114 MANUAL or BACTERIOLOGY. 



After the flagella have been stained, the bacteria appear some- 

 what larger than when stained by the ordinary methods. The 

 flagella upon the bacilli of typhoid fever are numerous and form 

 a very striking picture. 



Chemotaxis. — Motile bacteria possess the property of being 

 attracted by certain substances (positive chemotaxis) and of 

 being repelled by others (negative chemotaxis). Similar 

 properties are widely distributed among living cells both animal 

 and vegetable. 



Conditions Favorable fob. the Growth or Bacteria. 



Warmth. — ^Among the different kinds of bacteria forms 

 exist which multiply at temperatures as low as o° C, while there 

 are species that multiply at 70° C. Bacteria 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 necessarily 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 in- 

 dicated 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 afterward be inoculated into bouillon to 

 determine whether or not the organisms have been killed. 

 In the practical use of heat for sterilization or disinfection, the 

 exact thermal death-point is greatly exceeded. The time of 

 exposure is also longer than is absolutely necessarj' as deter- 

 mined by the results of the experiments. 



