Microbial Structures and Staining Reactions 105 



to 



at the same relative speed would turn over. Perhaps bacteria 

 do execute a "barrel roll" as they change direction. After all, 

 which side is up for a bacterium? 



When objects are very small they are bombarded by molecular 

 forces external to the cell, and the particles undergo a vibrating, 

 trembling-like motion called Brownian movement. This phe- 

 nomenon was reported in 1827 by Robert Brown, a botanist. As 

 the particle size diminishes, Brownian movement increases. To 

 distinguish this reaction from true movement caused by forces from 

 within the cell, more than mere vibration must be detected. True 

 motility means making progress through liquids— actually getting 

 somewhere rather than merely vibrating in one spot. Motion and 

 motility, in this sense, are not synonymous. The sluggish move- 

 ment of some bacteria makes the distinction between the two quite 

 difficult unless the culture is in the active stage of growth (less 

 than twenty-four hours old), and is examined at the optimum 

 temperature under the high dry objective of the microscope in a 

 preparation called a hanging drop. Drifting with the tide is 

 often mistaken for motility; bacteria must buck the tide and swim 

 upstream, as it were, before we can call the motion true motility. 



The natural slow movement of some microorganisms makes 

 distinction between motility and Brownian movement difficult, but 

 speed of the bacteria may be enhanced by examining hanging drop 

 preparations on a warm microscope stage with the culture in the 

 active phase of growth. Not all rods are flagellated, but rods ex- 

 hibiting motility have all been shown to possess flagella. Some 

 of the spiral forms of bacteria appear to move more by a twisting 

 of their elongated cells than by flagella, although this point is 

 still debatable. 



Because of their narrow width (about 0.03 ^ in diameter), 

 flagella are not visible when the usual staining technics are em- 

 ployed. The extreme fragility of these structures is another factor 

 complicating their observation. 



Spirals tend to have flagella located at the ends of the cells 

 but not on the sides. With straight rods, however, we find singles, 

 tufts, and dispersed flagella arranged around the cell's perimeter. 



