February 7, 1901] 



NA TURE 



361 



The majority possessed reducing powers upon nitrates and 

 decomposed proteid matter. In some instances cane sugar was 

 inverted and starch was diastased. These facts well illustrate 

 the full vitality of the organisms at these high temperatures, 

 whilst all the organisms isolated grew best at 55-65° C. A good 

 growth in a few cases occurred at 72° C. Evidence of growth 

 was obtained even at 74° C. They exhibited a remarkable and 

 unique range of temperature, extending as far as 30° of the 

 Centigrade scale. 



As a concluding instance of the activity of these organisms we 

 may cite their action upon cellulose. Cellulose is a substance 

 that is exceedingly difficult to decompose, and is therefore used 

 in the laboratory for filtering purposes in the form of Swedish 

 filter paper, on account of iis resistance to the action of solvents. 

 We allowed these organisms to act on cellulose at 60° C. The 

 result was that in ten to fourteen days a complete disintegration 

 of the cellulose had taken place, probably into CO2 and marsh 

 gas. . The exact conditions that may favour their growth, even 

 if it be slow at sublhermophilic temperatures, are not yet known 

 — they may possibly be of a chemical nature. 



Organisms may be gradually acclimathed to temperatures 

 that prove unsuited to them under ordinary conditions. Thus 

 the anthrax bacillus, with an optimum temperature for its de- 

 velopment of 37° C, may be made to grow at 12" C. and at 

 42° C Such anthrax bacilli proved pathogenic (or the frog 

 with a temperature of 12° C, and for the pigeon with a tem- 

 perature of 42" C. 



Let us in a very few words consider the inimical action of 

 temperature on bacterial life. An organism placed below its 

 minimum temperature ceases to develop, and if grown above 

 its optimum temperature becomes attenuated as regards its 

 virulence, etc. , and may eventually die. The boiling point is 

 fatal for non sporing organisms in a few minutes. The exact 

 thermal death-point varies according to the optimum and 

 maximum temperature for the growth of the organism in 

 question. Thus for water bacteria with a low optimum tem- 

 perature blood heat may be fatal ; for pathogenic bacteria 

 developing best at blood heat, a thermophilic temperature may 

 be fatal (60° C. ) ; and for thermophilic bacilli any temperature 

 above 75° C. These remarks apply to the bacteria during their 

 multiplying and vegetating phase of life. In their resting or 

 spore stage the organisms are much more resistant to heat. 

 Thus the anthrax organism in its bacillary phase is killed in one 

 minute at 70° C. ; in its spore stage it resists this temperature for 

 hours, and is only killed after some minutes by boiling. In the soil 

 there are spores of bacteria which require boiling for sixteen hours 

 to ensure their death. These are important points to be remem- 

 bered in sterilisation or disinfection experiments, viz. whether 

 an organism does or does not produce these resistant spores. 

 Most non-sporing forms are killed at 60° C. in a few minutes, 

 but in an air dry condition a longer time is necessary. Dry heat 

 requires a longer time to act than moist heat : is requires 140° C. 

 for three hours to kill anthrax spores. Dry heat cannot, therefore, 

 be used for ordinary disinfection on account of its destructive 

 action. Moist heat in the form ot steam is the most effectual 

 disinfectant, killing anthrax spores at boiling point in a few 

 minutes, whilst a still quicker action is obtained if saturated 

 steam under pressure be used. No spore, however resistant, 

 remains alive after one minute's exposure to steam at 140° C. 

 The varying thermal death-point ol organisms and the problems 

 of sterilisation cannot be better illustrated than in the case of 

 milk, which is an admirable soil for the growth of a large number 

 of bacteria. The most obvious example of this is the souring 

 and curdling of milk that occurs after it has been standing for 

 some time. This change is mainly due to the lactic acid 

 bacteria, which ferment the milk sugar with the production of 

 acidity. 



Another class of bacteria may curdle the milk without souring 

 it in virtue of a rennet-like ferment, whilst a third class pre- 

 cipitate and dissolve the casein of the milk, along with the de- 

 velopment of butyric acid. The process whereby milk is sub- 

 mitted to a heat of 65° to 70° C. for twenty minutes is known 

 as pasteurisation, and the milk so treated is familiar to us all as 

 pasteurised milk. Whilst the pasteurising process weeds out 

 the lactic acid bacteria from the milk, a temperature of 100' C. 

 for one hour is necessary to destroy the butyric acid organisms : 

 and even when this has been accomplished there still remain in 

 the milk the spores of organisms which are oiily killed after a 

 temperature of 100° C. for three to six hours. It will, therefore, 

 be seen that pasteurisation produces a partial, not a complete 



NO. 1632, VOL. 63] 



sterilisation of the milk as regards its usual bacterial inhabitants. 

