FOOD REQUIREMENTS OF BACTERIA 57 



it will not ignite J nor will it if the air, match, and surface on 

 which it is rubbed is absolutely dry. It is the same with the changes 

 going on within the cell. Water must be present or the cell reac- 

 tions will not proceed normally. (5) Water enters largely into 

 the composition of the cell, as bacteria consist of from 70 to 90 

 per cent water. (6) Water gives to the cell its shape. If we 

 place a cell in a strong salt solution water is drawn from it. If 

 we place it in distilled water it bursts. The force which causes 

 the bursting is called osmotic pressure. The extent of this pres- 

 sure varies with different cells. It averages about the same as that 

 of a 0.9 per cent salt solution. This is about 7.1 atmospheres. 

 In such a solution the tissues neither gain nor lose weight. 



Carbon Requirements. — All bacteria require carbon. But 

 in what queer form some microbes take their carbon ! There are 

 bacteria in the soil which utilize the poisonous carbon monoxid or 

 inert carbon dioxid. The microbe cannot get energy but only 

 building material from the latter. We use formaldehyde to de- 

 stroy disease-producing bacteria; yet there are soil organisms which 

 will flourish on dilute formaldehyde gas. Both the soil and ma- 

 nure contain bacteria which use urea. From some of it they con- 

 struct their bodies, but most of it is changed into ammonia which is 

 used by other tiny plants. The woody fiber of the plant yields its 

 carbon to bacteria, and there are writers who claim that coal is 

 produced by the action of bacteria upon plants. Be this as it may, 

 Renalt describes fossil bacteria, which are found in coal, and 

 today we know microorganisms which can get their required car- 

 bon from coal. Shall we say that of the various forms of carbon 

 that diamond and graphite alone are free from the attack of bac- 

 teria? Most bacteria, however, require their carbon in the form 

 of a carbohydrate. The sugars are especially accepted. The ni- 

 trogen-fixing organisms of the soil oxidise these to simple acids 

 and gases, and with the energy thus obtained convert nitrogen into 

 complex organic compounds. They seem to have a special prefer- 

 ence for the three- and six-carbon sugars. There are microorgan- 

 isms which will pick from a medium all of one compound leaving 

 others very similar untouched. This peculiar property is often 



