QUALITATIVE BACTERIOLOGICAL ANALYSIS. 165 



(2) The Absence of Gas in Lactose Media. By means of 

 this test the typical members of the Colon group are at once 

 excluded from the typhoid group. Lactose media are recom- 

 mended for this test, as some bacteriologists declare that the 

 B. typhosus may ferment glucose media. Lip to the present 

 time I have never found the B. typhosus produce gas bubbles 

 in a glucose-gelatine " shake " culture, or in a glucose-agar-stab. 



(3) The Absence of Indol In Witters Peptone and Salt Solution. 

 This test is not quite so exclusive as the tests just mentioned. 

 Two typical cultures of B. typhosus (one from Krai's laboratory) 

 in my possession give traces of indol after seven days growth at 

 37 C. in this medium. Also there are varieties of B. coli which 

 fail to produce indol when cultivated in this manner. Witters 

 peptone is a much better medium than broth for this test. I 

 have often found indol produced in the peptone medium when 

 not a trace could be discovered in broth. Muscle sugar is some- 

 times found in broth, and bacteria first convert this into acid 

 and then neutralise it by producing alkali. The indol reaction 

 is not obtained when the bacteria are unable to neutralise the 

 acid produced. Theobald Smith recommends the use of 

 dextrose-free broth, which is obtained by inoculating bouillon 

 with B. coli. After twenty-four hours the broth is sterilised 

 and filtered, and peptone and salt added in the ordinary 

 manner. 



(4) The Unchanged Appearance of Milk. I have never found a 

 true culture of B. typhosus which coagulated milk even after 

 incubation at 37 C. for one month. At the same time there 

 are many varieties of B. coli which do not change milk ; the 

 same is true of varieties of Gartner's bacillus. 



(5) The Amount of Acid Produced in Litmus-whey. After 

 incubation for one week at 37 C. the amount of acid is very 

 small, as compared with the typical members of the Colon group. 

 At the same time there are atypical coliform organisms which 

 produce less acid than the B. typhosus, so the test is not 

 conclusive. 



(6) The Growth on Potato. The colourless growth on potato 

 is a time-honoured test of great value, but the character of 

 the growth depends on the reaction of the potato. If the 

 potato has an acid reaction, the growth is colourless and trans- 



