142 SCHIZOMYCETES 



Food Stuffs. i. Organic Foods. 



(a) The pure parasites (e. g., B. leprae) will not live 

 outside the living body. 



(6) Both saprophytic and facultative parasitic bac- 

 teria agree in requiring non-concentrated food. 



(c) The facultative parasites need highly organised 

 foods; e. g., proteids or other sources of nitrogen and 

 carbon, and salts. 



(d) The saprophytic bacteria are more easily culti- 

 vated; e. g., 



1. Some bacteria will grow in almost pure distilled 

 water. 



2. Some bacteria will grow in pure solutions of the 

 carbohydrates. 



3. Water is absolutely essential to the growth of 

 bacteria. 



Food of a definite reaction is needed for the growth 

 of bacteria. As a general rule growth is most active 

 in media which react slightly acid to phenolphthalein 

 that is, neutral or faintly alkaline to litmus. Mould 

 growth, on the other hand, is most vigourous in media 

 that are strongly acid to phenolphthalein. 



Environment. The influence of physical agents 

 upon bacterial life and growth is strongly marked. 



1. Atmosphere. The presence of oxygen is necessary 

 for the growth of some bacteria, and death follows 

 when the supply is cut off. Such organisms are termed 

 obligate aerobes. 



Some bacteria appear to thrive equally well whether 

 supplied with or deprived of oxygen. These are termed 

 facultative anaerobes. 



A third class will only live and multiply when the 

 access of free oxygen is completely excluded. These 

 are termed obligate anaerobes. 



2. Temperature. Practically no bacterial growth 

 occurs below 5 C., and very little above 40 C. 30 C. 



