10 BACTERIA. 



of the casein, is frequently considerably increased ; so that 

 it would appear that the energy required for the building 

 up of the pigment substance was in this case diverted into 

 another channel, and lactic acid, and perhaps other sub- 

 stances, are produced in place of the usual pigment. 



This example may afford some idea of the complexity of 

 the problems that have to be grappled with by bacterio- 

 logists, and it may also help to explain how such different 

 results have been obtained by different observers, and how it 

 still may be possible to reconcile statements which at first 

 sight appear to be in direct opposition. 



It is a well-known fact that if the lactic acid fermentation 

 once obtains a footing, in a solution of sugar for example, 

 many of the other putrefactive bacteria find it impossible 

 to develop. On the other hand, whilst the lactic acid 

 organism cannot grow beyond a certain point, yeasts 

 are perfectly able to develop where there is a certain 

 quantity of acid present. This has been adduced as an 

 explanation of the fact that comparatively pure yeast 

 fermentation may go on even in the presence of the lactic 

 acid organism, when it is unable to make further head- 

 way and when other putrefactive organisms are unable to 

 grow at all. 



Another fermentation, the results of the careful studies 

 of which have been utilized in the commercial production of 

 vinegar, is that due to the acetic acid ferments Mycoderma 

 aceti and Mycoderma Pastorianum, by the action of either 

 of which alcohol is converted into acetic acid. 



In all the works on brewing, lists of micro-organisms that 

 give rise to such conditions as bitterness, muddiness, various 

 colorations red, yellow, green, etc. are given, and it is 

 pointed out at the same time that all appear to give rise 

 to some form of acid. It is supposed, for example, that 

 certain species of sarcinae are responsible for the sour and 

 bitter tastes which are sometimes developed in beer ; a similar 

 organism is also described as giving rise to the red colour 

 which is sometimes developed in white beer, though it may 

 be remarked that in this case a rod-shaped bacterium is also 

 frequently present, and may be answerable for the presence 

 of the substances that so seriously alter the taste of this 

 beverage. A series of other organisms, which Lindner 

 speaks of as Pediococci, all produce acid, and give rise to 



