THE PYOGENIC COCCI 157 



Staphylococcus epidermidis albus. It is probable that this 

 organism is identical with Staphylococcus albus. It is slightly 

 virulent and is frequently found in the upper layers of the epi- 

 dermis ; it is the common cause of " stitch abscesses " following a 

 surgical operation. 



Staphylococcus pyogenes citreus. This organism also differs 

 from aureus only in the color of its pigment, which is usually of a 

 bright lemon yellow. It is less often met with in wounds, however, 

 than either of the preceding cocci. A number of other staphylo- 

 cocci exist, few of which are in any degree pathogenic so far as 

 is known. They differ in minor details, such as ability to liquefy 

 gelatin or to form pigment. 



MICROCOCCUS TETRAGENUS 



In 1887 Gaffky isolated the organism from the pus of a tubercu- 

 lous patient. It has been observed associated with other organ- 

 isms in pulmonary tuberculosis, in acute abscesses, and also in 

 the pus of empyema following pneumonia. It is also frequently 

 found in the saliva of healthy persons, and it is generally assumed 

 that while it rarely incites disease its presence helps to contribute 

 to the progressive destruction of tissue in diseased conditions. 



Morphology and Staining. The cocci are somewhat larger 

 than staphylococci ; their diameter averages about 1 micron. They 

 are arranged regularly in groups of four or tetrads, and often when 

 first removed from pus appear to be surrounded by a capsule. 

 They are readily stained by the basic anilin dye and are Gram 

 positive. 



Cultivation. The optimum temperature is from 35 C. to 

 38 C. Growth is slow, but will occur both in the presence and 

 absence of oxygen. On agar the colonies appear as small, round 

 points, at first transparent and later becoming a grayish white. 

 Gelatin is not liquefied ; acid and coagulation is produced in milk. 



Pathogenesis. White mice are especially susceptible to in- 

 fection by the micrococcus tetragenus ; other animals are much 

 less so. In man it is usually non-pathogenic except in the condi- 



