202 THE ACUTE PNEUMONIAS 



bottom of the vessel as a slight dust-like deposit. On potatoes^ 

 as a rule, no growth appears. Cultures on such media may be 

 maintained for one or two months, if fresh sub-cultures are made 

 every four or five days, but they tend ultimately to die out. 

 They also rapidly lose their virulence, so that four or five days 

 after isolation from an animal's body their pathogenic action 

 is already diminished. Eyre and Washbourn, however, have 

 succeeded in maintaining cultures in a condition of constant 

 virulence for at least three months by growing the organisms on 



agar smeared with rab- 



bits' blood. The agar 



\ t must be prepared with 



X ' p% "' A s Witte's peptone, must 



\ \ % n t be nea ted over 100 



* i \ C., and after neutralisa- 



tion (rosolic acid being 



s ** V | > used as the indicator) 



i,,, \ '^jj must have '5 per cent of 



^ ^ normal sodium hydrate 



added. The tubes when 



. inoculated are to be kept 



^ ^ \ at 37' 5 C. and sealed to 



prevent evaporation. In 

 % ^. none of the ordinary arti- 



^ ficial media do pneumo- 



cocci develop a capsule. 

 FIG. 68. P raeukel s pneumococcus from a pure m , -,, 



culture on blood agar of twenty-four hours' Thev Dually appear as 



growth, some iu pairs, some in short chains, diplococci, but in pre- 



Stained with weak carbol-fuchsin. x 1000. parations made from the 



surface of agar or from 



bouillon, shorter or longer chains may be observed (Fig. 68). 

 After a few days' growth they lose their regular shape and size, 

 and involution forms appear. Usually the pneumococcus does 

 not grow below 22 C., but forms in which the virulence has 

 disappeared often grow well at 20 C. Its optimum temperature 

 is 37 C., its maximum 42 C. It is preferably an aerobe, but 

 can exist without oxygen. It prefers a slightly alkaline medium 

 to a neutral, and does not grow on an acid medium. These 

 facts show that when growing outside the body on artificial 

 media, the pneumococcus is a comparatively delicate organism. 

 There has been described by Eyre and Washbourn a non- 

 pathogenic type of the pneumococcus which may be found in 

 the healthy mouth, and which may also be produced during the 

 saprophytic growth of the virulent form. From the latter it 



