viii BACTERIA AS FERMENTS 91 



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, and of a carbonate or even 

 of carbon dioxide as a source of carbon : in other words, 

 they are able to live upon purely inorganic matter in spite 

 of the fact that they contain no chlorojihyll. Some species 

 may even multiply to a considerable extent in distilled water. 



But pari passu with their ordinary nutritive processes, 

 many Bacteria exert an action on the fluids on which 

 they live comparably to that exerted on a saccharine 

 solution by the yeast-plant. Such microbes are, in fact, 

 organized ferments (see p. 81). 



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

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

 lactic acid. 



^6^1206 = 2(C3Hg03), 

 Sugar. Lactic Acid. 



The transformation is brought about by the agency of 

 BaderiuTfi 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 — 



QHgO + O2 = HP + C^H.O^, 

 Alcohol. Oxygen. Water. Aceiic Acid. 



The ferment in this instance is Bacterium aceti, often 

 called Mycodenna 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 



