2IO INFLAMMATORY AND SUPPURATIVE CONDITIONS. 



in preparations stained by ordinary methods the difficulty of 

 recognising the capsule when it is present is due to the refrac- 

 tive index of the fluid in which the specimen is mounted being 

 almost identical with that of the capsule. This difficulty can 

 always be overcome by having the groundwork of the prepara- 

 tion tinted. 



The Cultivation of Fraenkel's Pneumococcus. — It is usually 

 difficult, and sometimes impossible, to isolate this coccus directly 

 from pneumonic sputum. On culture media it has not a vigor- 

 ous growth, and when mixed with other bacteria it is apt to be 

 overgrown by the latter. To get a pure culture it is best to 

 insert a small piece of the sputum beneath the skin of a rabbit 

 or a mouse. In about forty-eight hours the anirinal will die, with 

 numerous capsulated pneumococci throughout its blood. From 

 the heart blood, cultures can be easily obtained. 

 Cultures can also be got post mortem from the 

 lungs of pneumonic patients by streaking a 

 number of agar or blood-agar tubes with a 

 scraping taken from the area of acute con- 

 gestion or commencing red hepatisation, and 

 incubating them at 37° C. The colonies of the 

 pneumococcus appear as almost transparent 

 small discs which have been compared to drops 

 of dew (Fig. 80). This method is also some- 

 times successful in the case of sputum. 



The appearances presented in cultures by 

 different varieties of the pneumococcus vary 

 somewhat. It always grows best on blood 

 serum on Pfeiffer's blood agar or in milk. It 

 usually grows well on ordinary agar or in 

 bouillon, but not so well on glycerin agar. 

 In a stroke culture on blood serum growth 

 appears as an almost transparent pellicle along 

 the track, with isolated colonies at the margin. On agar media 

 it is more manifest, but otherwise has similar characters. The 

 appearances are similar to those of a culture of streptococcus 

 pyogenes, but the growth is less vigorous, and is more delicate 

 in appearance. A similar statement also applies to cultures in 

 gelatin at 22° C, growth in a stab-culture appearing as a row of 

 minute points which remain of small size ; there is, of course, no 



Fig. 80. — Stroke- 

 culture of Fraenkel's 

 pneumococcus o n 

 blood agar. The col- 

 onies are unusually 

 large and distinct. 

 Twenty-four hours' 

 growth at 37° C. 

 Natural size. 



