372 MANUAL OF BACTERIOLOGY. 



them from colonies of the colon bacillus and other bacteria which form acids 

 from lactose. Neutral-red has been used in the same manner, as it is not altered 

 by the typhoid bacillus, but is changed by the colon bacillus to a yellow color. 

 This medium is prepared by adding to neutral, plain agar .05 grams of neutral- 

 red to i liter of agar or i per cent, of a saturated aqueous solution of neutral 

 red, some also add 0.3 per cent, dextrose. The material to be examined is 

 shaken up with the melted agar and the tubes placed upright in the incubator 

 at 35-37 C. These constitute the "shake tubes" mentioned in books on water 

 examination. Neutral-red may also be used in the same proportions as an 

 addition to beef-broth, and the cultures made in fermentation tubes. 



On potato it usually forms what is called an invisible growth; 

 that is, although no development is apparent to the eye, 

 numerous bacilli may be shown under the microscope in smear 

 preparations made from the surface of potato inoculated 

 about forty-eight hours previously. Occasionally a slight 

 visible growth is seen on potato. 



The typhoid bacillus is killed at 60 C. in ten minutes moist 

 heat; though Clark and Gage found that individual bacilli 

 may withstand 80 C. in fluid cultures for 5 minutes. It resists 

 drying well. It can survive in soil and sewage a long time. . 



For a comparison of the properties of the typhoid bacillus 

 and the colon bacillus see the latter. 



There it probably no other organism associated with an 

 infectious disease which presents so much difficulty in its 

 identification in given cases as the typhoid fever bacillus, and 

 yet there is none which so frequently demands early and 

 prompt identification. Bacteriologists are constantly con- 

 fronted with the demand from communities, physicians, and 

 laymen to give a positive answer as to whether a specimen of 

 water, milk, feces or other material does or does not contain 

 the typhoid bacillus. This demand has led to many efforts 

 on the part of bacteriologists to work out special methods 

 for the isolation and identification of the organism. These 

 efforts have placed in the hands of those who have had long 

 training the means of saying with some probability, but 

 hardly yet with absolute certainty, in all cases at least, that the 

 typhoid bacillus is or is not present in the material examined. 



