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

THE LOWLY AND MINUTE

animals with characteristic smells for which they are directly responsible, but the smells of decaying things (not really a sign of death, but a sign of teeming bacterial life), the smell of a sea beach, the smell of a wood in winter, the smell of a stable, the smell of the earth after rain—these are all bacterial odours, produced by creatures too small to be seen. The most subtle and complicated and delicate perfumes that we know are produced in this way. The bouquet of a wine is due to the swarms of micro-organisms that have thrived in it, and it is at least possible that the subtler flavours of choice cigars are due to bacteria that have lived on the heaps of tobacco leaves when they were put out to dry.

The bacteria, indeed, are on the whole a much-abused group. They are usually spoken of simply as creatures which cause disease ; their more beneficial activities are less widely noised. It is, of course, true that there are disease-producing bacteria, and very important onescholera, typhoid, diphtheria, plague, pneumonia, tuberculosis—these are only a few of the major disorders for which bacteria are responsible. We shall return later to this side of their activities. But for the present we will note the existence of other kinds of bacteria, even friendly bacteria. For it is no exaggeration to say that, without bacteria, life on this world would be quite impossible.

Bacteria, for example, are very important in agriculture. There are a number of kinds of bacteria living on the earth which lay hold of the nitrogen gas in the air and turn it into substances in the soil which can be absorbed and used by plants. ‘The fertilizing action on the soil of crops of leguminous plants, such as beans and clover, is due to the fact that their roots are always swollen and warty from the presence of swarms of symbiotic bacteria—parasites, if you like, but parasites which benefit their host more than they harm it, for they use nitrogen from the air, which no higher plant can do, and build from it organic substances that the host absorbs. Again, bacteria are both enemies and allies to the dairy farmer. The taste of butter depends to a large extent on the bacteria that inhabited the milk in the time between milking and churning; there are some bacteria that make pleasant flavours and

Fig. 120. The bacilli forming chains on the left are two of many kinds that turn

wine into vinegar. ; lashing flagella, which causes butter and other fats to turn rancid.

others that make foul. For this reason it is customary in scientific butter-making to sterilize the milk as soon as possible after milking and then to add a “starter °a culture of the right kind of organism. In this way one is certain of getting a pleasant flavour. Moreover, the ‘starter’? usually includes the bacteria that turn milk sour, because it is found that the acidity of sour milk prevents the growth of undesirable bacteria. Further, the flavours and distinctive properties of cheeses (even their contained air-bubbles) depend on the organisms (chiefly bacteria and _ yeasts) that have inhabited them during the ripening

Three kinds of bacteria of economic importance.

On the right are two specimens of a bacterium with

process. The characteristic flavours of cheese from different districts, like the characteristic flavours of wines, are due to the fact that each district has its own races of the appropriate micro-organisms. Vinegar is made by the action of bacteria which turn alcohol into acetic acid; the slimy mass known as “‘ mére de vinaigre ” consists of these organisms embedded in the slime they produce. And many of the processes in leather mianufacture, such as_ the “ sweating ” process, which softens the hairs so that they are readily scraped off the skins, are really carefully watched bacterial decay. On a higher plane, bacteria have played a part in strengthening religious faith. ‘The prodigy of the bleeding Host, in which 183