STREPTOCOCCUS PNEVMONIM 599 



more acutely pointed. Some strains (particularly Type III) tend to form longer chains. 

 As seen in films from the tissues, the pneumococcus shows a well-marked capsule ; and 

 this capsule is frequently retained in cultures on suitable media. Non-motile. No spores. 

 Gram-positive. Not acid-fast. 



Growth Requirements. — Grows poorly on ordinary media, especially when first 

 isolated ; the addition of blood or serum to the medium greatly improves growth. Optimal 

 temperature 37° C, range of growth more restricted than with other species of Strep- 

 tococcus. Usually no growth on gelatin at 20° C. Aerobic and facultatively anaerobic. 

 Growth of some strains improved by incubation m 10 per cent. COj. 



Type of Growth. — On solid media. Small, raised, circular colonies, 0-5-1 mm. in 

 diameter, with a smooth surface, an entire edge, and very little differentiation. On a 

 favourable medium, such as blood agar, the colonies are often characteristic ; the surface 

 is flat and smooth, and the edges are sharply and steeply raised from the surface of the 

 medium. In some cases the edge may be raised above the surface of the colony, forming 

 a raised circumferential rmg. Several adjacent colonies may become confluent, forming 

 a raised area of growth with a flat, even surface and a sharply deUmited edge. With 

 longer periods of growth (48-72 hours) the central portion of the colon}^ often undergoes 

 autolysis. Some strains (particularly Tyjje III) give characteristic mucoid colonies. Old 

 laboratory strains of pneumococci, particularly when grown on a relatively unfavourable 

 medium such as ordinary nutrient agar, often give smaller colonies which lack the character- 

 istic appearance of a recently isolated strain grown on a favourable medium. The consis- 

 tency of the colonies is butyrous, and the growth emulsifies easily. In blood agar plates, 

 the colonies are surrounded by a zone of a-haemolysis showing the characteristic green 

 coloration. In a suitable fluid medium, a filtrable hsemolysin is formed, which is of the 

 oxygen-labile type. 



In broth, or serum broth, Str. pneumonicB gives a diffuse turbid growth, with a slight 

 deposit, increasing on prolonged incubation. No pellicle is formed. 



On gelatin very slight growth, usually none at or below 20° C. 



Gelatin stab — very slight growth along track, with minimal surface growth. No lique- 

 faction. 



Potato — growth slight, or absent. 



Heat Resistance and Viability. — Str. pneumonice is sensitive to heat, being killed 

 at a temperature of 55° C. in 20 minutes or less. It is a relatively delicate organism, 

 and dies out rapidly in artificial cultures unless maintained under particularly favourable 

 conditions, as, for instance, in semi-solid agar to which blood has been added. 



BiocHEjncAL Activities. — The pneumococcus produces acid, but no gas, from lactose, 

 saccharose, and inulin, and usually from raffinose. Salicin is rarely fermented ; when 

 fermented, acid is not usually produced for some days. Mannitol is not fermented. Milk 

 is acidified, and frequently clotted. Nitrates are not reduced. Indole is not formed. 

 Gelatin is not liquefied. The pneumococcus is soluble in bile. 



Antigenic Structure. — The pneumococcus possesses a species-specific carbohydrate 

 antigen, the presence of which is not detected by agglutination reactions carried out with 

 normal smooth forms. The species is divided into a number of antigenic types by type- 

 specific polysaccharide antigens contained in the capsules. Over seventy of these types 

 have so far been identified. 



Pathogenicity and Toxin Production. — The pneumococcus causes pneumonia and 

 certain other infections in man. It is highly pathogenic for mice and slightly less so for 

 rabbits. Guinea-pigs are rather more resistant, and cats, dogs, fowls and pigeons much 

 more resistant. 



The pneumococcus produces a soluble hsemolysin, and a leucocidin. It also pro- 

 duces a substance acting on rabbit fibrin or fibrinogen, and preventing the formation 

 of a clot. 



