MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. 1 25 



organism may look like a writhing mass of maggots or like 

 tadpoles in a pool. The motility is most active in young 

 cultures. The movement results from the vibration of little 

 processes, or ilagella (Fig. 45). Of these there may be one or 

 several, placed singly or in groups, at the end, or scattered 

 around the sides. They are extremely difficult to demonstrate 

 except by special staining methods, which, furthermore, are 

 quite uncertain of result. After the flagella have been stained, 

 the bacteria appear somewhat 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 prop- 

 erties are widely distributed among living cells, both animal 

 and vegetable. 



Favorable and Unfavorable Conditions for Growth. 



Warmth. — Among the different kinds of bacteria forms 

 exist which multiply at temperatures as low as 0° 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 

 best at ordinary temperatures. 



Sternberg's method for determining the thermal death-point 

 of a species of bacteria is to draw a portion of a pure culture of 

 the organism to be tested up into a capillary tube which has a 

 small bulb on one end, and after sealing the end of the capil- 

 lary tube in the flame the tubes are placed upon a glass plate 

 in a water-bath, whose temperature is indicated by a ther- 

 mometer, while a uniform temperature is secured by stirring. 



