408 



PATHOGENIC MICROORGANISMS 



colon, typhoid, and a few others to develop. This medium is at present 

 rarely used. 



Hiss ^ has employed with success an agar-gelatin mixture containing 

 one per cent of glucose, the preparation of which has been described in 

 detail in the section on media. The actual technique of the test is as 

 follows: One to two loopfuls of feces are transferred to a tube of broth, 

 making the broth fairly cloudy. From this emulsion five or six plates 

 are made by transferring in series one to five loopfuls of the emulsion 

 to tubes containing the melted plate medium, and then pouring the con- 

 tents of these tubes into Petri dishes. These dishes^ after the medium 



Fig. 90. — Bacilltjs typhosus. Deep colonies in Hiss plate medium. 



has hardened, are placed in an incubator at 37° C, and allowed to re- 

 main for eighteen to twenty-four hours, when they are ready for examina- 

 tion. If typhoid baciUi are present they will be found as small, usually 

 ghstening colonies with a fringe of threads growing out like flagella from 

 their peripheries (see Figs. 90 and 91). These colonies are smaller and 

 quite distinct from those of colon bacilli, which are heavier and darker 

 and do not display the fringing threads. Suspicious colonies may be 

 fished and transferred to the Hiss tube medium (see page 133) or iden- 

 tified by other reliable methods. 



A method which has been foimd useful, especially in Europe, 

 is that in which smears of diluted feces are made upon large plates 

 of the Conradi-Drigalski medium. (For preparation see page 135.) 

 The principles underlying the use of this medimn are the formation 

 of acid from the lactose by the colon bacilh and the inhibition of cocci 

 and many other bacteria by the crystal-violet. In practice, an emul- 



1 Hiss, Jour, of Exp. Med., ii, 1897; Med. News, May, 1901; and Jour. Med. Res., 

 N. S., iii, 1902. 



