CHAPTER VIII 

 STREPTOCOCCI 



Streptococci occur in air, water and soil, occasionally in horse manure and 

 cow manure, sometimes on the skin and in the mouth of healthy people. 



Morphology. Streptococci are spherical, unicellular organisms from 0.5 

 to i. on in diameter. They are arranged in chains. These chains may be short 

 or long, composed of three or four cocci, a dozen or more; they may be straight, 

 curved or tangled, especially the longer chains. Streptococci found in blood, 

 spinal fluid, and cultures from them often appear in pairs, but on further cultiva- 

 tion assume their regular arrangement in chains. 



Streptococci are non-motile, stain readily with anilin stains and are Gram 

 positive. 



Growth. Streptococci grow well on ordinary culture media, having a 

 slightly acid, neutral or slightly alkaline reaction. The optimum temperature 

 for growth is 37C., but though slowly, growth takes place at any temperature 

 between i5C. and 44C. 



Agar. Surface growth on agar appears in 1 8 to 24 hours. The colonies are 

 convex, raised, grayish-white, opalescent and have lace-like, slightly corrugated 

 edges. They are pin-point in size and usually do not coalesce nor enlarge as 

 the culture ages; after several days they tend to die out. 



Gelatin. Surface growth on gelatin is the same as that on agar, with the 

 exception that colonies are more distinctly white. 



Stab cultures in gelatin develop slowly, small, round, opaque, white colonies 

 appear along the stab in from 24 to 48 hours, and the culture dies when 5 or 6 

 days old. With the exception of the streptococcus fecalis and possibly a 

 few pathogenic streptococci, they do not liquefy gelatine. 



Blood Serum. On coagulated blood serum growth is the same as on agar, 

 except that it is usually more luxuriant. 



Potato. Growth apparent to the naked eye seldom, if ever, occurs on this 

 medium. 



Milk is acidulated; some streptococci coagulate it and others do not. 



Bouillon. Cultures incubated at 37C. show a light, white sediment in 24 

 hours; at first it sticks to the sides of the tube, later it falls to the bottom, in 

 most cases leaving the medium clear. Lactic acid is formed. In bouillon 

 streptococci find most favorable conditions for development of long chains. 



Both bouillon and agar containing blood serum or ascitic fluid favor a more 

 luxuriant growth of streptococci than plain bouillon and agar. 



Blood agar (2 parts agar mixed with 5 parts whole human blood). On this 

 medium different strains of streptococci show slight differences in growth. 



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