112 DISINFECTION 



alive and virulent for years under these conditions. The dried organisms are 

 resistant to quite high temperatures. Typhoid bacilli, for example, dried and 

 sealed in vacuo, are said to survive exposure to a temperature of 115° C. for over 

 30 minutes. Serum and complement can also be preserved satisfactorily by drying 

 from the frozen state (see Hartley 1936, Flosdorf and Mudd 1938). 



The method is now being used extensively for the preservation of stock cultures. 

 In practice 0-5-1-0 ml. quantities of a thick suspension of the organisms in broth 

 are distributed into suitable tubes. Drying is carried out as rapidly as possible 

 in vacuo in a desiccator over phosphorus pentoxide. The tubes are then evacuated 

 individually with an efficient pump, and sealed off in the flame. To recover the 

 organisms, an optimal medium is desirable for primary cultivation. 



Cold. — Very much less attention has been paid to the effect of cold on bacteria 

 than to the effect of heat. This may undoubtedly be attributed to the fact that, 

 although cold is an excellent means of preventing putrefaction, it has very 

 little germicidal action. Macfadyen (1900) exposed cultures of Bad. coli, Salm. 

 typhi, B. anthracis, V. cJioIeroe, Proteus vulgaris, and Staphylococcus aureus for 

 20 hours to liquid air at a temperature of —182° to — 190°C. After exposure 

 the organisms grew well on subculture, and manifested their usual biochemical 

 activities. He noticed that photogenic bacteria, when frozen, became non-lumin- 

 ous, but, when re-thawed, their luminosity returned with unimpaired vigour. In 

 another experiment he exposed the same organisms in broth suspensions enclosed 

 in fine quill tubing for 7 days to liquid air ; subsequently no structural altera- 

 tion could be detected in the bacteria, and all grew well on subculture. Mac- 

 fadyen and Rowland (1900) found that the same organisms in sealed glass tubes 

 withstood immersion for 10 hours in liquid hydrogen at a temperature of — 252° C. ; 

 microscopically and culturally the bacteria appeared to be unaltered. Paul and 

 Prall (1907) exposed staphylococci, which had been dried on garnets, to liquid 

 air, and found that under these conditions they retained their viability for several 

 months, and showed no appreciable alteration in their resistance to disinfectant 

 agencies. 



Haines (1938) found that rapid freezing with solid carbon dioxide at —70° C. 

 killed a high proportion of some organisms, but had little effect on others. If 

 the frozen organisms were subsequently stored at — 20° C, they died off very 

 slowly, but if they were stored at — 1° or — 2° C. they died rapidly. Evidence was 

 brought to suggest that at the latter temperatures denaturation and subsequent 

 flocculation of the bacterial protein occurred similar to the changes that have been 

 observed in muscle. In practice, a temperature of —70° C. is very useful for the 

 preservation of many bacteria and filtrable viruses. 



Dry Heat. — We have seen that disinfection by drying is influenced by numerous 

 Bmall factors ; in disinfection by heat, though numerous small factors may play 

 a part, the one factor, heat, is so important that it overshadows them. We can 

 therefore be more precise in our figures regarding this method of disinfection. 

 Koch and Wolff hugel (1881) were the first to make exact measurements of the 

 effect of heat on micro-organisms. They found that vegetative bacteria were 

 killed by a temperature of just over 100° C. in 1| hours ; many, of course, suc- 

 cumbed well within this interval, but this was the time necessary for complete 

 sterilization. Spores, on the other hand, were much more resistant, requiring a 

 temperature of 140° C. for 3 hours for destruction. On what this superior 

 power of resistance of spores depends is not known. Probably it is related to their 



