THE FOOD or BACTERIA 69 



therefore the oxidation of sulphuretted hydrogen to sulphates also 

 takes place, thus liberating another supply of energy. 



Now we must deal with the modes in which the carbon must be 

 supplied to the bacteria. The best are the various forms of sugar, 

 peptone, fats, and proteids. It has also been obtained by the addition 

 of alcohol to the nutrient media. Thus glycerine (which is one of the 

 alcohols) is a foodstuff to some bacteria, whilst the acetic-acid bacteria 

 can derive their carbonaceous supply by the decomposition of ethyl- 

 alcohol. Some of the fatty acids, e.g. formic and acetic acids, may be 

 employed in the cultivation of some bacteria, but their food value is 

 not great. In some combinations the carbon is quite useless to the 

 bacteria. Examples are the cyanide compounds, salts of oxalic acid 

 and urea. Of course all the compounds of carbon which are in any 

 way antiseptic are useless as food materials. 



Let us turn now to the nitrogen supply. As in the case of carbon, 

 provision must be made for this, but great care must be taken in the 

 nature of the supply, for the different kinds of bacteria require the 

 nitrogen in particular forms, otherwise they cannot use it. For the 

 cultivation of the majority of forms nitrogen is supplied by means of 

 flesh extracts, proteids, peptone, serum, etc. In the case of the nitrite- 

 bacteria, the nitrogen-supply must be an ammonium compound. The 

 nitrate-bacteria must obtain their nitrogen in the form of a nitrite. 

 Again, still another class obtain their nitrogen direct from the free 

 nitrogen of the atmosphere. These different kinds are discussed more 

 fully in later chapters. 



§4. CHIEF CONDITIONS REGULATING GROWTH. 



In considering the (growth of bacteria, we must treat them in the 

 same way as we treat all living organisms, for not only must we supply 

 food but also water, a suitable temperature, and, in the case of the 

 majority of bacteria, free oxygen. It has already been stated that 

 more than three-fourths of the protoplasm of all organic bodies consists 

 of water. An animal deprived of water and given plenty of solid food 

 succumbs much sooner than if supplied with plenty of water but with 

 no solid food. The same applies to bacteria. In making up nutrient 

 media, the amount of solid material in comparison to the amount of 

 water is very small, in fact, some bacteria seem to be able to find 

 enough food in distilled water. This question of the proportion of 

 solid food is extremely important for the reason already mentioned. 



