BACTERIAL DEVELOPMENT 21 



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. 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, anti- 

 septics, and disinfectants. The products produced in the 

 nutrient medium by the bacteria themselves also sooner 

 or later inhibit or stop further growth ; a familiar instance 

 of this is seen in the alcoholic fermentation of sugar by 

 yeast, which ceases when the amount of alcohol reaches 

 12 to 14 per cent. The same reason probably accounts 

 for the fact that growths of bacteria in culture tubes 

 frequently do not spread all over the surface of the nutrient 

 medium, and why our cultures sometimes die out more 

 rapidly than might be expected. 



Another point affecting bacterial life is the presence of 

 a mixture of organisms in the same nutrient medium. If 

 there be a very vigorous form, it may ultimately grow 

 and multiply to such an extent as to crowd out and finally 

 kill the other forms with which it is associated, and if the 

 nutrient medium equally favour two species, that one 

 which is in an excess at the beginning may outgrow the 

 other. The occurrence of what has been termed symbiosis 

 is of considerable interest in the life of micro-organisms, 



