46 BIOLOGY OF PNEUMOCOCCUS 



practically all strains. Pneumococci will grow in gelatin at 37° 

 but, because gelatin melts, it is not a useful medium for propagat- 

 ing pneumococci. 



On agar, Pneumococcus grows in very small, round, water-clear, 

 non-confluent colonies, resembling in many respects those of strep- 

 tococci, but differing from them in being more transparent, more 

 moist, and flatter. Under the microscope, colonies of Pneumococ- 

 cus are seen to be finely granular, with dark centers fading to 

 lighter zones nearer the periphery, which is regular, sometimes 

 slightly wavy, but never showing the intertwined convolutions such 

 as those seen in colonies of Streptococcus. In agar stab cultures 

 and sub-surface agar plate cultures, the colonies appear within 

 twenty-four to thirty-six hours, are smaller, sometimes almost in- 

 visible, and indistinguishable from colonies of Streptococcus. Un- 

 der low magnification they show partly as yellowish-brown, partly 

 as bright, oval, lenticular or whetstone-like forms. 



On blood-agar plates the colony may be surrounded by a zone of 

 methemoglobin formation with an exterior zone of greenish color 

 after twenty-four or more hours of incubation. Later a zone of 

 hemolysis may appear at the margin fainter than that encircling 

 streptococcal colonies, and this appearance may lead to error in 

 the differentiation of the two organisms. When grown on "mix- 

 tures of whole rabbits' blood and agar, the pneumococcus grows 

 well, and forms, after four or five days, thick, black surface colo- 

 nies, not unlike sun blisters on red paint. These colonies are easily 

 distinguished from those of streptococci, and are of considerable 

 differential value (Hiss)."* 



"Upon potato, thin, moist growth appears, scarcely visible and 

 indistinguishable from an increased moisture on the surface of the 

 medium," making this an unsuitable substrate. "Upon Loeffler's 

 coagulated blood serum, the pneumococcus develops into moist, 

 watery, discrete colonies which tend to disappear by a drying out 



* Paraphrase and direct quotation from Zinsser and Bayne-Jones.i579 



