BACTERIA 27 



organisms. They take an almost equal part in our life and in 

 our death, for whilst some are beneficent, many are malignant. 

 In a later chapter we shall see what a large part they play in 

 the soil, and we shall see how certain plants could not live 

 without their aid. In shape they may be spherical. Coccus; 

 or shaped like a rod, Bacillus; or spiral. Vibrio or Sjjirillum, 

 Sometimes they are grouped in packets, as in Sarcina. These 

 obscure plants are sometimes motile, being provided with 

 cilia which vary in number and position. Although there are 

 traces of nuclear matter, they have no definite nucleus. They 

 multiply by dividing in two, and at an appalling rate. The 

 ordinary hay bacillus, B. subtilis, divides about three times 

 an hour, so that in the course of eight hours a single specimen 

 will have produced over sixteen million offspring. They are 

 very susceptible to light and are readily destroyed by the blue- 

 violet rays of the spectrum. This, of course, has a vast influence 

 on public health, as many of the bacilli are disease-causing 

 germs. Their most important function is that, like other 

 parasites or saprophytes, they break down complex organic 

 compounds into simple ones; and although their methods of 

 do.ng this vary widely, the final products are chiefly water 

 and carbon dioxide. In the course of breaking down these 

 complex organic compounds many by-products are produced, 

 some of which are extremely harmful, and even fatal, to man 

 and other animals. 



They live in all sorts of queer places. One genus which 

 oxidizes iron sometimes chokes the water-mains of large cities. 

 Others will live in sulphur springs and separate the sulphur 

 from the sulphuretted hydrogen which makes the waters of 

 Harrogate, for instance, so unpalatable. They are the essential 

 organisms of putrefaction and they play a large part in com- 

 mercial products. The flavour of tobacco, butter and cheese 

 is dependent on them, and they take a large part in the 

 preparation of flax and hemp and indigo. Though we cannot 

 get on without them, in a large number of cases we cannot get 

 on with them. They are practically omnipresent, and we 

 breathe them with every breath of air. They infest all the 

 waters of the world, and are found throughout the superficial 

 layers in the soil. 



