GERMICIDAL ACTION OF FREEZING TEMPERATURES 425 



and have concluded in general that cold cannot be depended 

 upon to sterilize. 



Park (1901) records 100 per cent reduction with twenty- 

 different strains of typhoid subjected to — 5°C. for twenty-two 

 weeks. From their extensive work Sedgwick and Winslow 

 (1902) concluded that not only different species but different 

 ''races" within the same species exhibit marked variability in 

 their resistance to freezing temperatures. 



Keith, (1913) contrary to Prudden, has emphasized the impor- 

 tance of solid freezing as compared with cold in relation to the 

 death rate of bacteria. B. coli frozen solidly in water at — 20°C. 

 show 99 per cent killed in five days, but when not actually 

 frozen a large per cent remain alive for months; when frozen in 

 diluted milk the death rate increases with the dilution; when 

 suspended in aqueous mixtures of 5 to 42 per cent glycerine a 

 large percentage remain alive for six months at — 20°C. Keith 

 concluded that the important factors influenciaig the death rate 

 of bacteria at low temperatures are their rate of metabolism and 

 the mechanical protection offered by the medium. 



In 1915 one of the authors in collaboration with Torossian 

 and Stone published notes on the germicidal effect of freezing 

 and low temperatures. We suggested there that "bacteria may 

 be killed by the mere fact of low temperature, interfering with 

 metabolism; by freezing of the c^ll contents and rupture of the 

 membrane by internal pressure; by external pressure or grind- 

 ing developed during crystalhzation ; or by expansion of the 

 frozen medium within the receptacle; or by more or less pro- 

 longed suspension of metabolic activities, leading to slow death 

 from old age or starvation." Below we reproduce the tables as 

 given in these notes. The extension of the work since this time 

 has entirely confirmed this preliminary study as regards (a) 

 the rapid destruction of 90 per cent or more of B. coli when 

 frozen in tap water for three hours, (b) the greater viability of 

 spores {B. suhtilis) in frozen mixtures, (c) the greater germi- 

 cidal influence of intermittent freezing and thawing, (d) the 

 fact that depression of the temperature within certain limits 



