44 



some inhibitory action on the ordinary water forms and many soil forms. 

 This may be further increased by the addition of small quantities of phenol 

 to the agar medium. Chick found that 0.1 percent (1 part per thousand) 

 gave very excellent results. In using phenolated media, it must always 

 be borne in mind that there is danger of inhibiting some members of the 

 colon group as well. 



Endo agar consists of nutrient lactose agar containing 3 percent agar 

 with basic fuchsin decolorized by sodium sulphite as an indicator. The 

 lactose fermenting organisms produce red colonies often with a distinct 

 metallic luster, the medium itself being colorless or slightly pink. The 

 reaction is presumably due to the production of acid and aldehyde by the 

 lactose fermenting organisms. These products react with the fuchsin 

 sulphite combination liberating the fuchsin. The high concentration of 

 agar serves to check diffusion of acid and also to eliminate many of the 

 water forms. It was thought at one time that the indicator itself exerted 

 inhibitory action but this does not seem to have been adequately proven. 



Conradi-Drigalski agar consists of litmus lactose agar to which has 

 been added some nutrose and crystal violet. The crystal violet checks 

 the growth of many forms particularly the streptococci but is supposedly 

 non-inhibitory for the colon group in the concentration employed. Both 

 the Endo and the Conradi Drigalski have been found very valuable for 

 the isolation of the para typhoid and typhoid bacilli from stools but they 

 have never found favor in the United States although they are the media 

 of preference in Germany for direct isolation of the colon group from water. 



MacConkey agar (also known under the name Rebipelagar) consists 

 of peptone lactose agar with 0.5 percent bile salts (sodium glycocholate 

 and taurocholate), the function of the latter being to check saprophytic 

 forms. The inhibitory action of bile is well illustrated in Table XX. 



TABLE XX. SELECTIVE ACTION OF BILE SALTS. 

 (After Jackson 1906) 



*Bile diluted, 1:1. 



where it will be noticed that the bile salts exerted a much greater re- 

 straining action on forms generally found in water than on the organisms 

 (almost exclusively Bact. coli) present in a suspension of feces. The 



