340 BACTERIA PATHOGENIC TO MAN 



GROWTH ON SOLIDIFIED BLOOD SERUM. This is also an excellent 

 medium for the streptococcus. Tiny, grayish colonies appear twelve 

 to eighteen hours after inoculation. 



GROWTH IN MILK. All streptococci grow well in milk. As a rule, 

 when growth is luxuriant a marked production of lactic acid with 

 coagulation of the casein occurs. 



DEVELOPMENT IN BLOOD AGAR. Most streptococci produce abund- 

 ant hsemolytic substances. This is especially true of those from human 

 septic infections. As the pneumococci and other types of streptococci 

 produce them in a much less degree, blood-agar plates are a very useful 

 means for a probable identification. According to Rosenow if 1 c.c. 

 of fresh or defibrinated blood is added to 6 c.c. of melted agar at 

 40 to 45 C., well shaken, inoculated with the organisms and poured in 

 a Petri dish there will appear in twelve to twenty-four hours, if char- 

 acteristic streptococci are present, tiny colonies surrounded by clear 

 zones of about i to J inch in diameter. Pneumococci and some 

 varieties of streptococci on the other hand produce only narrow zones, 

 but instead a green pigment. 



DURATION OF LIFE OUTSIDE OF THE BODY. This is not, as a rule, 

 very great. When dried in blood or pus, however, they may live for 

 several months at room- temperature, and longer in an ice-chest, and 

 in gelatin and agar cultures they live for from one week to three months. 

 In order to keep streptococci alive and virulent, it is best to transplant 

 them frequently and to keep them in serum or ascitic fluid bouillon in 

 small, sealed glass tubes in the ice-chest. 



RESISTANCE TO HEAT AND CHEMICALS. The thermal death point of 

 the streptococcus is between 52 and 54 C., the time of exposure being 

 ten or twenty minutes. 



Mercuric chloride, 1:2500; sulphate of copper, 1:200; trichloride of 

 iodine, 1:750; peroxide of hydrogen, 1:50; carbolic acid, 1:300; 

 <cresol, 1:250; lysol, 1:300; creolin, 1:130, all kill streptococci within 

 a few minutes. 



Pathogenesis. The majority of test animals are not very susceptible 

 to infection by the streptococcus, and hence it is difficult to obtain any 

 definite pathological alterations in their tissues through the inoculation 

 into them of cultures of this organism by any of the methods ordinarily 

 practised. White mice and rabbits, under similar conditions, are the 

 most susceptible, and these animals are, therefore, usually employed 

 for experimentation. Streptococci, however, differ greatly in the effects 

 which they produce in inoculated animals, according to their animal 

 virulence, which is very different from human virulence. The most 

 virulent when injected in the minutest quantity into the circulation or 

 into the subcutaneous tissues of a mouse or rabbit, produce death by 

 septicaemia. Those of somewhat less virulence produce the same result 

 when injected in considerable quantities. Those still less pathogenic 

 produce septicaemia, which may be mild or severe, when injected into 

 the circulation; but when injected subcutaneously, they produce abscess 

 or erysipelas. The remaining streptococci, unless introduced in quan- 



