50 GENERAL CONDITIONS FOR GROWTH 



dry weight of the feces of animals and men is due to the 

 bacteria present. The urine is practically free from them 

 in health.^ 



While bacteria are thus found nearly everywhere, it is an 

 entirely mistaken idea to suppose that all are injurious to 

 man. As a matter of fact, those which are dangerous to man 

 are relati\'el}^ few and are for the most part found only in close 

 association with man. Most bacteria are harmless and the 

 vast majority are beneficial or even essential to man's 

 existence on the earth. These facts must be constantly 

 borne in mind, and it is hoped that the pages which follow 

 will make them clear. 



In order that any organism may thrive there are a number 

 of general environmental conditions which must be fulfilled. 

 These conditions vary more or less for each kind of organ- 

 ism. Bacteria are no exception to this general rule. These 

 conditions may be conveniently considered under the gen- 

 eral heads of moisture; temperature; light; oxygen supply; 

 osmotic pressure; action of electricity; of Roentgen and radium 

 rays; pressure; mechanical vibration; and chemical environ- 

 ment, including the reaction of the medium, the effect of injurious 

 chemicals, and especially the food requirements of bacteria. 

 For each of these conditions there is a maximum, meaning 

 the greatest amount of the given condition which the organ- 

 ism can withstand, a minimum, or the least amount, and 

 an optimum, or that amount which is most favorable for 

 development. Further, there might be distinguished a 

 maximum for mere existence and a lower maximum for 

 developmeiit; also a mimum for mere existence and a higher 

 minimum for development. These maxima, minima and 

 optima for bacteria have been determined with exactness 

 for only a very few of the general conditions and for com- 

 paratively few kinds. 



The student cannot too early in his course grasp the 

 idea that sufficient variation in any one of the environmental 

 conditions will modify the characteristics of an organism. 



1 B. Renault has found bacteria in fossil remains of various epochs, 

 Jurassic, Permian, Carboniferous, Devonian, Comp. Rend. Ac. Sci., 1894, 

 119, 377-379; 1895, 120, 162-164, 217-220; 1896, 122, 1226; 1898, 126, 1828- 

 1831. 



