MORPHOLOGICAL CHARACTERS. 321 



than the superficial colonies. On making cover -glass 

 preparations, the bacilli are found to present the same 

 microscopic 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. 85). The same is true of films 

 made from young gelatine colonies. Sometimes the 

 diversity in the length of the bacilli is such as to throw 

 doubts 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 tempera- 

 ture, the bacilli are plump, and the protoplasm stains 

 uniformly. In old cultures or in cultures which have 

 been exposed to change of temperature, the protoplasm stains 

 only in parts ; there may be an appearance of vacuolation 

 either at the centre or at the ends of the bacilli, or a 

 bacillus may resemble a string of irregular coccus-like 

 bodies. It is these appearances which have led some 

 to believe that the typhoid bacillus forms spores. Gaffky 

 described the latter as highly refractile bodies occurring at 

 the ends of the bacillus ; and others have thought that the 

 coccus-like bodies are spores. Cultures containing either 

 have, however, been found to be not more highly resistant 

 than those containing ordinary bacilli ; further, the staining 

 reactions of such bodies are not those of spores, so that 

 now it is generally believed that spore-formation does not 

 occur in the typhoid bacillus. 



Motility, In hanging-drop preparations the bacilli are 

 found to be actively motile. The smaller forms have a 

 darting or rolling motion, passing quickly across the field, 

 whilst some show rapid rotatory motion. The filamentous 

 forms have an undulating or serpentine motion, and move 

 more slowly. Hanging- drop preparations ought to be 

 made from agar or broth cultures not more than twenty- 

 four hours old. In older cultures the movements are less 

 active. 



Flagella. On being stained by the appropriate methods 



21 



