BACTERIA 9 



capsulated bacteria. The capsule is most easily demonstrated 

 in preparations made directly from animal tissues or fluids, where, 

 when stained, it can be seen surrounding the cell like a halo. 



Metachromatic Granules. In certain bacteria granules have 

 been observed which show a greater affinity for nuclear dyes than 

 does the surrounding protoplasm. They are called metachromatic 

 granules from the fact that by appropriate methods they will 

 retain one stain while the rest of the bacterial cell can be made to 

 take another. In young diphtheria bacilli they are often very 

 conspicuous and serve as an aid in diagnosis. Their nature and 

 significance have not yet been determined. 



Other granules have been described which have been shown 

 to consist of starch or fat or of other substances ; they probably 

 represent material in process of transformation into cell nutrition. 

 In certain bacteria which find their food supply in decaying organic 

 material granules of sulfur have been demonstrated, in others 

 iron granules. 



Motility. Many species of bacteria are capable of independent 

 movement. When seen in a fluid preparation through the micro- 

 scope their movements may appear of a darting or rolling nature 

 or they may be very sluggish and scarcely perceptible. Bacterial 

 motility is always a real progressive motion and not merely the 

 oscillating vibration exhibited by all finely divided particles sus- 

 pended in suitable fluid. The latter so-called " Brownian move- 

 ment " is a purely physical phenomenon which may be shown by 

 dead bacteria and inorganic substances. 



The speed with which certain bacteria move has been estimated ; 

 the cholera spirillum, for example, may travel for a short distance 

 at the rate of 18 cm., or about 7 inches, per hour. While this 

 does not seem very great it is considerable when one considers 

 the minute size of the organism. Most of the actively motile 

 bacteria are bacilli or spirilla. 



The motility of bacteria depends upon their possession of thin 

 hair-like appendages or flagella which are so extremely fine that 

 special staining methods are necessary to demonstrate them. 

 By means of their power of contractility they keep up a lashing 



