FOOD SUPPLY 17 



up of the protoplasm of the streptothrix into coccus- and bacillus- 

 like forms, links it to the other bacteria. 



GENERAL BIOLOGY OF THE BACTERIA. 



There are five prime factors in the growth of bacteria which 

 must be considered, namely, food supply, moisture, relation to 

 gaseous environment, temperature, and light. 



Food Supply. The bacteria are chiefly found living on the 

 complicated organic substances which form the bodies of dead 

 plants and animals, or which are excreted by the latter while 

 i li'-y are yet alive. Seeing that, as a general rule, many bacteria 

 grow side by side, the food supply of any particular variety is, 

 relatively to it, altered by the growth of the other varieties 

 present. It is thus impossible to imitate the complexity of the 

 natural food environment of any species. The artificial media 

 used in bacteriological work may therefore be poor substitutes 

 for the natural supply. In certain cases, however, the conditions 

 under which w r e grow cultures may be better than the natural 

 conditions. For while one of two species of bacteria growing 

 si<lu by side may favour the growth of the other, it may also 

 in certain cases hinder it, and therefore, when the latter is 

 grown alone it may grow better. Most bacteria seem to 

 produce exeretions which are unfavourable to their own 

 vitality, for, when a species is sown on a mass of artificial 

 food medium, it does not in the great majority of cases go on 

 growing till the food supply is exhausted, but soon ceases to 

 grow. Effete products diffuse out into the medium and prevent 

 growth. Such diffusion may be seen when the organism pro- 

 duces pigment, e.y. b. pyocyaneus growing on gelatin. In 

 supplying artificial food for bacterial growth, the general principle 

 ought to be to imitate as nearly as possible the natural surround- 

 iuu-s, though it is found that there exists a considerable adapt- 

 ability among organisms. With the pathogenic varieties it is 

 usually found expedient to use media derived from the fluids of 

 tin- animal body, and in cases where bacteria growing on plants 

 are being studied, infusions of the plants on which they grow 

 are frequently used. Some bacteria can exist on inorganic food, 

 but most require organic material to be supplied. Of the latter, 

 ><>me require proteid to be present for their proper nourishment, 

 while others can derive their nitrogen from a non-proteid such 

 as asparagin. All bacteria requre nitrogen to be present in 

 SMiiu- form, and many require to derive their carbon from 

 carl. ihydrat i >. Mineral salts, especially sulphates, chlorides, and 



