324 PATHOGENIC MICROORGANISMS 



As to the reaction of media, staphylococcus develops most favorably 

 upon those having a slightly alkaline titer. Moderately increased 

 alkalinity or even moderate acidity of media does not inhibit growth. 



On gelatin plates, growth occurs readily at room temperature, form- 

 ing within thirty-six to forty-eight hours, small, shining, pin-head 

 shaped colonies, appearing, at first, grayish- white, and later assuming a 

 yellowish hue, which intensifies into a light brown and often a bronze 

 color as the colony grows older. The intensity of the color differs con- 

 siderably in different races of staphylococci. Liquefaction of the gelatin 

 occurs, and, shallow, saucer-shaped depressions are formed about the 

 colonies after forty-eight hours or more. These zones of fluidification 

 grow larger as the colonies grow, finally becoming confluent. Micro- 

 scopically, the colonies themselves, before liquefaction has destroyed 

 their outline, are round, rather finely granular, with smooth edges. 

 They are not flat, but rise from the surface of the medium as the seg- 

 ment of a sphere. In gelatin stab cultures in tubes, fluidification leads 

 to the formation of a funnel-shaped depression, with, finally, complete 

 liquefaction of the medium and sedimentation of the- bacteria. Lique- 

 faction of gelatin by the staphylococcus is due to a ferment-like body 

 elaborated by it, which is spoken of as "gelatinase." This substance 

 can be obtained apart from the cocci by the filtration of cultures. 1 It 

 is an extremely thermolabile body. 



On agar plates the characteristics of the growth, barring liquefaction, 

 are much like those on gelatin. Colonies do not show a tendency toward 

 confluence, remaining discrete, and show a rather remarkable differ- 

 ence in the size of the colonies occurring upon the same plate. Upon 

 slanted agar in tubes, rapid growth occurs, at first grayish- white, but 

 soon covering the surface of the slant as a glistening, golden-brown 

 layer. 



In broth, growth is rapid, leading to a general, even clouding of the 

 medium, and giving rise, after forty-eight or more hours, to the formation 

 of a thin surface pellicle. As growth increases, the bacteria sink to the 

 bottom, forming a heavy, mucoid sediment. The odor of old cultures 

 is often peculiarly acrid, not unlike weak butyric acid. 



In milk, staphylococcus causes coagulation usually within three or 

 four days, with the formation of lactic and butyric acids. 



On potato, growth is abundant, rather dry and usually deeply pig- 

 mented. 



i Loeb, Cent. f. Bakt., xxxii, 1902. 



