THE GROUP OF THE DYSENTERY BACILLI 317 



the ends of the organisms stain somewhat more heavily than the 

 centre. All the organisms comprising this group are Gram-negative. 



Isolation and Culture. The dysentery bacilli grow well on ordinary 

 laboratory media. Colonies on agar, after eighteen to twenty-four 

 hours' incubation at the body temperature, are round, transparent 

 and colorless; frequently they attain a diameter of from 1 to 3 mm. 

 The colonies are indistinguishable from those produced by bacilli of 

 the typhoid and paratyphoid groups. There is moderate growth 

 along the line of inoculation in gelatin, but no liquefaction. In broth 

 after eighteen to twenty-four hours' growth a uniform turbidity 

 develops, somewhat more luxuriant in dextrose than in plain broth. 

 After several days' growth in plain broth a delicate pellicle frequently 



FIG. 45. Bacillus dysenterise. Shiga type, bouillon culture. X 1000. . 



appears on the surface of the latter medium. In milk moderate devel- 

 opment takes place with no coagulation. There is 'an initial acidity 

 followed after from two to five days by an alkaline reaction, which 

 increases somewhat in intensity with the age of the culture. On potato 

 the growth is very similar to that of the typhoid bacillus; on acid 

 potato the growth is almost invisible; on alkaline potato the growth 

 is brownish and of moderate luxuriance. 



The dysentery bacilli are aerobic, facultatively anaerobic bacilli 

 whose limits are approximate^fcthe following; minimum temperature 

 of growth 8 C.; maximum 42 to^ C.; optimum 37 C. 



Cultures of dysentery bacilli varyWimewhat in their resistance to 

 heat. The majority of cultures are killed by an exposure of ten min- 

 utes at 65 C. Some strains, however, are only killed by an exposure 

 of ten minutes at 70 C. The organisms are moderately resistant to 



