ISOLATION AND APPEARANCES OF CULTURES 321 



capsule is seared with a cautery to destroy all superficial con- 

 taminating organisms. A small incision is made into the organ 

 with a sterile knife, a little of the pulp removed by a platinum 

 needle, and agar or gelatin plates are prepared, or successive 

 strokes made on agar tubes. On the agar media the growths 

 are visible after twenty-four hours' incubation at 37 C. On 

 agar plates the superficial colonies are thin and film-like, circular 

 or slightly irregular at the margins, dull white by reflected light, 

 bluish-grey by transmitted light. Colonies in the substance of 

 the agar are small, and appear as minute round points. When 

 viewed under a low ob- 

 jective, the surface 

 colonies are found to be 

 very transparent (requir- 

 ing a small diaphragm 

 for their definition), finely 

 granular in appearance, 

 and with a very coarsely 

 crenated and well-defined 

 margin . The deep colonies 

 are usually spherical, 

 sometimes lenticular in 

 shape, and are smooth or 

 finely granular on the 

 surface, and more opaque 

 than the superficial 



colonies. On making 



FIG. 111. Typhoid bacilli, from a 



culture on agar, showing some filamentous 



forms. 



present the same micro- Stained with weak carbol-fuchsin. x 1000. 

 scopic appearances as are 



observed in preparations from solid organs, except that there 

 may be a greater number of the longer forms which may 

 almost be called filaments (Fig. 111). The same is true of films 

 made from young gelatin colonies. Sometimes the diversity in 

 the length of the bacilli is such as to throw doubt on the purity 

 of the culture. Its purity, of course, can be readily tested by 

 preparing plates from it in the usual way. As a general rule in 

 a young (twenty-four to forty-eight hours old) colony, grown at 

 a uniform temperature, the bacilli are plump, and the protoplasm 

 stains uniformly. In old cultures, or in cultures which have 

 been exposed to changes of temperature, the protoplasm stains 

 only in parts ; there may be an appearance of irregular vacuola- 

 tion either at the centre or at the ends of the bacilli. There 

 21 



cover -glass preparations, 

 the bacilli are found to 



young 



