CHAPTER XII. 



YEASTS, MOULDS AND STREPTOTRICES. 



The study of micro-organisms as the cause of diverse 

 fermentations and of disease would be incomplete if limited 

 to the group of bacteria. While it is true that the latter 

 are the most frequent causes of such conditions, it must be 

 remembered that there are other organisms entirely distinct 

 from bacteria which can induce similar changes. The 

 yeasts, moulds and the streptothrix forms are extremely 

 important in this respect. Furthermore, in practical lab- 

 oratory work yeasts and moulds are frequently met with 

 as contaminations of plate or tube cultures. Conse- 

 quently, it is advisable to understand the general charac- 

 teristics of these organisms in order to facilitate their 

 recognition. 



Yeasts. 



These are microscopic unicellular plants which differ 

 from bacteria in size and in manner of multiplication. 

 They are usually about 6 ii thick; in other words, they are 

 about as large or even larger than the red blood cell. Con- 

 sequently, a very simple examination will be sufficient to 

 distinguish the yeasts froin the bacteria. 



While bacteria multiply by dividing in two equal parts 

 the yeasts increase in number by a process of hudding. 

 That is to say, at some point on the surface of the cell a 

 minute protuberance develops which gradually enlarges and 

 forms a daughter cell. This budding may occur at several 

 places in the same cell and the young cells, thus formed, 



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