THE STAPHYLOCOCCUS GROUP 263 



to have prepared sera of marked agglutinating value, which clump 

 virulent strains in higher dilution than non- virulent strains. 



Precipitins. Specific precipitin reactions appear to have been 

 demonstrated in animals infected with staphylococci. 



Bacteriological Diagnosis. (a) Microscopic. A Gram stain of the 

 suspected material usually suffices to establish a diagnosis. It must 

 be remembered, however, that staphylococci from pus and exudates 

 may occur in pairs and even in short chains; they may, therefore, 

 be mistaken for streptococci. An absolute diagnosis can be made 

 only by the identification of pure cultures. 



(6) Cultural. Pure cultures of staphylococci are usually obtained 

 readily by "streaking out" or plating the organisms on agar. Blood 

 agar is preferable, if streptococci or pneumococci % are also suspected 



FIG. 33. Micrococcus tetragenus. X 800. 



to be present, otherwise the latter may be overlooked. The identi- 

 fication of the colonies on agar usually can be made by the examina- 

 tion of a Gram-stained preparation. Staphylococci are common on 

 the skin, and precautions must be taken to eliminate this source of 

 error before making cultures. 



(c) Animal Inoculation. The virulence exhibited by staphylococci 

 for animals is not a reliable index of their virulence for man. 



Dissemination and Prophylaxis. The wide distribution of staphy- 

 lococci on the mucous membranes, particularly on the skin and in 

 the hair follicles, makes the prevention of their introduction to under- 

 lying tissues through cuts and abrasions difficult. The customary 

 procedures of aseptic surgery are the best preventatives of infection. 

 The skin may be sterilized for operation (after thorough cleansing 

 and drying, which is imperative) by painting with freshly prepared 



