32 GROWTH OF BACTERIA 



the oxygen, there are large numbers of species which are 

 indifferent to the admission or exclusion of air, thriving 

 more or less satisfactorily under both conditions. Those 

 kinds, such as certain forms of lactic acid bacteria, and 

 many of the organisms causing putrefaction, which are 

 usually aerobic, but which can still grow and multiply 

 when the supply of atmospheric oxygen is diminished, 

 are described as facultative anaerobic bacteria ; they can 

 live as anaerobes when necessity arises. The species 

 which generally live as anaerobes, but are yet capable of 

 existing under conditions which admit of the access of 

 fresh air, are termed facultative aerobic bacteria. 



5. Food. Food is essential to the life of all forms 

 of living things. Out of it they are able to construct 

 the material used in building up their bodies and in 

 repairing the wear and tear which goes on within them. 

 Moreover, the energy needed to maintain their vital 

 activity is derived chiefly from chemical changes induced 

 in the food which they consume. The substances 

 utilized as food by bacteria are always taken from 

 watery solutions, the dissolved materials diffusing into 

 the interior of the organism through the surrounding 

 cell-walls, although there are no visible openings in' the 

 latter. Such passage or diffusion of dissolved sub- 

 stances through membranes which have no apparent 

 openings in them is termed osmosis. While a few species 

 of bacteria are known which are able to develop when 

 supplied with very simple chemical compounds, such 

 as carbon dioxide, nitrogen, water, and inorganic salts, 

 the majority of them are unable to grow and multiply 

 unless they are provided with more complex organic 

 compounds, such as sugar, proteins, and similar sub- 

 stances. In respect of the source from which the 



