646 PATHOGENIC MICROORGANISMS 



by a slight moist glistening, an appearance which is in marked contrast 

 to the grayish-yellow or even brown and abundant growth of colon 

 bacilli. If the potato medium is rendered neutral or alkaline, this 

 distinction disappears, the typhoid bacillus growing more abundantly. 



In milk, typhoid bacilli do not produce coagulation. In litmus-milk, 

 during the first twenty-four hours, the color is changed to a reddish or 

 violet tinge by the formation of acid from the small quantities of mono- 

 saccharid present. Later the color becomes deep blue owing to the 

 formation of alkali. 



In Dunham's pepton solution no indol is produced. According to 

 Peckham, however, continuous cultivation in rich pepton media may 

 lead to eventual indol formation by typhoid bacilli. This fact has no 

 bearing on the value of the indol test, as indol is never have formed 

 under the usual cultural conditions. 



Tested for its power to form acid from sugars commonly used in 

 differential tests, typhoid bacilli form add, but no gas, on the mono- 

 saccharides, on mannit, maltose and dextrin, and neither add nor gas on 

 lactose and saccharose. (See Table, p. 718.) 



In the Hiss tube medium (formerly employed extensively) the 

 typhoid bacillus within eighteen to twenty-four hours produces an even 

 clouding by virtue of its motility, but does not form gas. In contradis- 

 tinction to this, dysentery bacilli grow only along the line of inocula- 

 tion, while bacilli of the colon group move in irregular sky-rocket-like 

 figures away from the stab, at the same time breaking up the medium 

 by the formation of gas-bubbles. Some actively motile colon bacilli 

 cloud the medium, but the ruptures caused by the gas are always 

 evident. 



The differentiation of the typhoid bacillus in pure culture from similar 

 microorganisms by means of its growth upon media has been the sub- 

 ject of many investigations. It is neither practicable nor desirable to 

 enumerate all the various media which have been devised and reported. 

 The aim has been chiefly the differentiation of typhoid bacilli from the 

 bacilli of the colon group, and most of the media have been devised with 

 this end in view. (See section on Media.) 



Rothberger 5 devised a mixture of glucose agar to which is added 

 1 per cent of a saturated aqueous solution of neutral-red. Shake- 

 cultures or stab-cultures are made in tubes of this medium. The typhoid 

 bacillus causes no changes in it, while members of the colon group, by 



5 Rothberger, Cent, f . Bakt., xxiv, 1898. 



