THE SMOOTH 



ROUGH VARIATION 



299 



Variations in Pigment Production. — It has long been recognized that different 

 strains of a particular bacterial species, which gives rise to a coloured growth on 

 the ordinary laboratory media, may vary widely in their power of pigment produc- 

 tion ; and that any particular strain may lose this power as the result of repeated 

 subculture, and may regain it for no apparent reason at some later period. In 

 many cases it has been demonstrated that particular environmental conditions 

 are favourable or unfavourable to pigment production, but there are many cases 

 on record in which we cannot reasonably attribute the loss of pigmentation to 

 such external influences. 



An interesting series of observations have been recorded by Rettger and Sherrick 

 (1911), who studied a strain of the red Chromohacterium prodigiosufn, which had 

 partially lost its pigment-producing capacity. By successive subcultures from 

 growths on solid media, using in one series the most pigmented part of the growth 

 and in another that part which showed least pigmentation, they were able to 

 separate a strain which produced an intensely coloured growth, and a strain which 

 gave almost colourless colonies. The segregation of these two types occurred early 

 in the series of subcultures, and there appeared to be some tendency for the highly 

 pigmented variant to revert to the slightly pigmented type, though the property 

 of intense pigmentation was successfully maintained by selection through a long 

 series of subcultures. There was, however, no apparent tendency for the non- 

 pigmented variant to acquire the property of pigment production. Within recent 

 years there have been numerous records of colourless variants of species that are 

 normally pigmented. In some cases, at least, these variants differ sharply from 

 the parent strain in the form of colony produced, as well as in the absence of pigment, 

 and in such instances the variation appears to have much in common with the type 

 discussed in the succeeding section. 



Antigenic Variations and the Changes in Colonial and other Characters Associated 

 with Them : the Smooth-Rough Variation. 



The Smooth — >- Rough (8 — > R) type of variation has already been referred to 

 in our discussion of antigenic structure (pp. 277, 278), but it is of such fundamental 

 importance in the general problem with 

 which we are here concerned that it is 

 necessary to consider it in considerably 

 greater detail. 



Arkwright (1920, 1921, 1924) described 

 variants of bacteria belonging to the 

 coh-typhoid-dysentery group which were 

 characterized by the formation of rough 

 or granular colonies on solid media (see 

 Fig. 45), by giving granular growths in 

 broth or peptone water, and in manv 

 cases by undergoing spontaneous agglut- 

 ination in the presence of 0-85 per cent, 

 sodium chloride. 



These properties — colonial roughness, 

 granular growth in fluid media, and 

 instability in saline — are associated, in 

 some species at least, with recognizable 

 changes in morphology (Wilson 1930) and 



Fig. 45. — Salm. typhi. 

 Smooth and rough colonies, 24-hours' 



growth on agar (X 8). 



