34 MORPHOLOGY AND CULTURE OF MICROORGANISMS. 



to four spores are formed in a cell. These spores are often produced in 

 the fermenting liquid. The fermentative power is high and a large per- 

 centage of alcohol may be formed. 



Several other species of this genus have been isolated from grapes and 

 from Jamaica rum. 



PSEUDO YEASTS. Budding cells often occur in fermenting liquids 

 which have all the characteristics of yeast except that of producing endo- 

 spores. They are grouped together under the name of Torula. They 

 are usually small, spherical or slightly elongated. Some species produce 

 a little alcohol and some none. They seldom occur in sufficient quantities 

 to be harmful and one form is accredited with producing the special 

 flavor of some English beers. 



The groups of forms included under Mycoderma resemble yeast in 

 form but produce little or no alcohol, are strongly aerobic and do not pro- 

 duce endospores. Their most noticeable characteristic is that they grow 

 only on the surface of the liquid, where they produce a thick film. They 

 cause complete combustion of the alcohol and other organic matters, 

 making beer and wine vapid and finally spoiling them. 



CULTURE OF YEASTS. 



PURE CULTURES. Yeast can be properly studied only in pure cultures. The 

 media used are either the liquids in which the yeasts are to be used as wort, cider, 

 grape juice, or a special medium devised for a special investigation. An example of 

 the latter is Laurent's medium: 



Ammonium sulphate, 4. 71 g. 



Potassium phosphate, .75 g. 



Magnesium sulphate, . 10 g. 



Water, i L. 



To this is to be added any carbohydrate to be studied. Media may be made solid 

 by the addition of gelatin or agar. 



Pure cultures can be made, rarely, by inoculation from a naturally pure source, 

 such as the sporangium of a Mucor. 



Physiological Separation. The first attempts at purifying mixed cultures were by 

 means of physiological differences. Pasteur freed yeast from bacteria by growing it 

 in a medium containing 2 per cent of tartaric acid. Effront used fluorides in the same 

 way. These methods may be made more effective by repeated transfers of the culture. 

 Each transfer will contain a larger proportion of the form most suited to the condi- 



