252 ESSENTIALS OF BACTERIOLOGY 



Form. Small narrow rods about three times longer than 

 they are broad; usually found in pairs. The ends are 

 rounded. 



Properties. They are very motile; do not liquefy gelatin. 

 A bluish-gray pigment is formed outside of the cell, 

 around the medium. The less alkaline the medium, the 

 deeper the color. It does not act upon the milk otherwise 

 than to color it blue. 



Growth. Grows rapidly, obligate aerobe. 



Colonies on Plate. Depressed center, surrounded by ring 

 of porcelain-like bluish growth. Dark-brown appearance 

 under microscope. 



Stab-culture. Grows mainly on surface; a nail-like growth. 

 The surrounding gelatin becomes colored brown. 



Potato. The surface covered with a dirty blue scum. 



Attenuation. After prolonged artificial cultivation loses 

 the power to produce pigment. 



Staining. By ordinary methods. Gram positive. 



Red milk and yellow milk are due to other chromogenic 

 organisms, as, for instance, B. erythrogenes. 



Examination of Milk. American Standard. Some bac- 

 teria are found in all milk as ordinarily handled. Strepto- 

 cocci and colon group, when present, always regarded with 

 suspicion. A high-cell leukocyte count, when accompanied 

 by chain bacteria, is an indication of udder disease. There 

 should be several samples taken one week apart and an 

 average made. Bacteria present may be counted in one of 

 three ways. 



Stewart-Slack Method. Centrifuge i to 2 c.c. of milk; 

 smear sediment on slide, and stain with Jenner or Wright 

 stain and count bacteria in field. 



Prescott-Breed Method. In a special capillary tube y^-jj 

 c.c. of milk is sucked up and spread over a square centimeter 

 on a microscopic slide, dried and fixed with methyl-alcohol. 

 Flood with xylol to dissolve fat, stain with methylene-blue 

 or Jenner, and decolorize slightly with alcohol. Focus 15 

 mm. of the specimen and count bacteria and cells present. 



