MORPHOLOGY 



561 



usually occur in chains of eight or more cocci and often form long chains. Others 

 again, such as the strains of the viridans type, are markedly variable in this respect, 

 occurring both as very short chains, or pairs, and as chains of enormous length. 



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Fig. 118. — Viridans streptococci. 



From 24-hours' culture in broth, showing 

 short chains ( X 1000). 



Fig. 119. — Str. pneumonice. 



From 24-hours' culture on agar, showing 

 diplococci and short chains ( x 1000). 



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The shape of the individual cocci forming the chain also varies, and there is a 

 tendency for the cells of short-chained streptococci to be ovoid, with the long axis in 

 the axis of the chain. When, as in the pneumococcus, a majority of the cocci occur 

 in pairs, each cell of a pair may be definitely lanceolate, the blunt ends being 

 adjacent. The individual cells of long- 

 chained streptococci tend to approach 

 more closely to the spherical form ; or 

 they may sometimes be compressed in 

 such a way that the longer axis of the cell 

 lies at right angles to the axis of the chain 

 as a whole. Cell shape, like modal chain 

 length, may vary from one species of 

 streptococcus to another, but it forms no 

 better criterion for systematic purposes. 

 Occasional strains may take on a diph- 

 theroid type of morphology, which may 

 lead to errors in identification (see 

 Lamanna 1944). 



Capsule formation, when it occurs, is 

 of greater value for classification. It is 

 almost constantly shown by Str. pneu- 

 monice when growing within the tissues 

 (see Fig. 120) and is absent in most other 



species, though not in all. Seastone (1934, 1943), however, has reported the 

 presence of capsulation in Str. pyogenes during the first 2-2| hours of growth in 

 serum broth, though the capsules have usually disappeared by the 3rd or 4th hour. 

 Morison (1940) likewise finds that all recently isolated strains of this organism are 



Fig. 120. — Str. pneumonice. 



In peritoneal exudate of mouse, showing 

 capsulation ( x 1000). 



