CHAPTER VIII. 



THE EFFECT OF VARIOUS DELETERIOUS INFLUENCES UPON 



BACTERIA. 



Influence of Electricity on Bacteria. The majority of the observations 

 heretofore made on this subject would seem to indicate that there is no 

 direct action of the galvanic current on bacteria ; but the effect of heat 

 and the electrolytic influence on the culture liquid may produce changes 

 which finally sterilize it. 



Influence of Agitation. Meltzer has shown that the vitality of bac- 

 teria is destroyed by protracted and violent shaking, which causes a 

 disintegration of the cells. Appel found that moderate agitation of the 

 bacteria caused no injury, even when long continued. 



Influence of Pressure. D'Arsonval and Charrin submitted a culture 

 of bacillus pyocyaneus to a pressure of fifty atmospheres under car- 

 bonic acid. At the end of four hours cultures could still be obtained, 

 but the bacillus had lost its power of pigment production. A few 

 colonies were developed after six hours' exposure to this pressure, but 

 after twenty-four hours no development occurred. 



Influence of Light. A large number perhaps the majority of 

 bacteria are inhibited in growth by the action of bright daylight, all are 

 by that of direct sunlight, and when the action of the latter is prolonged 

 they lose their power of developing when later placed in the dark. 



In order to test the susceptibility of bacteria to light, it is best, accord- 

 ing to H. Buchner, to suspend a large number of bacteria in nutrient 

 gelatin or agar arid pour the media while still fluid in Petri dishes, 

 upon which has been pasted a strip of black paper on the side upon 

 which the light is to act. The action of heat may be shut off by allow- 

 ing the ray of light to first pass through a layer of water or alum of 

 several centimetres' thickness. After the plates have been exposed to 

 the light for one-half, one, one and a half, two hours, etc., they are 

 taken into a dark room and allowed to stand at 20 or 35 C. a sufficient 

 length of time to allow of growth, and then examined to see whether 

 there are colonies anywhere except on the spot covered by the paper; 

 when the colonies exposed to the light have been completely destroyed 

 there is a sharply defined region of the shape of the paper strip crowded 

 with colonies lying in a clear sterile field. 



Dieudonne, in experiments upon the bacillus prodigiosus, found 

 that direct sunlight in March, July, and August killed these bacilli in 

 one and a half hours; in November in two and a half hours. Diffuse 

 daylight in March and July restrained development after three and a 

 half hours' exposure (in November four and a half hours), and com- 



