336 BACTERIA PATHOGENIC TO MAN 



undivided cells, in pairs of oval elements, and in groups of three and 

 four (Fig. 102). When the division is complete they remind one of 

 sarcina? in appearance, except that they do not divide in three direc- 

 tions and are not built up like diminutive cotton bales. 



Staining. This micrococcus stains readily with the ordinary aniline 

 dyes; the transparent gelatinous envelope is only feebly stained. It is 

 not decolorized by Gram's method. 



Biology. The growth of this micrococcus is slow under all condi- 

 tions. It grows both in the presence and absence of oxygen; it grows 

 best from 35 to 38 C., but may be cultivated also at the ordinary 

 room-temperature about 20 C. 



Growth on Gelatin. On gelatin plates small, white colonies are 

 developed in from twenty-four to forty-eight hours, which, when exam- 

 ined under a low-power lens, are seen to be spherical or lemon-shaped, 



FIG. 102 FIG. 103 



-,* 



"** 



Micrococcus tetragenus. Micrococcus tetragenus in peritoneal 



X 1000 diameters. fluid. (After Zettnow.) 



grayish-yellow disks, with a finely granular or mulberry-like surface, 

 and a uniform but somewhat roughly dentated border. When the deep 

 colonies push forward to the surface of the gelatin they form white, 

 elevated, drop-like masses, having a diameter of 1 to 2 mm. In gelatin 

 stick cultures the colonies may be either isolated or confluent, in the 

 case forming a thick, white, slimy mass, filling out the fissures and 

 hollow spaces all along the line of puncture; on the surface a broad, 

 thick layer of 4 to 5 mm. in extent is apparent. The gelatin is not 

 liquefied. 



Growth on Agar and Blood Serum. The colonies appear as small, 

 transparent, round points, which have a grayish-yellow color and are 

 slightly elevated above the surface of the medium. 



Pathogenesis. Subcutaneous injections of a culture of this micro- 

 coccus in minute quantity is usually fatal to white mice. The micro- 

 cocci are found in comparatively small numbers in the blood of the 

 vessels and heart, but are more numerous in the spleen, lungs, liver, 



