THE FOOD OF BACTERIA 67 



are too concentrated, they tend to withdraw water from the cells, 

 with disastrous results. It is, therefore, always preferable to risk 

 making the solutions too dilute than too concentrated. Again, a 

 solution which, when dilute, may be a nutrient material, is often an 

 antiseptic in the concentrated form. An example of this is sugar, 

 which up to 10 per cent, can serve as food to bacteria, at 50 per cent. 

 is a strong antiseptic. 



Bacteria have been known to thrive in a liquid containing only 

 sugar. In fact, it was stated by Fermi that he had obtained bacteria 

 which had no nitrogen in their composition, because he claimed to 

 have cultivated them in a solution of water and sugar. It has since 

 been shown that Fermi's solution had in it very small quantities of 

 foreign matter derived from various sources, and these quantities, 

 though extremely small, were yet sufficient to supply the nitrogen 

 required. This shows that bacteria can thrive in extremely dilute 

 solutions. 



Of the various elements necessary for the building up of the 

 bacterial cell oxygen and hydrogen are amply supplied by the 

 water. They are also supplied by the various compounds which 

 are used as food-material for bacteria, and into whose composition 

 they enter largely. With the exception of two, carbon and nitrogen, 

 we may assert that the same applies to all the other necessary 

 elements. 



With regard to carbon, this substance is necessary, because it forms 

 a large proportion of the body of the cell. Also, the energy which the 

 bacteria require to perform the various functions of life, division, multi- 

 plication, growth, etc., is best obtained by the breaking down of some 

 carbonaceous compound like sugar, or a similar carbohydrate. In 

 making our medium we must, therefore, for most purposes, make 

 provision for this supply. In most of the nutrient media that are 

 used for the cultivation of bacteria, carbon is supplied in the form of 

 flesh-extract, peptone, sugar, etc. The necessary energy can, however, 

 be supplied in other ways than by the breaking down of carbonaceous 

 organic material. For instance, the sulphur-bacteria derive additional 

 energy by the oxidation of sulphuretted hydrogen, first into sulphur, 

 and later into a sulphate. It, therefore, follows that we can cultivate 

 these bacteria with a very small quantity of carbonaceous material. 

 Another class, the nitrate-bacteria, which transform nitrites into 

 nitrates, can be cultivated without having carbon supplied to them 

 in the organic form, in fact, they seem rather to suffer when such 

 material is presented to them. 



