TYPHOID FEVER. 259 



typhoid bacillus (Fig. 75) and its near congener, the 

 Bacillus coli communis (Fig. 76). 



The colonies that develop upon such gelatin plate- 

 cultures are seen under the microscope to be brownish- 

 yellow in color, spindle-shaped, and sharply circum- 

 scribed. When superficial they are larger and form a 

 bluish iridescent layer with notched edges. The centre 

 of the superficial colonies is the only portion which 

 shows the yellowish-brown color. The margins of the 

 colony appear somewhat reticulated. The gelatin is not 

 liquefied. 



Unfortunately, the appearances of the colonies of the 

 Bacillus typhi and the Bacillus coli communis are iden- 

 tical, and make it next to impossible to select a single 

 colony of either with any certainty. The only solution 

 of the problem is to transfer a large number of colonies 

 to some culture-medium in which a characteristic of one or 

 the other species is manifested, and then study the growth. 



When transferred to gelatin puncture-cultures the 

 bacilli develop along the entire track of the wire, with 

 the formation of minute confluent spherical colonies. A 

 small thin whitish layer develops upon the surface near 

 the centre. The gelatin is not. liquefied, but sometimes 

 is slightly clouded in the neighborhood of the growth. 

 The growth upon the surface of obliquely solidified gela- 

 tin, agar-agar, or blood-serum is not very luxuriant. It 

 forms a thin, moist, translucent, non-characteristic band 

 with smooth edges. 



Upon potato a characteristic growth takes place. When 

 the potato is inoculated and stood in the incubating oven, 

 no growth can be detected at the end of the second 

 day, unless the observer be skilled and the examina- 

 tion thorough. If, however, the medium be touched 

 with a platinum wire, it is discovered that its entire sur- 

 face is covered with a rather thick, invisible layer of a 

 sticky vegetation which the microscope shows to be made 

 up of bacilli. No other bacillus gives the same kind of 

 growth upon potato. Unfortunately, it is not constant, 



