MORPHOLOGY, REPRODUCTION, ETC. 13 



value. When stained, the capsule takes the ordinary anilin dyes less 

 deeply than does the bacterial cell body, and varies greatly in thickness, 

 ranging from a thin, just visible margin to dimensions four or five 

 times exceeding the actual size of the bacterial body itself. This struc- 

 ture is perfectly developed in a limited number of bacteria only in which 

 it then becomes an important aid to identification. Most prominent 

 among such bacteria are Diplococcus pneumoniae. Micrococcus tetra- 

 genus, the bacilli of the Friedlander group, and B. aerogenes capsulatus. 

 The development of the capsule seems to depend intimately upon the 

 environment from which the bacteria are taken. It is most easily de- 

 monstrable in preparations of bacteria taken directly from animal tis- 

 sues and fluids, or from media containing animal serum or milk. If 

 cultivated for a prolonged period upon artificial media, many otherwise 

 capsulated microorganisms no longer show this characteristic structure. 



Capsules may be demonstrated on bacteria taken from artificial 

 media most successfully when albuminous substances, such as ascitic 

 fluid or blood serum, are present in the culture media, or when the 

 bacteria are smeared upon cover shp or slide in a drop of beef or other 

 serum. 1 Most observers believe that the capsule represents a swelling 

 of the ectoplasmic zone of bacteria. By others it is regarded as an 

 evidence of the formation of a mucoid intercellular substance, some of 

 which remains adherent to the individual bacteria when removed from 

 cultures. It is noticeable, indeed, that some of the capsulated bacteria, 

 especially Streptococcus mucosus and B. mucosus capsulatus, develop 

 such slimy and gelatinous colonies that, when these are touched with a 

 platinum wire, mucoid threads and strings adhere to the loop. Exactly 

 what the significance of the capsules is cannot yet be decided. 



There is, however, definite reason to believe that there is a direct 

 relation between virulence and capsulation; capsulated bacteria are 

 less easily taken up by phagocytes than are the non-capsulated mem- 

 bers of the same species. Also, as Forges and others have shown, 

 capsulated organisms are not easily amenable to the agglutinating action 

 of immune sera. Many bacteria (plague, anthrax) which are habitu- 

 ally uncapsulated on artificial media acquire capsules within the in- 

 fected animal body. Also in some species (pneumococci), the loss 

 of capsule formation as cultivated on the simpler media is accompanied 

 by a diminution of virulence. 



Organs of Locomotion. — When suspended in a drop of fluid many 

 bacteria are seen to be actively motile. It is important, however, in 



1 Hiss, Jour. Exp. Med., vi, 1905. 



