18 THE BIOLOGY OF BACTERIA 



isolated from the intestine, horse manure, etc., grow at 60-70 C. 

 On the other hand, investigations have shown that bacteria can 

 withstand exceedingly low temperatures. Koch showed that the 

 cholera vibrio was not killed by a temperature of 32 C. In 1900, 

 Swithinbank exposed cultures of the tubercle bacillus to the 

 temperature of liquid air (193 C.) for continuous periods varying 

 from six hours to forty-two days, without their vitality being affected ; 

 and in the same year MacFadyen and Eowland found that Proteus 

 vulgaris, B. coli, and several other species were not killed after an 

 exposure of ten hours to a temperature of liquid hydrogen ( 252 C). 

 It will thus be seen that bacteria can withstand great alternations of 

 temperature. From a public health point of view, it is important 

 to remember that organisms can exist in freezing mixtures and ice, 

 retaining their vitality and virulence. For example, B. coli and the 

 typhoid bacillus can exist from the low temperatures above mentioned 

 to 80 C., although the usual thermal death-point for these species 

 is between 50-60 C.* 



Moisture has been shown to have a favourable effect upon the 

 growth of microbes. Drying will of itself kill many species (e.g. the 

 spirillum of cholera), and other things being equal, the more moist a 

 medium is, the better will be the growth upon it. Thus it is that the 

 growth in broth is always more luxuriant than that on solid media. 

 Yet the growth of Bacillus subtilis and some other species are an 

 exception to this rule, for they prefer a dry medium. Desiccation as 

 a rule diminishes virulence and lessens growth. But some species 

 can withstand long-continued drying without injury. 



Light acts as an inhibitory, or even germicidal, agent. This 

 fact was first established by Downes and Blunt in a memoir to the 

 Eoyal Society in 1877. They found by exposing cultures to different 

 degrees of sunlight that the growth of the culture was partially or 

 entirely prevented, being most damaged by the direct rays of the 

 sun, although diffuse daylight acted prejudicially. Further, these 

 same investigators proved that the rays of the spectrum which acted 

 most inimically upon bacteria were the blue and violet rays, next to 

 the blue being the red and orange-red rays. The action of light, 

 they explain, is due to the gradual oxidation which is induced by the 

 sun's rays in the presence of oxygen. Duclaux, who worked at this 

 question at a later date, concluded that the degree of resistance to 

 the bactericidal influence of light, which some bacteria possess, 

 might be due to difference in species, difference in culture media, and 

 difference in the degrees of intensity of light. Tyndall tested the 

 growth of organisms in flasks exposed to air and light on the Alps, 



* For the latest researches on this point, see Proc. Roy. Soc., 1900 and 1901 ; and 

 the Thirty-fourth Annual Report of the State Board of Health, Massachusetts, 1903, 

 pp. 269-281. Dewar commenced experiments of this character in 1892. 



