74 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



They are usually smaU, spherical or slightly elongated. Some species 

 produce a httle alcohol and some none. They seldom occur in suf- 

 ficient quantities to be harmful and one form is accredited with pro- 

 ducing the special flavor of some EngHsh beers. 



The forms included under Mycoderma resemble yeast in shape 

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

 produce 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 or Yeasts 



Pure Cuxtures. — Yeast can be properly studied only in pure cultures. The 

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

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

 the latter is Laurent's medium: 



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 difEerences. Pasteur freed yeast from bacteria by growing it i 

 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 conditions, until finally a pure culture may be obtained. The principle of these 

 methods is of great use in practical fermentation, but is of little use in rigidly separat- 

 ing forms. Methods of general application for the latter purpose must be such that 

 a single cfeU can be isolated in a sterile medium and a culture propagated from 

 this single cell. ' 



Separation by Dilution in Liquid Media.— A mixed culture is diluted with steri- 

 " lized water until on the average every two drops contain one cell. A large number 

 of 1 flasks of a sterilized nutrient medium is then inoculated from the dilution, one 

 drop in each flask. If the dilution has been properly made, about half of the flasks 

 will remain sterile and half will show growth. Many or most of the latter will 

 contain pure cultures. 



Separation by Dilution in Solid Media. — If we dip a sterilized platinum wire into 

 a mixed culture and then draw it repeatedly over the surface of a solid culture medium 



