404 PATHOGENIC MICROORGANISMS 



by an inexperienced bacteriologist, he may assume for some time 

 that his culture has been contaminated. These swollen individuals 

 may probably be interpreted as involution forms. 



Streptococci are easily stained by the usual anilin dyes. Stained 

 by the method of Gram, the pyogenic streptococci are not decolorized 

 and invariably retain the gentian-violet. Certain species found in 

 stools and described as Gram-negative, are rare and are non- 

 pathogenic. Others of the "Streptococcus brevis" variety, and 

 purely saprophytic, may stain irregularly by the Gram method. 



Cultivation. The pyogenic streptococci are easily cultivated 

 upon all the richer artificial media. While meat extract-pepton 

 media may suffice for certain strains, it is usually better to employ 

 those media which have the beef or veal infusion for a basis. For 

 the cultivation of more delicate strains of streptococci, especially 

 when taken directly from the animal or human body, it is well to 

 add to the media animal albumin in the form of whole blood, blood 

 serum, or ascitic or pleural transudates. Glucose, added in propor- 

 tions of one to two per cent, likewise renders media more favorable 

 for streptococcus cultivation. Prolonged cultivation of all races upon 

 artificial media renders them less fastidious as to cultural require- 

 ments. The most favorable reaction of media for streptococcus cul- 

 tivation is moderate alkalinity (two-tenths to five-tenths per cent 

 alkalinity to phenolphthalein) . Growth may be readily obtained, 

 however, in neutral media or even in those slightly acid. The 

 optimum temperature for growth is at or about 37.5. Above 43 

 to 45 C., development ceases. At from 15 to 20 C., growth, while 

 not energetic, still takes place, an important point in the differen- 

 tiation of these microorganisms from pneumococci. While the free 

 access of oxygen furnishes the most suitable environment for most 

 races of streptococci, complete anaerobiosis does not prevent develop- 

 ment in favorable media. Strictly anaerobic streptococci have been 

 cultivated from the human intestinal tract by Perrone 8 and others. 



In alkalin bouillon at 37.5 C., pyogenic streptococci grow 

 rapidly, form long and tortuous chains, and have a tendency to 

 form flakes which rapidly sink to the bottom. Diffuse clouding occurs 

 rarely and is a characteristic rather of the shorter so-called Strepto- 

 coccus brevis. When sugar has been added to the broth the rapid 

 formation of lactic acid soon interferes with extensive development. 



8 Perrone, Ann. de Pinst. Pasteur, xix, 1905. % 



