The effect of physical agents on bacterial life / Sir James Crichton-Browne, M.D. LL.D. F.R.S., Treasurer and Vice-President, in the chair. Allan Macfadyen, M.D., Bsc., Director of the Jenner Institute of Preventive Medicine.

  • Crichton-Browne, James, 1840-1938
Date:
[1900?]
Catalogue details

Licence: In copyright

Credit: The effect of physical agents on bacterial life / Sir James Crichton-Browne, M.D. LL.D. F.R.S., Treasurer and Vice-President, in the chair. Allan Macfadyen, M.D., Bsc., Director of the Jenner Institute of Preventive Medicine. Source: Wellcome Collection.

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    75° C. These remarks apply to the bacteria during their multipl ing and vegetating phase of life. In their resting or spore staj, the organisms are much more resistant to heat. Thus the anthn organism in its bacillary phase is killed in one minute at 70° C.; : its spore stage it resists this temperature for hours, and is only kilh after some minutes by boiling. In the soil there are spores bacteria which require boiling for sixteen hours to ensure their deat's These are important points to be remembered in sterilisation < disinfection experiments, viz. whether an organism does or n produce these resistant spores. Most non-sporing forms are killed 60° C. in a few minutes, but in an air-dry condition a longer time necessary. Dry heat requires a longer time to act than moist hea it requires 140° C. for three hours to kill anthrax spores. Dry he cannot therefore be used for ordinary disinfection on account of i destructive action. Moist heat in the form of steam is the mo, effectual disinfectant, killing anthrax spores at boiling point in a fe: minutes, whilst a still quicker action is obtained if saturated stea:' under pressure be used. No spore, however resistant, remains air after one minute’s exposure to steam at 140° C. The varying therm; death-point of organisms and the problems of sterilisation cannot 1 better illustrated than in the case of milk, which is an admirable so for the growth of a large number of bacteria. The most obvioi example of this is the souring and curdling of milk that occurs aftc it has been standing for some time. This change is mainly due 1 the lactic acid bacteria, which ferment the milk sugar with the pr< duction of acidity. Another class of bacteria may curdle the milk without sourin it in virtue of a rennet-like ferment, whilst a third class preeipital and dissolve the casein of the milk, along with the development < butyric acid. The process whereby milk is submitted to a heat < 65° to 70° C. for twenty minutes is known as pasteurisation, and tl milk so treated is familiar to us all as pasteurised milk. Whik: the pasteurising process weeds out the lactic acid bacteria from tb milk, a temperature of 100° C. for one hour is necessary to destro the butyric acid organisms: and even when this has been accon plished there still remain in the milk the spores of organisms whic are only killed after a temperature of 100° C. for three to six hour; It will therefore be seen that pasteurisation produces a partial, nc a complete sterilisation of the milk as regards its usual bacteris inhabitants. The sterilisation to be absolute would require si hours at boiling point. But for all ordinary practical purpose pasteurisation is an adequate procedure. All practical hygienic re quirements are likewise adequately met by pasteurisation, if it i properly carried out, and the milk is subsequently cooled. Mil may carry the infection of diphtheria, cholera, typhoid and scarle fevers as well as the tubercle bacillus from a diseased animal to th human subject. For the purpose of rendering the milk innocuous freezing and the addition of preservatives are inadequate method