CHAPTER VIII 



THE BACTERIA AND VIRUSES 



There is probably no group of organisms which has a closer association 

 with man than the Bacteria. Owing to their ubiquitous occurrence he is 

 brought into constant and unavoidable contact with them. They are to be 

 found on all external parts of the human and animal body, and the respiratory 

 and digestive passages contain a large and varied assemblage of forms, some 

 of which are specially characteristic of these areas. Many of the diseases 

 from which we suffer are brought about directly or indirectly through their 

 influence, so that Bacteriology has become an important part of medical 

 science. 



Only a minority of Bacteria are harmful, however. The decomposition 

 of organic substances which they bring about is essential to the normal 

 circulation of nutrient materials, and it is not too much to say that but for 

 their activities the continuance of life would be impossible. Man has also 

 learned to take advantage of their chemical powers. He has utilized some 

 fermentation processes from time immemorial, but in modern times Bacteria 

 are being brought increasingly into use as agents in the manufacture of many 

 industrial products, for which the rapidity and cheapness of their actions 

 recommend them. 



The real nature of Bacteria has only been understood comparatively 

 recently, and even at the present time their structure is still obscure. This 

 is chiefly due to their extremely small size, for they are the smallest of all 

 living organisms. An average bacterial cell is only one twenty-thousandth 

 of an inch (1-25^) in diameter. There is considerable variation in this 

 respect, the largest Bacteria ranging up to as much as 20/x in diameter, while 

 the smallest are no more than o-i^a. Organisms of this order of minuteness 

 can only be observed with the highest available magnifications, while the 

 smallest lie at the extreme limit of microscopic visibility. Naturally their 

 study therefore was impossible with the older forms of microscopes. 



The relationships of the Bacteria are uncertain. The great majority are 

 unicellular and possess no chlorophyll, their mode of nutrition being either 

 parasitic or saprophytic. From this we might conclude that their nearest 

 allies are the Fungi, but it is impossible to point to any group of the Fungi 

 with which they show any relationships. Indeed in some ways they show a 

 nearer approach to the Cyanophyceae among the Algae, which might have 

 had a common ancestry with them. At present we can only regard them 

 as a remarkably isolated group. 



In spite of their small size it would be a mistake to assume that they are 

 primitive organisms ; on the contrary they are highly specialized to their 



343 



