the products of countless soil-bacteria. A lifeless world, we may note, would be completely odourless except perhaps for such cold, unemotional scents as chlorine and ozone and the oxides of nitrogen. And most of the odours in this world are due to microscopic creatures. There are, of course, many flowers and