CULTIVATION OF THE PNEUMOCOCCUS. 211 



liquefaction of the medium. On agar plates colonies are almost 

 invisible to the naked eye, but under a low power of the micro- 

 scope appear to have a compact finely granular centre and a pale 

 transparent periphery. In bouillon, growth forms a slight tur- 

 bidity, which settles to the bottom of the vessel as a slight dust- 

 like deposit. Qn 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 ulti- 

 mately 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 rabbits' blood. The agar must 

 be prepared with Witte's peptone, must not be heated over 

 1 00 C, and after neutralisation (rosolic acid being used as the 

 indicator) 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 artificial media 

 do pneumococci develop a capsule, but in milk cultures cap- 

 sules are usually to be demonstrated. They usually appear as 

 diplococci, but in preparations , 



made from the surface of agar V ' - ' 



or from bouillon, shorter or 

 longer chains may be observed 

 (Fig. 8 1). After a few days' 

 growth they lose their regular f i ^ ~i 



shape and size, and involution ^. ,/ 



forms appear. Usually the - 

 pneumococcus does not grow X, Wf 



below 22 C., but forms in which ** ^ ^- _ ' 

 the virulence has disappeared *^. 



often grow well at 20 C. Its 



optimum temperature is 37C., FIG. Si. Fraenkel's pneumococcus from 



, . o /-i T a pure culture on blood ag[ar of twenty-four 



its maximum 42 C. It is pre- <. growth> some in p3n, some in short 



f erably an aerobe, but Can exist ch ains. Stained with weak carbol-fuchsin. 



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 



