54 OUTLINES OF BACTERIOLOGY 



sample of sea-water at 0° C. which contained, when freshly taken, 150 

 germs per c.c, had 520 after 24 hours, and 1750 after four days. 



Although their power of reproduction does not extend much below 

 0°C., their power of remaining unharmed on exposure to very low 

 temperatures is considerable. Pictet and Yung found that some 

 bacteria could be kept at -70°O. for 108 hours, and at -130°C. for 

 20 hours, without losing their power of growth when afterwards 

 transferred to a suitable medium. It was also found that tubercle 

 bacillus can be exposed to the temperature of liquid air ( - 193° C.) for 

 continuous periods, varying from 6 hours to 42 days, without its 

 vitality being affected. MacFadyen and Kowland experimented with 

 Proteus vulgaris, Bac. coli communis, and others, and found them 

 unimpaired after exposure of 10 hours to the temperature of liquid 

 hydrogen ( - 252° C). It will thus be seen that exposure to low 

 temperatures is quite useless for purposes of sterilisation. The ques- 

 tion is always one of penetration of the membrane of bacteria, for it 

 must not be imagined that the protoplasm itself can stand these 

 temperatures, but rather that the cell-membrane is a very bad con- 

 ductor of heat and thus prevents the heat contained in the protoplasm 

 from being dissipated. 



Bacteria are very much more sensitive to heat than to cold, 

 because, probably, the expansion of the membrane by heat facilitates 

 the entrance of the penetrating hot fluid. The average thermal 

 death-point is about 55° C, hence when non-sporing bacteria are placed 

 in boiling water (100° C.) they are killed almost instantaneously. The 

 thermophilic bacteria, however, can stand higher temperatures. Bac. 

 thermophilus is a type of this class. It can grow actively at 70° C, 

 which being 15° C. above the normal thermal death-point, is fatal to 

 animal cells and to protoplasm generally. Miguel describes this form 

 as producing short rods about 1/x in thickness at 50° C, the rods 

 becoming longer as the temperature rises. At 70° C. the whole field 

 is occupied by these long threads. Its minimum temperature is 42° C. 

 and maximum about 72° C. This species is very frequent in sewage 

 and in the alimentary canal of human beings and other mammals. 

 It is a saprophyte so far as is known, as it feeds on dead organic 

 matter. 



Another heat-loving form is Bacterium phosphorescens, found in the 

 West Indies, which ceases to live below 15° C. Again, other bacteria 

 have been found in boiling springs, the temperature of which was 

 04° C, or 9° C. above the normal thermal death-point. Again, in the 

 hot sulphur springs at Ilidze, near Sarajevo in Bosnia, two forms, called 



