THE DIPLOCOCCUS OF PXEUMOMA 353 



Pathogenesis in Animals. Most strains of the micrococcus lanceolatus 

 are moderately pathogenic for numerous animals; mice and rabbits are 

 the most susceptible, indeed some strains are intensely virulent for 

 these animals, while guinea-pigs and rats are much less susceptible. 

 Pigeons and chickens are refractory. In mice and rabbits the subcuta- 

 neous injection of small or moderate quantities of pneumonic sputum in 

 the early stages of the disease, or of a twenty-four hour ascitic broth cul- 

 ture from such sputum, or of a pure, virulent ascitic broth culture of the 

 micrococcus, usually results in the death of these animals in from twenty- 

 four to forty-eight hours. The course of the disease produced and the 

 post-mortem appearances indicate that it is a form of septicaemia what 

 is known as sputum septicaemia. After injection there is loss of appetite 

 and great debility, and the animal usually dies some time during the 

 second day after inoculation. The post-mortem examination shows a 

 local reaction, which may be of a serous, fibrinous, hemorrhagic, necrotic, 

 or purulent character; or there may be combinations of all of these 

 conditions. The blood of inoculated animals immediately after death 

 often contains the micrococci in very large numbers. For microscopic 

 examination they may be obtained from the blood of the veins, 

 arteries, or cavities of the heart, and usually from the pleural and 

 peritoneal exudations when these are present. 



True localized pneumonia does not usually result from subcutaneous 

 injections into susceptible animals, but injections made through the 

 thoracic walls into the substance of the lung may induce a typical 

 fibrous pneumonia. This was first demonstrated by Talamon, who 

 injected the fibrinous exudate of croupous pneumonia, obtained after 

 death or drawn during life from the hepatized portions of the lung, 

 into the lungs of rabbits. Wadsworth showed that by injecting virulent 

 pneumococci into the lungs of rabbits which had been immunized, a 

 typical lobar pneumonia was excited, the general bactericidal property of 

 the blood being sufficient to prevent the general invasion of the bacteria. 



Attenuation of Virulence. This may be produced in various ways. 

 The loss of virulence which occurs when the micrococcus is trans- 

 planted through several generations in culture fluid containing no 

 blood has already been referred to. An apparent attenuation of 

 virulence takes place also spontaneously in the course of pneumonia. 

 It has been shown by daily puncture of the lung in different stages of 

 the pneumonic process that the virulence of the organism diminished 

 as the disease progresses, and that the nearer the crisis was approached 

 the more attenuated it became. This attenuation is probably only 

 apparent. So many more micro-organisms are living in each cubic 

 centimetre of fluid during the early stages of a pneumonia that much 

 smaller quantities kill. If a little sputum be taken at different periods 

 in the disease and planted in ascitic bouillon the resultant cultures will 

 not vary greatly in virulence. The average virulence of cultures made 

 from pneumonic sputum is greater than in those from normal sputum. 



Restoration and Increase of Virulence. The simplest and perhaps the 

 most reliable method of restoring lost virulence for any susceptible 



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