REACTIONS OF B. TYPHOSUS AND B. COLL 327 



The fermentation of lactose by the B. coli may also be demonstrated 

 by means of Petruschky's litmus-whey. The preparation of this 

 medium, which is somewhat difficult, is as follows. Fresh milk is 

 slightly warmed, and sufficient very dilute hydrochloric acid is added 

 to cause precipitation of the casein, which is now filtered off. Dilute 

 sodium hydrate solution is added up to, but not beyond, the point of 

 neutralisation, and the fluid steamed for one to two hours, by which 

 procedure any casein which has been converted into acid albumin by 

 the hydrochloric acid, is precipitated. This is filtered off, and a clear, 

 colourless, perfectly neutral fluid should result. Its chief constituent, 

 of course, will be lactose. To this, 5 per cent of a saturated alcoholic 

 solution of litmus is added, the medium is put into tubes and then 

 sterilised. After growth has taken place, the amount of acid found 

 can be estimated by dropping in standardised soda solution, till the 

 tint of an uninoculated tube is reached. 



The fermentation of sugars is a very important effect of 

 the growth of the B. coli. In a culture on a medium 

 equally rich in lactose, for example, and peptone, the former 

 will be broken up and the latter be left practically un- 

 affected. According to the first results of Chantemesse 

 and Widal, the B. typhosus did not ferment lactose. 

 Petruschky, however, states that it can do so in litmus whey. 

 Much seems to depend upon what other constituents are 

 present in the medium. Pere has confirmed the earlier 

 view, but finds that the typhoid bacillus, though it cannot 

 ferment cane sugar or lactose, can originate such a change 

 in arabinose, galactose, levulose, and glucose. The fer- 

 mentative power of the typhoid bacillus is thus, though 

 existent, much less active than that of the B. coli ; and as 

 a matter of practical experience the formation of bubbles 

 of gas in Chantemesse and Widal's lactose medium is 

 rarely observed. The test may, therefore, be taken in 

 conjunction w r ith others, as of use in diagnosing the identity 

 of the bacillus. 



Curdling of Milk by the B. Coli. This probably depends 

 on the fermentation of the lactose of the milk, and the 

 throwing down of the casein by the resulting lactic acid ; 

 but the action may be a more complicated one, as milk 

 can be curdled by organisms which do not possess acid- 

 forming properties. 



