BACTERIOLOGICAL TECHNIQUE 19 



black. According to the form of the hyphae the moulds are 

 divided into four divisions, viz. : 



1. Mucorineae ; 



2. Aspergillinae ; 



3. Penicilliaceae ; 



4. Oidaceae. 



All three classes of organisms, bacteria, yeasts, and moulds, 

 occur very widely distributed in nature. They are always 

 most abundant where there is the needful food supply. It is 

 a matter of common knowledge that meat goes bad if long 

 exposed to the air, that jam if uncovered develops mould, 

 that milk becomes sour, that sewage or excretal matter becomes 

 offensive if allowed to accumulate. These changes are due to 

 micro-organisms either originally present in the decomposing 

 substance, or carried in air and deposited on substances capable 

 of putrefactive change, which themselves thus become sources 

 of infection. The presence of bacteria in the air can be demon- 

 strated by exposing a slice of potato for some time in a room. 

 In the course of forty-eight hours or so small spots or 

 centres of growth will appear, which can be recognised as 

 colonies of bacteria or as moulds by methods shortly to be 

 described. 



Certain organisms are capable of producing chemical 

 changes in the bodies of higher living organisms, and have been 

 found to accompany the development of specific diseases ; such 

 organisms are termed pathogenic. 



Other organisms perform exceedingly useful functions. 

 It is scarcely necessary to refer to the technical importance of 

 yeasts in the brewing industries. Special varieties of bacteria 

 are concerned in the production of vinegar and the ripening 

 of cheese, or are useful at certain stages in the manufacture of 

 leather in the tannery. The harmless disposal of refuse matter 

 from men and animals is effected largely by the activity of 

 bacteria, and the processes of agriculture are increasingly 



found to depend upon the activity of the organisms in the soil ; 



c2 



