50 VETERINARY BACTERIOLOGY 



the blues and violets to be most powerful, while the reds and 

 yellows of the other end have little or no effect. It will be re- 

 membered that the blue and violet rays are the ones which affect 

 the photographic plate most intensely. The germicidal action 

 of light on the pathogenic bacteria is of the greatest practical 

 importance. It renders infection through the medium of the air 

 in most cases a remote possibility. 



Effect of Electricity on Bacteria 



Strong direct currents of electricity passed through a solution 

 containing bacteria will sterilize it. It is difficult, however, 

 to dissociate the physical effect of the current directly upon the 

 bacteria from the action of the chemicals produced by electrolysis. 

 No practical use has been made of the destructive action of 

 electricity upon microorganisms, as the method is difficult to apply 

 and is inefficient at best. The Rontgen rays (x-rays) do not 

 destroy bacteria even when the latter are exposed for consid- 

 erable periods. 



RELATIONSHIPS OF MICROORGANISMS. TO CHEMICALS 



Microorganisms are profoundly affected both in growth and 

 movement by the chemicals with which they come in contact. 

 They may be attracted or repulsed, stimulated to increased growth, 

 their development inhibited, or they may be destroyed when certain 

 substances are present. 



Chemotaxy. Motile microorganisms are attracted or repulsed 

 by certain chemicals. The first is known as positive chemotaxy, 

 the latter as negative. Certain protozoa and bacteria are attracted 

 by oxygen and may be observed to swim about air bubbles under 

 the microscope. From their movements it is evident that differ- 

 ent species prefer varying amounts of oxygen. This results in a 

 grouping of the different kinds in concentric circles about the 

 bubble. This type of chemotaxy is called curotaxy (not ae'ro- 

 tropism) . The avidity of certain bacteria for oxygen has been used 

 in the laboratory for their isolation from water, particularly the 

 Asiatic cholera organisms. Peptones and meat extractives at- 

 tract many kinds of bacteria. This phenomenon may be rcndily 

 demonstrated by introducing the tip of a minute capillary tube 



