374 PATHOGENIC BACTERIA. 



ical or of a whetstone form, and by transmitted light are 

 darker, more opaque, and less refractive than the typhoid 

 colonies. By reflected light, to the unaided eye they are 

 pale yellow. The surface-colonies are large, round, irreg- 

 ularly spreading, and are brown or yellowish-brown in 

 color. Hiss claims that by the use of these reagents the 

 typhoid bacillus can be readily detected in typhoid stools. 



When transferred to gelatin puncture-cultures the ba- 

 cilli 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 center. 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 growth formerly regarded as character- 

 istic 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 examination thorough. If, however, the 

 medium be touched with a platinum wire, it is discovered 

 that its entire surface is covered with a rather thick, in- 

 visible 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, for occasionally there will be encountered 

 a typhoid bacillus which will show a distinct yellowish 

 or brownish color. The typical growth seems to take 

 place only when the reaction of the potato is acid. 



In bouillon the only change produced by the growth of 

 the bacillus is a diffuse cloudiness. 



In milk a slight and slow acidity is produced. The 

 growth in milk is not accompanied by coagulation. 



The chief hindrance to the ready isolation of the 

 typhoid bacillus is the closely-allied Bacillus coli com- 

 iniiiiis. This organism, being habitually present in the 



