CULTURAL AND BIOLOGIC PROPERTIES 199 



permanent characteristic of certain varieties. It often depends upon 

 differences in occurrence, environment, and culture media. Certain 

 stems of streptococci, however, generally show a tendency to form short 

 chains of only four, six, or eight cocci, while others have the opposite 

 tendency. Virulent forms in tissues often appear in short chains; 

 these generally have a capsule. The Streptococcus pyogenes stains 

 well with the ordinary watery anilin dyes, and keeps Gram's stain. 

 There are, however, other pathogenic streptococci, such as the 

 streptococcus found in abscess of the udder in cows (Nocard's 

 streptococcus), that are Gram negative, as are also some saprophytic 

 streptococci. 



Cultural and Biologic Properties. Pyogenic streptococci grow and 

 exhibit the typical chain form much better in fluid than in solid cul- 

 ture media. The Streptococcus pyogenes longus clouds the bouillon 

 diffusely; the Streptococcus pyogenes brevis produces less clouding. 

 A faintly alkaline reaction of the nutrient bouillon is best. An in- 

 crease of the peptone of the bouillon from 1 to 3 to 5 per cent, and 

 the addition of 0.2 to 1 per cent, glucose is also favorable to the growth 

 of the organism. Too much glucose, however, lessens the virulency 

 of the streptococcus and causes it to die out sooner, since the acid 

 formed from the sugar changes the reaction of the culture soil. 



Blood serum, generally used with the addition of one part of nutri- 

 ent bouillon to three parts of serum, is an excellent medium for the 

 growth of pyogenic streptococci with preservation of their virulency. 

 Human, rabbit, or horse serum may be used. The serum must not 

 be solidified, as the organism does not grow very abundantly on solid 

 media. On agar plates kept in the incubator, small grayish or yel- 

 lowish gray, finely granular colonies develop, which generally do not 

 exceed 0.5 mm. in size. The deep colonies are brownish, round, or 

 oval. Agar streak and stick cultures are not characteristic. Gelatin 

 kept at 20 to 22 C. is generally not liquefied, but gelatin which can 

 be kept at 29 C. is sometimes. Saprophytic streptococci, cultivated 

 from dust and from the contents of the intestines, liquefy gelatin. 

 Pyogenic streptococci only occasionally grow on potatoes. The 

 optimum temperature of growth is in the neighborhood of the blood 

 temperature. At 12 to 15 C. the development is poor, at 24 C. good, 

 and best at 35 to 37 C. ; at 40.5 C. it falls off and at 42.3 C. it ceases 

 entirely. Slightly different figures are given by other authors. Strep- 

 tococci can grow in the presence of oxygen, but they do not require it, 

 and in fact some varieties evidently grow better anaerobically. All 

 streptococci form acid in their growth, chiefly lactic acid, and coagu- 

 late milk, in which they generally grow poorly because of the acid 

 formation. Some stems form a brownish-yellow pigment, partic- 

 ularly in gelatin and in the sediment of bouillon cultures. While 

 pyogenic streptococci generally grow for two to three days only on 

 artificial culture media and then die out, certain stems may remain 

 alive longer, occasionally for several weeks. 



