DIPLOCOCCUS PNEUMONIA 367 



Differentiation of Pneumococcus from Streptococcus. — Pneumococci 

 and streptococci which do not differ in morphology from their classic 

 types can usually be differentiated from each other and identified by 

 their morphological characters without difficulty; but it is equally true 

 that certain cultures of these organisms, either at the time of their 

 isolation or after cultivation on artificial media, approach the type of the 

 other so closely that it may be impossible to identify them by their mor- 

 phology alone. When such morphological variations occur there are no 

 constant and distinctive cultural or pathogenic characters as yet de- 

 monstrated which can with certainty be depended upon as distinguish- 

 ing marks between these organisms. 



This lack of distinct cultural differences between pneumococci and 

 streptococci has not infrequently led to confusion, and that uncertainty 

 should exist and mistakes be made in identification is not surprising 

 when one considers the characters usually depended upon to distinguish 

 pneumococci from streptococci. Chief among these, as has just been 

 implied, are the morphological features which are, in the case of pneu- 

 mococci, a slightly lancet or elongated form rather than the more typical 

 coccus form characteristic of the streptococci, and an arrangement of 

 such cocci in pairs rather than in chains; added to these features is the 

 possession of a more or less well-defined capsule. All of these char- 

 acters are subject to variation or may be absent. Compared with the 

 morphological, the cultural characters are of minor importance and are 

 variable. They consist in a more moist and flatter appearance of the 

 pneumococcus colonies on coagulated blood serum and on agar, and 

 in the usual inability of the freshly isolated pneumococcus to develop 

 readily or at all on gelatin at temperatures below 22° C. 



The distinctness of the capsule of the pneumococcus in the body 

 fluids of man and animals, and at times when this organism is culti- 

 vated artificially on blood serum, milk, or serum agar, has really been 

 depended upon as the chief distinguishing and diagnostic character. 



Nevertheless, from time to time, instances have been reported of 

 distinct capsule formation by organisms which had either been pre- 

 viously identified as Streptococcus pyogenes, or at the time of their 

 isolation could not be definitely identified by their discoverers as be- 

 longing to either this group or to the pneumococci, but were considered 

 intermediate in their character.^ 



I Brief Description of Organisms Reported as Capsulated Streptococci. — Bordet 

 {Bordet, Ann. de I'inst. Pasteur, 1897, xi, p. 177), working with an organism previously 



