BIOLOGICAL CONDITIONS 1 9 



only grow when deprived of free oxygen are known as obligate 

 anaerobes, while those that require the presence of oxygen are 

 called obligate aerobes. Those that grow under either conditions 

 are named facultative anaerobes. Free oxygen is needed for spore 

 formation by certain bacteria. Anaerobes obtain oxygen as 

 they need it by breaking up their foodstuffs. 



Nutriment is most important for the growth of bacteria, 

 nitrogenous compounds (albumins) particularly being required. 

 Simple aquatic forms of bacteria can live and grow in distilled 

 water. The addition of the various sugars is of advantage in 

 the cultivation of many bacteria, and glycerine for the growth 

 of some members of the Mycobacteriacea. Blood serum or whole 

 blood is required by some pathogenic organisms. The foodstuffs 

 must be in a form that can diffuse through the cell wall. 



The temperature of the medium in which various bacteria 

 grow is most important. Bacterial growth is possible between 

 oC. and 7oC., some varieties thrive at the one extreme, and 

 others at the other. 



Psychrophilic bacteria, are those that grow at i5C., with a 

 maximum of 3oC. and a minimum of oC. Water bacteria of 

 the polar seas belong to this group. 



Mesophilic grow best at 37C. the temperature of the body 

 and thrive from ioC. (minimum) to 45C. (maximum). All 

 pathogenic bacteria belong to this group. 



Thermophilic (min. temp. 4oC., max. 6o-7oC.) are most 

 prolific at 50-5 5 C. To this class belong bacteria of the soil. 

 All of this class are spore-bearing. 



Darkness favors bacterial growth. 



Association of different kinds of bacteria is of some importance 

 in their growth and welfare and when thus associated, they some- 

 times benefit each other. Such combination is called symbiosis. 

 Antibiosis is the condition when one or more of a mixture of 

 organisms suffers by the presence of others, e.g., the destruction 

 of putrefactive germs in the intestinal tract by lactic acid bacilli. 



