212 RELATIONSHIPS OF THE COCCACE^E 



ment production. Furthermore, the red and yellow forms, 

 while alike in the characters just enumerated, differ from 

 each other in nitrate reduction, and in action upon gelatin, 

 as well as in chromogenesis. These considerations led us 

 to separate the parasitic cocci under the genera Aurococ- 

 cus and Albococcus and the red forms under the genus 

 Rhodococcus, denning them as described-irrChapter IV. 



Thus purged of extraneous forms, the genus Micro- 

 coccus presents a much clearer and more definite picture. 

 The commonest yellow coccus is recognized by all bac- 

 teriologists as a saprophytic type, occurring in water, 

 air, and earth, generally Gram-negative and with slight 

 fermentative powers. Similar forms are occasionally 

 found in water or earth, in which the power of pigment 

 production is so weakened that their growth may appear 

 white. Yellow forms occur, on the other hand, living as 

 parasites on the surface of the body. By far the common- 

 est combination in nature, however, is the yellow sapro- 

 phytic form; and this marks a well-defined type from 

 which the others deviate. The full definition of the ge- 

 neric type is as follows: 



GENUS MICROCOCCUS (Hallier, Cohn) Winslow and 

 Rogers. Facultative parasites or saprophytes. Cells in 

 plates or irregular masses (never in long chains or packets'). 

 Generally decolorize by Gram. Growth on agar abundant, 

 with formation of yellow pigment. Dextrose broth slightly 

 acid, lactose broth generally neutral. Gelatin frequently 

 liquefied. Nitrates may or may not be reduced. 



