64 



The advantages claimed for the eosine-methylene-blue medium are (1) 

 ease of preparation, as neither filtration nor adjustment of the reaction 

 is required, and (2) relative permanency. (Petri dishes poured have 

 been used after a week or longer, if stored in the ice box) and (3) the 

 medium affords an excellent differentiation between Bact. coli and Eact. 

 aerogenes, as described in Table XXVII above. 



The method of preparation of the test tubes is briefly as follows : 

 The medium prepared as above is poured aseptically into sterile test 

 tubes and allowed to solidify so as to give a long slant without a butt. 

 To confirm the presumptive test, inoculation is made on the surface of 

 these slants with a straight platinum needle. If desired the medium may 

 be placed in test tubes and sterilized in the Arnold or in the auoctlave 

 in the ordinary manner. In that case it will be found that the agar, when 

 first removed from the sterilizer, is fluorescent like eosin but as it cools 

 the typical wine color returns. 



The reliability of the differentiation of Bact. coli and Bact. aerogenes 

 is indicated in the following table: 



TABLE XXVIII. RELIABILITY OF PRESUMPTVE DIFFERENTIATION OF 

 BACT. COLI AND BACT. AEROGENES ON EOSIN-METHYLENE BLUE AGAR. 



Thus in 1917, of 122 colonies fished, as probably Bact. coli, 96.9 per- 

 cent were proved to be such and in 1918 and 1919, 94.2 percent of 87 

 colonies fished were correctly designated from their appearance on eosin- 

 methylene blue agar. The proportion of Bact. aerogenes, correctly indenti- 

 fied was 82.4 percent and 85.5 percent in the series in Iowa and France 

 respectively. The medium therefore appears to be quite reliable for the 

 routine presumptive differentiation of these two groups of colon bacteria 

 and may therefore be of considerable value if in the future it is consid- 

 ered desirable to distinguish these forms because of their probable differ- 

 ent sanitary significance. 