 The sterilisation to be absolute would require six hours at 

 boiling point. But for all ordinary practical purposes pasteur- 

 isation is an adequate procedure. All practical hygienic 

 requirements are likewise adequately met by pasteurisation, if 

 it is properly carried out and the milk is subsequently cooled. 

 Milk may carry " the infection of diphtheria, cholera, 

 typhoid and scarlet fevers as well as the tubercle bacillus 

 from a diseased animal to the human subject. For 

 the purpose of rendering the milk innocuous, freezing 

 and the addition of preservatives are inadequate methods 

 of procedure. The one efficient and trustworthy agent we 

 possess is heat. Heat and cold are the agents to be j.nntly 

 employed in the process, viz a temperature sufficiently high to 

 lie fatal to organisms producing a r.ipid decomposition of milk, 

 as well as to thosevvhich produce disease in man; this to be 

 followed by a rapid cooling to preserve the fresh flavour and to 

 prevent an increase of the bicteria that still remain alive. The 

 pasteurising process fulfils these requirements. 



In conjunction with Dr. Hewlett, I had occasion to investigate 

 in how far the best pasteurising results might be obtained. 

 We found that 60° to 68" C. applied for twenty minutes weeded 

 out about 90 per cent, of the organisms present in the milk, 

 leaving a 10 per cent, residue of resistant forms. It was found 

 advisable to fix the pasteurising temperature at 68° C. in order to 

 make certain of killing Hny pathogenic organisms that may happen 

 to be present. We passed milk in a thin stream through a coil 

 of metal piping, which was heated on its outer surface by water. 

 By regulating the length of the coil, or the size of the tubing, or 

 the rate of flow of the milk, almost any desired temperature could 

 be obtained. The temperature we ultimately fixed at 70° C. 

 The cooling was carried out in similar coils placed in iced water. 

 The thin stream of milk was quickly heated and quickly cooled 

 as it passed through the heated and cooled tubing, and, whilst 

 it retained its natural flavour, the apparatus accomplished at 

 70° C. in thirty seconds a complete pasteurisation, instead of 

 in twenty minutes, i.e. about 90 per cent, of the bicteria 

 were killed, whilst the diphtheria, typhoid, tubercle and pus or- 

 ganisms were destroyed in the same remarkably short period of 

 time, viz. thirty seconds. This will serve to illustrate how the 

 physical agent of heat may be employed, as well as the sensitive- 

 ness of bacteria to heat when it is adequately employed. 



Bacteria are much more sensitive to high than to low temper- 

 atures, and it is possible to proceed much further downwards 

 than upwards in the scale of temperature, without impairing 

 their vitality. Some will even multiply at zero, whilst others 

 will remain alive when frozen under ordinary conditions. 



I will conclude this discourse by briefly referring to experi- 

 ments recently made with the mo~t remarkable results upon the 

 influence of low temperatures on bacterial life. The 

 experiments were conducted at the suggestion of Sir James 

 Crichton-Browne and Prof. Dewar. The necessary facilities 

 were most kindly given at the Royal Institution, and the 

 experiments were conducted urider the personal supervision of 

 Prof. Dewar. The action of liquid air on bacterii was first tested. 

 A typical series of bacteria was employed for this purpose, pos- 

 sessing varying degrees of resistance to external agents. The 

 bacteria were first simultaneously exposed to the temperature of 

 liquid air for twenty hours (about - I90°C.). In no instance could 

 any impairment of the vitality of the organisms be detected as 

 regards their growth or functional activities. This was strikingly 

 illustrated in the case of the phosphorescent organisms tested. The 

 cells emit light which is apparently produced by a chemical process 

 of intracellular oxidation, and the phenomenon ceases with the 

 cessation of their activity. These organisms, therefore, furnished 

 a very happy test of the influence of low temperatures on vital 

 phenomena. These organisms when cooled down in liquid air 

 became non-luminous, but on re-thawing the luminosity returned 

 with unimpaired vigour as the cells renewed their activity. The 

 sudden cessation and rapid renewal of the luminous properties of 

 the cells despite the extreme changes of temperature was re- 

 markable and striking. In further experiments the organisms 

 were subjected to the temperature of liquid air for seven days. 

 The results were again nil. On re-thawing the organisms re- 

 newed their life processes with unimpaired vigour. We had not 

 yet succeeded in reaching the limits of vitality. Prof. Dewar 

 kindly afforded the opportunity of submitting the organisms to 

 the temperature of liquid hydrogen — about - 250° C. The same 

 series of organisms was employed, and again the result was nU. 

 This temperature is only 21" above that of the absolute zero, a 



