63 



Boil ingredients until dissolved and make up any loss due to evapora- 

 tion. Place measured quantities in flasks and sterilize at 15 pounds for 

 15 minutes. 



Just prior to use, add to each 100 c. c. of the melted agar, prepared 

 as above, the following constituents: 



Sterile (20%) lactose solution 1 gm. or 5 c. c. 



Aqueous (2.0%) eosin (yellowish) solution.. ..2 c. c. 

 Aqueous (0.5%) methylene blue solution 2 c. c. 



Pour medium into petri dishes, allow, it to harden in incubator and 

 inoculate in the ordinary way. Smearing the surface with a glass rod 

 seems preferable to the streaking method sometimes employed. 



There is no adjustment of reaction and nitration of medium is not 

 necessary. 



Test tubes may be substituted for petri dishes if desired. The value 

 of such a change is (1) the reduction of expense, as only 3 or 4 c. c. of 

 medium is needed for every test tube while about 15 c. c. is usually em- 

 ployed with petri dish, and (2) test tubes may be stored for long periods 

 whereas the medium in petri dishes would have to be prepared at intervals 

 of a week or less. 



TABLE XXVII. DIFFERENTIATION OF BACT. COLI AND BACT. AEROGENES 

 ON EOSIN-METHYLENE BLUE AGAR. 



(1) Two other types have been occasionally encountered: One resembles the type de- 

 scribed, except that there is no metallic sheen, the colonies being wine colored. The other 

 type of colony is somewhat larger (4 m. m.) grows effusly, and has a marked crenated or 

 irregular edge, the central portion showing a very distinct metallic sheen. These two 

 varieties constitute about 2 or 3 percent of the colonies observed. 



(2) A small type of aerogenes colony about the size of the colon colonies, which 

 shows no tendency to coalesce, has been occasionally encountered. 



