STAPHYLOCOCCUS AUREUS 271 



cells in a healthy wound can dispose of a few organisms, 

 if the latter are abundant or in masses they may gain the 

 mastery. 



In this chapter the pathogenic micrococci and strep- 

 tococci and B. pyocyaneus are dealt with. The anaerobic 

 bacilli which play such a part in dirty wounds and gun- 

 shot injuries are discussed in Chapter XIII. 



Micrococcus pyogenes, var. aureus (Staphylococcus 

 pyogenes aureus) [Staphylococcus aureus] 



Morphology and biology. A coccus measuring about 

 0-75 /A in diameter. It generally occurs in more or less 

 irregular groups, but may be met with singly or in pairs 

 (Plate I, c). It is non-motile, does not form spores, and 

 stains well with all the anilin dyes and also by Gram's 

 method. It is aerobic and facultatively anaerobic, 

 will develop in vacuo, and grows well and rapidly on all 

 the usual culture media at temperatures from 18 to 

 37 C. On agar-agar it forms a thickish, moist, shining 

 growth, cream-coloured at first, but after a day or two 

 developing a characteristic orange-yellow colour. It 

 grows in the same manner on blood-serum without lique- 

 faction of the medium. Gelatin is rapidly liquefied, the 

 liquefied gelatin being at first somewhat turbid from 

 yellowish masses of organisms ; these later on subside 

 and form an orange-yellow sediment (Plate I, d). In 

 gelatin plates the colonies form at first small whitish, 

 granular points, developing in two or three days into 

 circular areas of liquefaction with yellowish masses of 

 the organism floating in them. On potato it forms a 

 growth similar to that on agar. When grown in milk it 

 produces coagulation. Acid production (lactic and 

 butyric acids) can be demonstrated by growing on a 

 neutral litmus glucose-agar. When grown in broth or 



