kept in a virulent condition by repeated passage through animals 

 and maintenance in culture on favourable media. The medium 

 upon which the special differential features of the colonies are 

 best shown is the chocolate -coloured serum agar with chloro- 

 formed blood cells added. On ordinary agar, or on agar smeared 

 with fresh blood, the different varieties of colonies are much less 

 readily distinguished. 



When the blood from a mouse which has died from pneumo- 

 coccus septicemia, or a young blood-broth culture, is plated 

 on the above medium and incubated overnight, all the discrete 

 colonies are of the same type. They are circular, dome-shaped, 

 with a smooth shiny surface and an almost watery consistency ; 

 when autolytic changes take place they become flattened, and 

 finally shaped like a button or draught. The characteristic 

 feature of the variant or rough type of colony is the surface, 

 which is dull or finely granular ; the colony is more coherent than 

 the smooth type, and may either break into pieces on being 

 touched or stick as a whole to the point of the needle. The 

 difference in appearance between the two varieties of colonies 

 on the same plate is often very striking, but it is essential that 

 the colonies should be well spaced. At first the rough variant 

 is generally smaller than the smooth colony : it is equally bile- 

 soluble. 



Such are the two types of colonies which can be grown from 

 a normal virulent strain cultivated in homologous immune 

 serum. There is a close resemblance between the changes de- 

 scribed above and those described by Arkwright as occurring in 

 old cultures of various bacilli of intestinal origin. In the case 

 of pneumococci, as with these, rough colonies can be produced 

 also by growing cultures on agar media or in liquid media con- 

 taining glucose. Arkwright, as mentioned earlier, designated 

 his types of colonies S (smooth) and R (rough) respectively, 

 and this seems quite appropriate for the pneumococcal colonies, 

 the S being the normal and the R the variant. There is, however, 

 a slight distinction, which is probably one of degree, namely, 

 that the rough pneumococcal cultures grow uniformly in broth 

 and form stable suspensions, whereas Arkwright's R colonies 

 form clumps in any medium containing • 85 per cent, salt solu- 

 tion. The rough cultures in broth are, however, a little more 

 granular than the smooth ; they clear more completely by centri- 

 fuging, and the deposit packs into a mass, which is not so readily 

 emulsified as the deposit from the smooth culture. 



Stability of R and S Characters. 



The smooth characters of the normal colonies remain unaltered 

 so long as the strains are kept virulent by passage through animals 

 and growth in blood broth. They persist even after prolonged 

 culture on solid media. For example, a number of cultures were 

 allowed to remain in sealed tubes of ordinary agar in the incubator 

 at a temperature of 37° C. At the end of three months all were 



B 4 



