394 BACTERIOLOGY. 



man and animals are due to these organisms. Other 

 pathogenic species have been found, though but rarely, in 

 man and in the lower animals. Many of the streptotrices 

 are found in the air and in water. The members of this 

 group, like the moulds and yeasts, are stained by Gram's 

 method. 



Laboratory work. The student will inoculate ordinary gelatin, or 

 better glucose gelatin, with the several yeasts. The Saccharomyces 

 cerevisiae, or the white yeast is to be poured into Petri dishes, while 

 the red or black yeasts are to be used for making Esmarch roll- 

 tubes. 



Ordinary baker's or brewer's yeast should be examined in the 

 hanging-drop and in stained preparations. If compressed yeast is 

 employed it will be well to distinguish the budding yeast-cells from 

 the very large starch grains that are present. 



The yeasts are fixed on the cover-glass and stained in the same 

 way as bacteria. 



The moulds grow best on slightly acid media. They can, there- 

 fore, be grown to advantage on the surface of potatoes or on moist 

 bread. The addition of about 2 per cent, of cane-sugar or of glycerin 

 to agar renders this a very good medium. The Oidium lactis should 

 be grown on gelatin plates. 



Preparation of Bread-flasks. 



A bread powder is usually first prepared. For this 

 purpose the bread is cut up into slices and heated in a 

 dry-heat oven till over-toasted. It is then finely crushed 

 and preserved in a bottle. The dry powder can be kept 

 indefinitely in a well stoppered bottle. 



Six small, 30-50 c.c. Erlenmeyer flasks' are cleaned, 

 plugged and sterilized. The dry, powdered bread is filled 

 into each flask to a depth of about i inch, and water is then 

 added so as to render the mass thoroughly moist. The 

 bread-flasks are now steamed for half an hour on each of 

 three successive days. 



