VI] GENERAL CHARACTERS OF BACTERIA 117 



gradually enlarge— «.^., bacillus subtilis, bacillus fluorescens, 

 some species of proteus, and many spirilla ; other species 

 possess this swarming propensity and therefore the first 

 colonies do not remain well defined, but gradually extend 

 in lines and irregular streaks in different directions. But it 

 is not correct to conclude that a bacillus is motile if its 

 colonies do not remain defined, that is, if they extend 

 in the shape of threads or irregular streaks on or through 

 the medium, for there exist several well studied species 

 which do this {e.g., bacillus anthracis, bacillus filamentosus), 

 though their bacilli are not motile, as will be more 

 minutely described when speaking of the cultural char- 

 acters of bacilli. 



When certain bacilli show only slight motility it may be 

 extremely difficult to distinguish this from Brownian mole- 

 cular movement, but no locomotion can be ascribed to 

 bacilli unless one or the other individual can be distinctly 

 seen to show a darting or spinning movement. As men- 

 tioned above, the easiest and best way to see locomotion is 

 to examine the fresh bacilli in a fluid, as sterile broth or 

 sterile salt solution in the " suspended drop.'' 



The motility of bacilli and spirilla is due to their pos- 

 sessing at one, and occasionally at both ends, or also over the 

 general surface, fine flagella or cilia, the movement of which 

 causes the motility of the microbe. Where two or more 

 microbes are connected into a chain or thread, only the 

 terminals have the flagella. Although the flagellum has not 

 been stained and photographed hitherto in all bacilli and 

 spirilla, there can be no doubt that all motile organisms do 

 possess the flagellum, for without it motility would not be 

 possible. Micrococci are not possessed of motility, but 

 recently Ali-Cohen has isolated from drinking-water a 

 species of micrococcus {Micrococcus agilis) which forms an 



