CHAPTER XL VI 



SACCHAROMYCETES 



(BLASTOMYCETES) 



Yeasts are widely distributed in nature, having much the same occurrence 

 as molds. They are the common cause of fermentation and with the exception 

 of a few of the pathogenic species produce an endo-enzyme that splits sugars 

 into alcohol and CO2- In addition many of them produce other ferments that 

 reduce starches and other complex substances. 



Torula or wild yeasts are chiefly of interest as frequent and annoying 

 contaminants. 



The cultivated varieties play an important role in various industries, espe- 

 cially wine-making, brewing and baking. 



As is the case with bacteria and hyphomycetes 

 only a few yeasts are pathogenic. 



Morphology. Saccharomycetes as commonly ob- 

 served are round or oval unicellular organisms, 

 presenting a distinct cell wall some species having 

 a double, concentric wall. The protoplasm generally 

 contains numerous small structureless granules, and, 

 less frequently, one or several vacuoles. 



They occur singly, in chains and irregular masses. 

 As a whole, individual cells are sufficiently larger 

 than cocci and the spores of molds to be distinguish- 

 able from them on sight, but exceptions to this occur: 

 some are as little as 0.5/4 in diameter, they average 

 10 /* to 40 /i in diameter. 



Different species differ in size but individual differences in size in various 

 cultures of a single species under different conditions is great. 



Saccharomycetes are non-motile and quite distinctly discernible when not 

 stained. They stain readily with the usual dyes and are Gram positive. 



Growth. There are both aerobic and anaerobic species. Isolated patho- 

 genic species are aerobic and grow well in all the ordinary culture media. Incu- 

 bated at 37C., growth becomes apparent in from 24 hours to 5 weeks and con- 

 tinues for a long time. 



Young colonies on agar are round, dry, smooth, waxy, white or yellowish, 

 firm and adherent and may attain a diameter of i centimeter. Later they 

 become wrinkled, moist and develop aerial hyphae. 



Reported studies of pathogenic species of saccharomyces indicate that many, 

 if not all, under certain conditions form mycelia and aerial hyphae. 



164 



ff. 



FIG. 33. YEAST CELL. 

 (Marshall.) 



