EXPERIMENT STATION BULLETINS 221 



through the air, traces of nitrogenous compounds are found. No matter 



in what direction the attention is turned, there is sufficient 

 Oxygen. nitrogen for the needs of bacteria. 



Oxygen is also essential to bacteria. Many of them require 



thefiee oxygen of the air the same as man; but others are 

 Asrobicaad poisoued by this same free oxygen", that is, there are those bac- 

 Anasrobic. teria which will not grow in the presence of air. Two classes 



of bacteria therefore spring from their reactions to oxygen of 

 the air, the one requiring this oxygen, the aerobic bacteria; the other 

 poisoned by this free oxygen, the anaerobic bacteria. From this the con- 

 clusion must not be drawn that the free oxygen of the air is the only 

 source for the aerobic bacteria, and that the anaerobic bacteria do with- 

 out oxygen in any form; on the contrary both classes utilize oxygen in 

 combination with other elements as found iu organic matter. 



Bacteria as a rule are unable to absorb carbonic acid gas as 

 Carbon. the higher plants with their chlorophyll, yet some of them are 



able to employ it in their functional processes. For their own 



sustenance they obtain their carbon along with the other ele- 

 Hydrogen, meuts in organic substances. This is also true of hydrogen, 

 Phosphorns, phosphorus, sulphur, and inorganic salts necessary to their 

 saiphnr, etc. development. 



No bacterial development occurs without the presence of 

 Moisture, moisture. Whenever the micro-organism finds a dry habitat it 



ceases to grow, although it may remain alive for days and 

 months. Bacteria stand in need of moisture as much as the wheat plant. 

 The spore, the most resistant form of the micro-organism, may lie dormant 

 for years if moisture is absent. Like the grain of wheat, it will not germi- 

 nate unless sufficient moisture prevails. Dust contains life, but it is not 

 until it reaches a humid condition that it gives activity to the life 



within. 

 Temper- Bacteria differ widely in their relation to temperature, but 



ature. every species has its minimum, optimum, and maximum; that 



is, there is a point so low at which a certain species will cease 

 to grow and one so high that it will not develop, and between these ex- 

 tremes a point will be found at which this species will flourish best. Each 

 species has its own minimum, optimum, and maximum temperature. 

 Some bacteria will grow at 8" C. [46° F.] and others at 70° C. [166° F.]. 

 As a rule, however, those found outside of tbe animal body will grow best 

 at about 28° C. [82° F.] to 30° 0. [86° F.]. and those connected with 

 infectious diseases require 'dl^° C. [98° F. ] for their optimum. The 

 tubercle bacillus develops only within a range of two degrees and 

 others have a very narrow range of temperature, as the bacillus of glanders 

 and the micrococcus of pneumonia; still most bacteria have a latitude of 

 15° or 20° in which they will grow. Meat kept below 15° 0. [59° F.] will 

 not decompose rapidly and milk will remain sweet much longer at this 

 temperature, simply because the bacteria lie dormant and will not grow. 

 Change in temperature is quickly noticed by bacteria. 



Bacteria will not bear a strongly acid or alkaline solution. 

 Reaction. They may remain alive but refuse to grow. Some bacteria will 



grow in a slightly acid medium, but the reaction most favorable 

 to bacteria as a class is neutral or slightly alkaline. 



