26 A MANUAL OF BACTERIOLOGY 



Free oxygen is essential to the growth of some organ- 

 isms ; these are termed strictly aerobic. Others will 

 not develop in its presence, strictly anaerobic ; others, 

 again, while preferably aerobic or anaerobic, will grow in 

 the absence, or in the presence, of oxygen, and are 

 respectively termed facultative anaerobic or facultative 

 aerobic. The distinction between aerobic and anaerobic 

 organisms though in the main true has limitations, for 

 it is possible to " educate " strictly anaerobic forms to 

 grow aerobically. Some organisms are strictly parasitic 

 on animals or plants ; others live in water, soil, decaying 

 matter, etc. these are termed saprophytes ; and many 

 are able to exist either as parasites or as saprophytes. 



Bacterial development is much influenced by the 

 presence of foreign substances in the nutrient medium. 

 A number of metallic and other salts, chlorine, bromine, 

 and iodine, carbolic acid, salicylic acid, etc., have an 

 injurious effect upon bacterial life, inhibiting or stopping 

 growth, or killing the organisms outright ; these are of 

 considerable practical importance and are known as 

 germicides, antiseptics, and disinfectants. The products 

 produced in the nutrient medium by the bacteria them- 

 selves also sooner or later inhibit or stop further growth ; 

 a familiar instance of this is seen in the alcoholic fermenta- 

 tion of sugar by yeast, which ceases when the amount of 

 alcohol reaches 12 to 14 per cent. It is this accumulation 

 of metabolic products that probably limits the growth of 

 bacteria in culture tubes and is the cause of the ultimate 

 death of organisms in cultures. 



The individual organisms of a culture vary in their 

 resistance to germicidal agencies, so that while the 

 majority of the organisms of a particular culture may be 

 destroyed by a certain procedure, a few may survive. 



Another point affecting bacterial life is the presence of 

 a mixture of organisms in the same nutrient medium. If 



