192 BACTERIA PATHOGENIC TO MAN 



neutral bouillon and many in alkaline bouillon produce for twenty- 

 four or forty-eight hours a more or less diffuse cloudiness, and fre- 

 quently a film forms over the surface of the broth. On shaking the 

 tube this film breaks up and slowly sinks to the bottom. This film is 

 apt to develop during the growth of cultures which have long been 

 cultivated in bouillon, and, indeed, after a time the entire development 

 may appear on the surface in the form of a friable pellicle. The 

 diphtheria bacillus in its growth causes a fermentation of meat-sugars 

 and glucose, and thus if these are present changes the reaction of the 

 bouillon, rendering it distinctly less alkaline within forty-eight hours, and 

 then, after a variable time, when all the fermentable sugars have been 

 decomposed, more alkaline again through the progressing fermenta- 

 tion of other substances. Among the products formed by its growth 

 is the diphtheria toxin. 



GROWTH IN ASCITIC OR SERUM BOUILLON. Diphtheria bacilli grow 

 well in this medium, even when first removed from the throat. They 

 almost always form a slight pellicle at the end of twenty-four to forty- 

 eight hours. To the nutrient bouillon 25 per cent, ascitic fluid or blood 

 serum is added. This culture medium is, as pointed out by Williams, 

 of the greatest value in attempts to get pure cultures of the diphtheria 

 bacillus from solidified serum cultures containing few bacilli. 



GROWTH ON GELATIN. The growth on this medium is much slower, 

 more scanty, and less characteristic than that on the other media men- 

 tioned, on account of the lower temperature at which it must be used. 



GROWTH IN MILK. The diphtheria bacillus grows readily in milk, 

 beginning to develop at a comparatively low temperature (20 C.). 

 Thus, milk having become inoculated with the bacillus from some cases 

 of diphtheria may, under certain conditions, be the means of conveying 

 infection to previously healthy persons. The milk remains unchanged 

 in appearance. 



Pathogenesis. The diphtheria bacillus is pathogenic for guinea- 

 pigs, rabbits, chickens, pigeons, small birds, and cats; also in a lesser 

 degree for dogs, goats, cattle, and horses, but hardly at all for rats and 

 mice. In spite of its pathogenic qualities for these animals true diph- 

 theria occurs in them with extreme rarity. As a rule, supposed diph- 

 theritic inflammations in them are due to other bacteria which cannot 

 produce the disease in man. 



The virulence of diphtheria bacilli from different sources, as meas- 

 ured by their toxin production in bouillon, varies enormously, but in 

 ascitic fluid it is more alike. Thus 0.002 c.c. of a forty-hour bouillon 

 culture of one bacillus will kill a guinea-pig, which it would require 1 c.c. 

 of the culture of another bacillus to kill. This difference frequently 

 depends on the unequal growth of the bacilli; one culture having fifty 

 times as many bacilli as the other. The same marked variation occurs 

 in the amount of toxin produced by different bacilli in their growth 

 outside of the body. Moreover, the diphtheria bacilli differ greatly 

 in the tenacity with which they retain their virulence when grown 

 outside the body. The bacillus that we have used in the labora- 



