58 GENERAL CONDITIONS FOR GROWTH 



temperature for growth in the air much higher than is ever 

 reached in their natural habitat, but they do grow at a 

 moderate temperature under anaerobic conditions (page 53) 

 When these organisms are grown with aerobes or facultative 

 anaerobes they thrive at ordinary temperatures. These 

 latter organisms use up the oxygen and keep the tension 

 low. This explains how thermophil organisms may grow in 

 the soil. 



These organisms illustrate the fact that relationships to 

 one environmental condition are greatly modified by some 

 other environmental condition. In this case growth at 

 high temperatures is dependent upon abundant oxygen 

 supply while growth at low temperatures can occur only 

 with diminished oxygen supply. Such interrelations of 

 environmental conditions are the rule and not the exception. 



OSMOTIC PRESSURE. 



Like all living cells bacteria are very susceptible to changes 

 in the density of the surrounding medium. If placed in a 

 medium less concentrated than their own protoplasm water 

 is absorbed and they tend to "swell up;" while if placed in a 

 denser medium water is given off and the cytoplasm shrinks 

 (plasmoptysis or plasmolysis) . Should these differences be 

 marked or the transition be sudden, the cell walls may even 

 burst and the organisms be destroyed. If the differences are 

 not too great or if the transition is made gradually, the organ- 

 isms may not be destroyed, but will either cease to grow and 

 slowly die out, or will show very much retarded growth, or 

 will produce abnormal cell forms. This is illustrated in the 

 laboratory in attempting to grow bacteria on food material 

 which has dried out. 



A practical application of osmotic effects is in the use of 

 a high percentage of sugar in preserving fruits, etc., and 

 in the salting of meats. Neither the cane-sugar nor the 

 common salt themselves injure the bacteria chemically, 

 but by the high concentration prevent their development. 

 In drying material in order to preserve it there are two 

 factors involved: first, the loss of water necessary for 

 growth and second, the increased osmotic pressure. 



