MACROFORMATIONS I4I 



F: SMOOTH COLONIES 

 (1.2) 



There is little to be said ot the structure and mode of formation of the 

 colonies of smooth bacterial variants. The constituent bacteria separate 

 completely after cell division (Chapter III), and produce a structureless colony. 

 It tends to be less flat than the medusa-head colony, because its edges lack the 

 cohesion which is necessary to force their way out over the medium. Con- 

 sequently it is a less efficient colony, because so great a proportion of the 

 component cells lose contact with the substrate. Such colonies are often far 

 from truly smooth in appearance, as surface concentrations of hydrophobe 

 lipo-proteins or insoluble polysaccharides may give them a dusty or even a 

 rocky appearance. Alternatively, like rough colonies, they may be enveloped 

 in a mass of mucoid capsular material. 



G; ROUGH AND SMOOTH COLONIES OF 

 STREPTOCOCCI 



Streptococci and pneumococci, like rod-shaped bacteria, may grow in 

 the form of threads and chains, or may separate completely after cell division. 

 Accordingly, the former produce a modified medusa-head colony, and the 

 latter a relatively structureless colony. The phenomenon of smooth^rough 

 variation in the pneumococcus, as usually described, is not, however, con- 

 cerned with this change, but with the loss o{ the capsule, which exposes the 

 rough structure of the long-chained colony (Section E above). No distinction 

 is drawn, by most bacteriologists, between S->R variation, as it is termed, in 

 bacteria and in pneumococci. Both are associated with an antigenic change 

 in the surface material, and both with an alteration in the appearance of the 

 colony, as seen by the naked eye ; but the reason for this difference is not the 

 same in each case, and the employment of the same expression to describe 

 two phenomena which are analogous without being homologous, is un- 

 fortunate and has given rise to a certain amount oi confusion. 



