Conditions Prejudicial to Growth of Bacteria 41 



exceedingly small, however, Bacillus prodigiosus being 

 able to develop successfully upon crackers and dried 

 bread. Artificial culture media should not be too concen- 

 trated ; at least 80 per cent, of water should be present. 



(d) Reaction. Should the pabulum supplied to bacteria 

 contain an excess of either alkali or acid, the growth of the 

 organisms is inhibited. Most true bacteria grow best in a 

 neutral or feebly alkaline medium. There are exceptions 

 to this rule, however, for Bacillus butyricus and Sarcina 

 ventriculi can grow well in strong acids, and Micrococcus 

 urea can tolerate excessive alkalinity. Acid media are 

 excellent for the cultivation of molds. 



Conditions Prejudicial to the Growth of Bacteria. 

 (a) Light. Most bacteria are not influenced by the pres- 

 ence or absence of ordinary diffused daylight. The direct 

 rays of the sun, and to a less degree the rays of the electric 

 arc-light, retard and in numerous instances kill bacteria. 

 Certain colors are distinctly inhibitory to their growth, blue 

 being especially prejudicial. Some of the chromogenic 

 forms produce their colors only when exposed to the ordinary 

 light of the room. Bacillus mycoides roseus produces its 

 red pigment only in the dark. The virulence of many 

 pathogenic bacteria is gradually attenuated if they are kept 

 in the light. 



(b) Electricity, X-rays, etc. Very powerful currents 

 of electricity passed through cultures of bacteria have 

 been found to kill the organisms and change the reaction 

 of the culture medium; rapidly reversed currents of high 

 intensity to destroy the pathogenesis of the bacteria and 

 transform their toxic products into neutralizing protective 

 bodies (antitoxin?). Attention has been called to this sub- 

 ject by Smirnow, d'Arsonval and Charin, Bolton and Pease, 

 Bonome and Viola, and others. 



The most thorough and important contribution upon the 

 "Effect of Direct, Alternating, Tesla Currents and X-rays 

 on Bacteria" is by Zeit.* The technical methods adopted 

 make it worth while for the student to refer to the original 

 paper. The conclusions to which the author comes are as 

 follows : 



i. A continuous current of 260 to 320 milliamperes passed 

 through bouillon cultures kills bacteria of low thermal 

 death-points in ten minutes by the production of heat 

 *"Jour. Amer. Med. Assoc.," Nov. 30, 1901. 



