104 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



Capsules* A considerable number of the bacteria regularly, or 

 under certain conditions, form what are known as capsules (Fig. 81). 

 These are mucilaginous envelopes which in width frequently exceed 

 that of the organism itself. In microscopical preparations of bacteria 

 it is important to differentiate these from artifacts, since by ordinary 

 staining methods the capsules are not colored but appear as colorless 

 areas surrounding the bacteria. If, due to shrinkage of the bacteria, 

 or other material on the preparation, clear spaces are formed, it is 

 readily seen that these might be confused with the real capsule. It is 





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FIG. 81. Capsules. Bad. pneumonia (Friedlander). (After Weichselbaum from 



Frost and McCampbell.) 



possible to stain the capsules by special methods; these must be used in 

 order to determine positively the existence of the capsules. The 

 bacteria which grow in the bodies of animals frequently contain these 

 capsules but fail to show them when grown upon artificial culture media. 

 It is difficult, therefore, to determine whether or not an organism has a 

 capsule by mere examination of cultures. Some culture media, how- 

 ever, do cause a formation of capsules in the case of capsulated bacteria. 

 These are blood serum, sometimes, and milk, usually. Beautiful cap- 

 sules can be obtained by growing such bacteria as the Bact. pneumonia, 

 Bact. capsulatum, and Bact. Welchii in milk cultures. Strept. mesen- 

 teroides is a bacterium which grows in the syrup of the sugar refineries 

 and forms abundant capsules. This organism changes the char- 



* Prepared by W. D. Frost. 



