COCCACE^ AND THEIR~PARASITIC RELATIONSHIPS 267 



positive. In cultures the capsules are less well developed and 

 often cannot be demonstrated at all. The individuals are often 

 less pointed and frequently resemble short bacilli in form. They 

 may remain attached together in chains of six to eight cells. 



Cultures may be obtained on ordinary media but they are 

 prone to die out quickly. Blood-agar, serum agar or asci tic-fluid 

 agar are the best solid media, but even with these weekly trans- 

 plantation is usually necessary. Broth to which serum or ascitic 

 fluid has been added forms an excellent medium. There is prac- 



FiG. io8.- — Pneumococcus, shoTidng capsule, from pleuritic fluid of infected rabbit, 

 stained by second method of Hiss. 



'tically no growth below 25° C. On blood agar, the colony is 

 surrounded by a zone of greenish discoloration, a character of 

 great value in the early recognition of the pneumococcus isolated 

 from the body. The virulence of the microbe diminishes very 

 rapidly in artificial culture. Virulent material is best kept in 

 stock by preserving in a desiccator dried blood taken from a 

 rabbit dead of pneumococcus infection. The fluid blood may also 

 be kept in sealed capillaries in the refrigerator. By these methods 

 the virulence may be preserved for months. Rabbits, mice and , 

 young ral^s are the most susceptible animals. 



