INFLUENCE OF TEMPERATURE 125 



a living host, thrive best at or about blood heat, namely 36" to 39° C. 

 {c) The temperature of the thermal deatJi pobit varies considerably- 

 according to species, but lies not uncommonly between 52° and 58° C 

 Yet some species will flourish at 70^ C, and an exception to these 

 figures is of course found in the thermophylic bacteria, which have 

 been isolated from the intestinal tract, excreta, or sewage, and 

 which do not flourish below 60" C It should be understood that 

 while growth and multiplication do not take place as a general rule 

 below or above the standards mentioned, it by no means always 

 follows that death will take place immediately outside those limits. 

 Organisms can withstand cold and heat beyond and below the 

 minimum, or beyond the maximum, without being actually killed, 

 although their virulence and biological characters may be modified. 

 As is well known, many of the non-thermophilic spore-bearing 

 bacilli occurring more or less frequently in milk can withstand a 

 temperature of 80^ C. for as long as fifteen minutes (which would 

 kill all non-spore-bearing bacilli) without being killed, e.g. Bacillus 

 enteritidis sporogenes (Klein). Now whilst it is true that this resist- 

 ance is a resistance of the spore — which by its concentrated proto- 

 plasm and more resistant capsule is of tougher substance than the 

 adult bacillus — it must not be forgotten that it has a practical issue 

 of the utmost importance. For, as Koch has stated, the standard of 

 sterilisation should always be the highest resistant power possible of 

 either bacillus or spore. Hence milk is only sterilised by heat when 

 the heat is of such a degree that the most resistant spores succumb. 

 Following these general remarks, we may briefly consider the 

 effect of temperature on bacteria in milk as shown by a large 

 number of researches. The majority of bacteria met with in milk 

 grow best at about 25° C, but they will multiply at a temperature 

 as low as 4° C. 



DO appreciable effect on the vitality of the micro-organisms tested. At such 

 temperatures it must be assumed that the chemical metabolism of the cell 

 ceases, in the absence of heat and moisture. Bacteria were suspended in small 

 loops of platinum wire or on cotton-wool swabs, and directly immersed in liquid 

 air. The yeast, washed and pressed, was wrapped in rice-paper, and likewise 

 I directly immersed in the liquid air. Samples were taken and tested at intervals 

 I for a total period of six months. In no instance could any impairrrient of the 

 vitality of the organisms be detected. Judging by the results, the experiments 

 might have been prolonged for a much longer period than six months without 

 appreciable influence on the vitality of the organisms in question. A further 

 note on our own work in relation to this subject will be found in a subsequent 

 chapter dealing with the tubercle bacillus. All such experiment is, of course, 

 of a tentative character only. 



