90 BACTERIA 



forms such as Bacterium termo, and species of Bacillus, 

 Vibrio, &c., will, however, flourish in Pasteur's solution, in 

 which they obtain their nitrogen in the form of ammonium 

 tartrate instead of decomposing proteid. It has also been 

 shown that some Bacteria can go further and make use of 

 nitrates as a source of nitrogen. But part passu with their 

 ordinary nutritive processes, many Bacteria exert an action 

 on the fluids on which they live comparable to that exerted 

 on a saccharine solution by the yeast-plant. Such microbes 

 are, in fact, organized ferments. 



Every one is familiar with the turning sour of milk. This 

 change is due to the conversion of the milk-sugar into 

 lactic acid. 



C G H 12 (! = 2(C 3 H 6 0) 3 . 



Sugar. Lactic Acid. 



The transformation is brought about by the agency 01 

 Bacterium lactis, a microbe closely resembling B. termo. 



Beer and wine are two other fluids which frequently turn 

 sour, there being in this case a conversion of alcohol into 

 acetic acid, represented by the equation 



C 2 H 6 + 2 = H 2 + C 2 H 4 2 , 

 Alcohol. Oxygen. Water. Acetic Acid. 



The ferment in this instance is Bacterium aceti, often 

 called My coder ma aceti, or the " vinegar plant." It will 

 be noticed that in this case oxygen enters into the reaction : 

 it is a case of fermentation by oxidation. 



Putrefaction itself is another instance of fermentation 

 induced by a microbe. Bacterium termo the putrefactive 

 ferment causes the decomposition of proteids into simpler 

 compounds, amongst which are such gases as ammonia 

 (NH.,), sulphuretted hydrogen (H 2 S), and ammonium 



