62 GENERAL CONDITIONS FOR GROWTH ' 



beings and animals; on clothing, utensils; in dwellings, 

 stables, outhouses, etc. From one-foiu'th to one-half of the 

 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 everjTvhere, it is an 

 entirely mistaken idea to suppose that all are injurious to 

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

 relatively few and are for the most part found only in close 

 association \\'ith 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 sitpply; 

 osmotic pressure; action of electricity; of Rontgen 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 organism can withstand, a minimum, or the least amount, 

 and an optimum or that amount which is most favorable for 

 development. Further, there might be distmguished a maxi- 

 mum for mere existence and a lower maximum for develop- 

 ment; also a minimum for mere existence and a higher mini- 

 mum for development. These maxima, minima, and optima 

 for bacteria haxe been determined with exactness for only 

 a very few of the general conditions and for comparatively 

 few kinds. 



