VIL] MICROCOCCUS. 41 



(c) Chromogenic micrococci (Schroter, Cohn). These micro - 

 cocci are characterised by their power of forming pigment of 

 various colours. They grow well at ordinary temperatures, 

 and occur chiefly as zoogloea ; they differ from one another by 

 forming different pigments. The thicker the layer the more 

 marked is the pigment. This is either soluble in water or it 

 is insoluble, and therefore remains limited to the cells and 

 their interstitial substance. The cells are spherical (Micro- 

 coccus prodigiosus, chlorinus, fulvus) or slightly elliptical 

 (M. luteus, auriantiacus, cyaneus, violaceus). They are all 

 aerobic and produce this pigment only when there is free 

 access of air. They grow best on boiled potato, bread, paste, 

 and boiled-egg albumen. They can be transplanted, and 

 always produce the same pigment. When growing and kept 

 in the depth of a solid nourishing material, i.e. removed from 

 the free surface, they grow as colourless micrococci. They 

 abound in the air in some localities and at certain seasons 



FIG. 13. OVAL MICROCOCCI WHICH POSSESS A BLUE COLOUB, MICBOOOCCUS 

 CYANEUS, SINGLY AND IN DUMB-BELLS. 



more than at others, (a) Micrococcus prodigiosus is blood- 

 red, the colour is lodged not in the micrococci but in the 

 interstitial substance, and is insoluble in water, soluble in 

 alcohol ; it occurs chiefly as zoogloea, in the shape of smaller or 

 larger droplets. The cells are the smallest of all pigment- 

 micrococcL (/3) Micrococcus luteus is yellowish, and the pigment 

 is insoluble in water. It occurs also in fluid nourishing 

 material, forming a pellicle. I have met with it in the air, and 

 have sown it in fluid pork broth, where it grew very abundantly 

 at a temperature of 32 38 C. It was found as single cells 

 or dumb-bells, and formed a thick pellicle on the surface, 

 which after some time sank down into the fluid, the pellicle 

 retaining a pale yellow colour. (?) Micrococcus auriantiacus 

 grows on boiled -egg albumen, chiefly as zoogloea. The 

 pigment is soluble in water. (S) Micrococcus cyaneus, 

 violaceus, chlorinus, and fulvus, produce blue, violet, green, 

 and brown pigment respectively. The first two grow well 

 as zoogloea of elliptical cells on boiled potatoes, the third 

 on boiled-egg albumen, and the last is met with on horses' 

 dung. 



