8 AIDS TO BACTERIOLOGY 



destructive to the typhoid bacillus than continuous freez- 

 ing. S. C. Keith states that living bacteria are hardly 

 ever found in clear ice, though they are comparatively 

 abundant in snow ice and bubbly ice. Cooling is valuable 

 in the preservation of putrescible material, because it 

 inhibits bacterial growth. 



Desiccation. Moisture is absolutely necessary for the 

 growth of bacteria. Ordinary drying in the air has a 

 detrimental effect on the vegetative forms of bacteria, 

 spores suffering less or not at all. Resistance varies with 

 the species. The tubercle bacillus, which retains its 

 virulence after five months' desiccation, is much more 

 resistant than the cholera spirillum, which is incapable of 

 development after three hours' drying in the form of a very 

 thin film. In spite of the large numbers of colon bacilli 

 continually being deposited in our streets, Gordon failed 

 to find the organism in 500 litres of air of the East Central 

 district of London. 



Heat. The Thermal Death-point (Eyre) for vegetative 

 forms is ' that temperature which with certainty kills a 

 watery suspension of the organisms in question after an 

 exposure of ten minutes.' Eyre defines the moist t.d.p. 

 for spores as ' that time exposure to a fixed temperature 

 of 100 C. necessary to effect the death of all the spores 

 present in a suspension.' Eyre's dry thermal death- 

 point for both vegetative forms and spores is the tem- 

 perature that kills each form in a thin film after a time 

 exposure of ten minutes. The thermal death-point can be 

 determined with a fair degree of accuracy for each species, 

 and is much higher for spores than for the vegetative 

 forms. Heat may be applied either in the absence or 

 presence of moisture, known respectively as ' dry heat ' 

 and ' moist heat.' 



Dry heat is much less efficient than moist heat, the 

 death of the protoplasm taking place more readily in the 

 presence of moisture. In practical disinfection moist heat 

 is therefore used where possible, especially where penetra- 

 tion of fabrics is required (see p. 245). Steam may be 

 applied under pressure, as in an autoclave, when a fifteen 

 minutes' exposure to 125 C. suffices to destroy all 

 known organisms. When moisture is present, most vege- 

 tative forms are killed by an exposure to 65 C. for ten 

 minutes, while an exposure for an hour and a half at 



