580 Introduction to the Study of Science 



288. External conditions affecting bacteria. The conditions 

 upon which bacteria depend for life and multiplication are 

 physical, chemical, and biological. The physical conditions are 

 generally those considered for molds and yeasts, namely, mois- 

 ture, temperature, light, nutritive materials, and possibly 

 electricity. These need only summary statement. While 

 moisture is necessary to growth, bacteria are very resistant to 

 drought, and their spores remain alive for years in extreme dry- 

 ness. Two facts make this clear: first, the spores contain 

 practically no moisture, and second, the protective membrane of 

 spores and bacteria is almost impermeable to water. 



Temperature varies for different kinds and for different 

 individuals of a kind. Some are rendered inactive by freezing, 

 as those in milk ; hence by keeping a liquid frozen the number 

 may be diminished. But those surviving resume growth as 

 soon as sufficient warmth is provided. Many kinds resist 

 extremely low temperatures. Some investigators have kept 

 certain species in a temperature of 252 C. for twenty hours 

 with no injury to the organisms. 



High temperatures have a deterrent influence upon certain 

 kinds. Pasteurizing milk, or keeping it from fifteen to thirty 

 minutes at a temperature of 140 to 160 F., checks the growth 

 of bacteria and kills any of the disease-causing variety, as those 

 causing typhoid fever. But bacteria of the " heat-loving " 

 variety thrive only at a high temperature. Some of these grow 

 rapidly in a temperature of 160 F. and become inactive when the 

 temperature falls much below 125 F. Such bacteria are found 

 in compost, in vegetable matter in silos, in hay, and tobacco, all 

 of which develop in fermentation a high temperature. The 

 spontaneous heat of moist cotton waste is due to bacteria of the 

 heat-loving variety. Many kinds, including those causing 

 diseases in animals and men, are adapted to the temperature of 

 the bodies in which they are found. 



Destruction of bacteria by heat is possible, but not always 

 practicable. In food materials cooking usually destroys them, 



