440 THE BACILLUS OF TYPHOID FEVER. 
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, asa 
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, ete. And when a bacillus having these 
characters is obtained in a pure culture from the spleen of a typhoid 
cadaver the student may be very sure that he has the typhoid bacillus. 
But in the presence of various similar bacilli, as in faeces, very careful 
comparative researches will be required to determine in a definite 
manner that a non-liquefying bacillus obtained in pure cultures by 
the plate method is really the one now under consideration—espe- 
cially so as the cultures of the typhoid bacillus in the same medium 
may differ considerably at different times, and a number of bacilli 
are known which resemble it so closely that it is still uncertain 
whether they are to be considered as varieties of the typhoid bacillus 
or as distinct species. Thus Babes, in an extended research, found in 
the organs of typhoid cases, associated with the true typhoid bacillus, 
other bacilli or varieties very closely resembling it. He has also 
described three varieties (?), obtained by him from other sources, 
which could only be differentiated from the true typhoid bacillus by 
very careful comparison of cultures made side by side in various 
media. 
