BACTERIA 33 



It may be possible for a human being to live without 

 bacteria in the alimentary tract, but some of those present 

 are beneficial in effect. A perfectly healthy young animal 

 may be born without bacteria in the intestines, but organ- 

 isms soon gain entrance with air and food, since practically 

 no object in the world of life is free of them. The ordinary 

 saprophytes of the intestinal tract assist in making fats 

 more easily assimilable and destroy some of the pathogenic 

 bacteria. 



Bacteria require for their life moisture some degree of 

 heat and a variety of foodstuffs. 



The amount of moisture is of little importance provided 

 sufficient is available to make up the physical bulk of the 

 organism and assist in the passage of foodstuffs through 

 the cell wall. The substances used by bacteria in nutrition 

 are dissolved or suspended in water. Temperature require- 

 ments are, however, more exact, and every class has its own 

 preferred degree. Those which commonly inhabit the animal 

 body require a temperature of 98 F. (37 C.), while those 

 living naturally in soil or water thrive best at 60 to 70 F. 

 (15-21 C.). Foodstuffs must contain the same substances 

 as for the growth of other plants, but the organisms which 

 infest the animal body grow most luxuriantly when animal 

 tissue or fluid is present. 



The reaction of the material upon which they are growing 

 is of no small importance. Nearly all bacteria live best 

 when the medium is about neutral or of faintly alkaline or 

 acid reaction. All need carbon, oxygen, nitrogen, hydrogen, 

 and salts. Some organisms cannot live in the presence of 

 free oxygen, but obtain it as they need it by breaking up, 

 or reducing, substances containing this element. These are 

 called anaerobic bacteria, such as the tetanus bacillus. Micro- 

 organisms that can live in the presence of atmospheric 

 oxygen are called aerobic; most pathogenic forms have this 

 power. 



The foodstuffs presented to bacteria are seldom in a pure 

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