MORPHOLOGY AND PHYSIOLOGY OF BACTERIA, 125 



and of being repelled by others (negative chemotaxis). 

 Similar properties are widely distributed among living 

 cells, both animal and vegetable. 



CONDITIONS FAVORABLE FOR THE GROWTH OF 

 BACTERIA. 



Warmth. Among the different kinds of bacteria forms 

 are said to exist which multiply at temperatures as low as 

 o C, while there are species that multiply at 70 C. Bac- 

 teria which flourish at a very high temperature (maximum 

 about 70 C.) are called thermophilic. The pathogenic 

 bacteria usually flourish better at a point somewhere near 

 the temperature of the human body. This is not necessa- 

 rily the case with the non-pathogenic species. Ordinary 

 water bacteria thrive better at ordinary temperatures. 



Sternberg's method for determining the thermal death- 

 point of a species of bacteria is to draw portions of a pure 

 culture of the organism into capillary tubes with expanded 

 ends, when the tubes are sealed in the flame. The tubes 

 are supported upon a glass plate placed in a water-bath, 

 whose temperature is indicated by a thermometer, while a 

 uniform temperature is secured by stirring. The time of 

 exposure is, as a rule, ten minutes. The tubes should be 

 removed quickly to cold water. Their contents should 

 afterwards be inoculated into bouillon to determine whether 

 or not the organisms have been killed. 



Moisture is indispensable to the growth of bacteria, and 

 drying causes the death of certain kinds, as, for instance, 

 the spirillum of cholera. 



Food. There are a few species of bacteria that contain 

 chlorophyll, but it is wanting in most forms. On account 

 of the absence of chlorophyll, bacteria require, as part of 

 their food, organic compounds containing carbon, such as 

 sugar. They are unable, with possibly a very few excep- 



