56 MICROBES AND TOXINS 



times it is of cellulose, sometimes it resembles chitin, some- 

 times it is impregnated with fats or waxes like the bacillus 

 tuberculosis. In the membrane of certain bacteria iron and 



silicon have been 

 found. Sometimes 



in ld cultures 



one can see mem " 



branes without 

 typhoid V&rto Of Tetanus contents which re- 



Baxi&tts QtoUlU BatittttS se mble empty 



FIG. 25. Flagella of bacteria. sheaths. 



Certain bacteria 



possess besides Brownian movement an independent motility ; 

 they can be seen performing long journeys, pirouetting 

 on their own axis. It has been calculated that the cholera 

 vibrio can travel 18 centimetres in the hour, which is 

 equivalent to ten or fifteen times its length in a second. 

 This motility is due to cilia or flagella analogous to those 

 of the vibratile epithelial cells or the ciliated infusorians. They 

 are often longer than the bacterium which carries them. In 

 old cultures they become detached and scattered through the 

 medium. It is doubtful whether they are fine expansions of 

 cellular protoplasm which have passed through pores in the 

 enveloping membrane or whether they are expansions 

 of this membrane itself; whether they are not mere 

 propelling organs but rather tentacles of some sort which 

 increase the surface of the bacterium and thus play a role in 

 nutrition, especially in the young bacteria. No definite 

 reply can be given to these questions. According to 

 G. de Rossi's experiments on agglutination and immunisation 

 the cilia behave like protoplasm. Their action ceases at very 

 low temperatures and above 50 C. When a bacterium 

 provided with cilia only at one end (certain vibrios) is 

 attracted towards a definite substance (positive chemiotaxis), 

 the cilium or cilia take the front position and mark the head- 

 end of the bacterium just as the flagellum does in the case of 

 the trypanosomes. 



