132 AGEICULTUKAL AND INDUSTRIAL BACTERIOLOGY 



bacteria are repulsed by solutions containing alcohol or 

 strong acids. 



The white blood corpuscles of the blood are attracted 

 under certain conditions to the invading bacteria. The 

 cells have some power of independent motion. This posi- 

 tive chemotaxis toward bacteria is one of the most im- 

 portant items in the list of body defenses against disease. 



Some bacteria are markedly attracted by oxygen, a phe- 

 nomenon termed aerotaxis. If a drop of sewage swarming 

 with bacteria be mounted under a cover glass on a glass 

 slide, using care to include a few small air bubbles, 

 within a short period of time the microorganisms, both 

 bacteria and protozoa, will be found to arrange them- 

 selves in quite regular concentric lines about these 

 bubbles. Apparently each motile species influenced by 

 oxygen concentration seeks to remain within that zone 

 where the oxygen concentration is most favorable. This fact 

 has been put to use by bacteriologists in isolating certain 

 species of bacteria. For example, the organism which causes 

 Asiatic cholera may be detected in f eces by adding a small 

 amount of the stool to broth. The many kinds of bac- 

 teria present will multiply but the cholera vibrio is 

 attracted most by oxygen and will be present, therefore, 

 in relatively much larger numbers in the surface layer. 

 An examination of a drop from the surface will show the 

 vibrios to be numerous if the test is positive. 



EFFECT OF CHEMICALS ON DIRECTION OF GROWTH 



The effect of an external agency upon the direction of 

 growth has already been defined as a tropism. The effect of 

 a chemical is correspondingly termed a chemotropism. 

 That type of chemotropism most frequently observed is the 

 attraction or repulsion exerted by water, termed liydrotrop- 

 ism. Most species of mold show this to a marked degree. 

 Those hyphas which are differentiated for spore production, 



