340 BACTERIOLOGY. 



are nearly circular. With a high-power lens the edges 

 show sprouting bacilli. The colonies are gray or 

 grayish-white by reflected light and pure gray with 

 an olive tint by transmitted light. 



The growth of the diphtheria bacillus upon agar 

 presents certain peculiarities which are of practical 

 importance. If a large number of the bacilli from a 

 recent culture are implanted upon a properly prepared 

 agar plate a certain and fairly vigorous growth will 

 always take place. If, however, the agar is inoculated 

 with an exudate from the throat which contains but 

 few bacilli, no growth whatever may occur, while the 

 tubes of coagulated blood-serum inoculated with the 

 same exudate contain the bacilli abundantly. Again, 

 agar prepared from broth made from different speci- 

 mens of beef or to which different peptones have been 

 added, varies as to its suitability for the growth of the 

 bacilli. Because of the uncertainty, therefore, of ob- 

 taining a growth by the inoculation of agar with bacilli 

 unaccustomed to this medium, agar is a far less reliable 

 medium than blood-serum for use in primary cultures 

 for diagnostic purposes. If used the agar should at 

 least be tested by means of a culture before being em- 

 ployed. A mixture composed of two parts of a 1J 

 per cent, nutrient agar and one part of sterile ascitic 

 fluid makes a medium upon which the bacillus grows 

 much more luxuriantly but not so characteristically. 

 The mixture is made by adding the warmed ascitic 

 fluid to the tubes containing the melted agar cooled to 

 60. After shaking the Petri plates are filled. 



The Isolation of the Diphtheria Bacillus from Plate 

 Cultures. Nutrient plain or glycerin-agar, with or 

 without the addition of ascitic fluid, is, however, the 



