THE BACILLUS OF TYPHOID FEVER. 361 



or at the room temperature in three or four days, the surface of 

 the potato has a moist, shining appearance, but there is no visible 

 growth such as is produced by many other bacteria upon this me- 

 dium. A simple inspection would lead to the belief that no growth 

 had occurred; but if with a platinum needle a little material is 

 scraped from any portion of the shining surface and a stained pre- 

 paration is made from it, numerous bacilli will be seen, some of 

 which are likely to be in the form of quite long threads, while others 

 are short and have rounded extremities. This "invisible growth" 

 has been shown by the researches of Buchner and others to be most 

 characteristic upon potatoes having a decidedly acid reaction, as is 

 usually the case. When cultivated upon potatoes having an alkaline 

 reaction a thin, visible film of a yellowish-brown color and of limited 

 extent may be developed. Inasmuch as several common and widely 

 distributed bacteria closely resemble the typhoid bacillus in form and 

 in their growth in nutrient gelatin, this character of invisible growth 

 upon potato is very important for its differentiation, especially as the 

 common bacilli referred to Bacillus coli communis, bacillus of Em- 

 merich produce a very distinct and rather thick, yellowish- white 

 mass upon the surface of potato. But recent researches show that 

 this invisible growth, although not a common character, does not 

 belong exclusively to the typhoid bacillus (Babes). 



This bacillus in its development in culture media produces acids 

 according to Brieger small quantities of volatile fat acids, and, in 

 presence of grape sugar, lactic acid. It also grows readily in a de- 

 cidedly acid medium, and this character has been employed as a test 

 for differentiating it from other similar bacilli ; but some of these 

 also grow in a decidedly acid medium, and too much reliance cannot 

 be placed upon this test. 



Brieger has shown that indol is not produced in cultures of the 

 typhoid bacillus, and Kitasato has proposed to use the indol test for 

 differentiating this from other similar bacilli which are said, as a 

 rule, to give the indol reaction. This test consists in the addition to 

 ten cubic centimetres of a bouillon culture which has been in the in- 

 cubating oven for twenty-four hours, of one cubic centimetre of a 

 solution of sodium nitrite (0. 02 gramme to one hundred cubic centi- 

 metres of distilled water), together with a few drops of concentrated 

 sulphuric acid. If indol is present a red color is developed. 



None of the above-mentioned tests are entirely reliable, but, taken 

 together with the morphological and biological characters above de- 

 scribed, they may enable the bacteriological expert to give a tolerably 

 confident opinion as to the presence of this bacillus in a water supply 

 suspected of contamination, etc. And when a bacillus having these 

 characters is obtained in a pure culture from the spleen of a typhoid 



