164 GENERAL BIOLOGY OF MICRO-ORGANISMS 



mo us development of knowledge gained by the first- mentioned 

 method, which quite over-shadows our knowledge of morph- 

 ology and structure in this group of organisms. 



THE INFLUENCE OF ENVIRONMENTAL FACTORS. 



Moisture. Moisture is indispensable to the growth of mi- 

 cro-organisms. A few species will grow and multiply in almost 

 pure distilled water. Drying causes the death of the majority of 

 the vegetating cells, of some more readily than others, while the 

 spore forms may remain alive in a dry condition for many years. 



Heim 1 found that pathogenic bacteria resist drying much 

 longer when contained in pathological tissues or exudates from 

 animals which have succumbed to the disease, than when they 

 are taken from artificial cultures. 



Organic food. One species of bacteria, Nitrosomonas of 

 Winogradsky, lives, grows and multiplies without organic food, 

 utilizing the gases of the atmosphere as its source of carbon and 

 nitrogen. From the standpoint of nutrition this organism is 

 among the most primitive of beings. Other bacteria are known 

 which may grow in water containing only mineral salts and a 

 simple sugar, utilizing large quantities of atmospheric nitrogen. 

 These are known as nitrogen-fixing bacteria. Most of the bac- 

 teria, yeasts and molds require a small amount of nitrogenous 

 organic matter as food, such as the ammo-acids or albumoses, and 

 many of them flourish better when furnished a fermentable 

 carbohydrate such as dextrose. The complex organic molecules 

 are utilized in part to build up the substance of the bacteria, but 

 a much larger part of them is broken down into simpler and more 

 stable substances, such as carbon dioxide, simple fatty acids, 

 ammonia and water, with the liberation of energy. Sapro- 

 phytic organisms are those which grow on dead organic matter. 

 Micro-organisms of still narrower adaptibility grow well in artifi- 

 cial culture only if they be furnished abundant protein or nucleo- 



1 Zeitschriftf. Hygiene, Apr. 4, 1905, Bd. L, No. i, p. 123. 



