6 APPLIED BACTEKIOLOGY 



Twenty years ago it would have seemed chimerical to 

 have said that we could cultivate at will, in the laboratory, 

 the very living essence and cause of such diseases as 

 cholera, diphtheria, typhoid, tuberculosis, and others, as a 

 gardener does his plants, and from the knowledge thus 

 gained plan new and efficient methods for combating and 

 preventing disease. This new field of study opened up 

 during the last twenty-five years by medical men such as 

 Davaine, Koch, Virchow, Klein, Sanderson, Lister, Loffler, 

 and by chemists like Pasteur, Kitasato, Frankland, and 

 others, has not yet been by any means perfectly ex- 

 plored. The results so far obtained, however, are so im- 

 portant that it has become an absolute necessity for those 

 concerned in the study and treatment of disease to have 

 some knowledge of this branch of science. But enough 

 has been said to pave the way for the better appreciation of 

 the marvellous manner in which these investigations have 

 led and are leading to the most important and far-reaching 

 results in medicine. 



Structure of Micro-organisms. — The bacteria appear under 

 the microscope as pale, translucent bodies ; they consist of 

 unicellular organisms composed of protoplasm surrounded 

 with a membrane, or skin, of a body allied to cellulose. 

 This outer skin swells up in some cases to form a jelly-like 

 casing, by which the internal protoplasm is covered. 



The cells sometimes contain a nucleus which is readily 

 stained with the usual staining reagents. 



Types of Organisms. — The organisms vary very much in 

 shape and size. Many are globular or spherical in shape, 

 and are generally known as micrococci ; others, on the other 

 hand, are rod-like bodies, hence are termed bacilli ; whilst 

 others, having a spiral or corkscrew shape, are known as 

 spirilla. Some spirilla sometimes appear in a much 

 shorter form, and resemble the shape of a comma. Very 



