THERMAL DEATH POINT OF BACTERIA 115 



found that the addition of sufficient alkali to render the solution N/7,000 alkaline increased 

 the mean rate of disinfection at 54° C. about 1-5 to 2-fold ; a similar addition of acid 

 increased it 5 to 7-fold. Fm'ther addition of alkaU influenced the rate of disinfection but 

 little, whereas further addition of acid rendered it too rapid for study. 



Other factors are the age of the culture and the nature of the suspending 

 medium. Young organisms are generally more susceptible to the action of heat 

 and of chemical disinfectants than old, while the presence of protein in the sus- 

 pension, or of sugar in considerable concentration (Fay 1934), tends to protect 

 the organisms to some extent. (For general information on sterilization by steam, 

 see Underwood 1934, Konrich 1938, and on the sterilization and testing of dressings, 

 in particular, see Hayes 1937, Savage 1940, Chisholm 1941, Eeport 1942.) 



Thermal Death Point of Bacteria.— The mode of action of heat on bacteria 

 appears to be one of protein coagulation. Chick and Martin (1910) showed that 

 heat coagulation of proteins is an orderly process, the rate of which varies with 

 the alteration of temperature, reaction of the medium, and other conditions. The 

 actual process of coagulation consists of two stages : in the first, known as denatura- 

 tion, the water reacts with the protein ; in the second, known as agglutination, 

 the altered protein separates out in a particulate form. In the case of haemoglobin 

 the coagulation occurs logarithmically, the rate at any moment being proportional 

 to the concentration of uncoagulated protein. Very much the same law appears 

 to be applicable to bacteria. The higher the temperature to which they are sub- 

 mitted, the more rapidly is their cellular protein coagulated. Between different 

 organisms there are considerable variations ; thus some vegetative bacteria, such 

 as the gonococcus, are destroyed by heat at 47° C. in a few minutes ; others, such 

 as the enterococcus, withstand a temperature of 60° C. for nearly an hour. It 

 muc^fc not, however, be supposed that these temperatures are to be regarded as 

 specific thermal death points, irrespective of the time of exposure. Chick (1910) 

 has shown that the death of bacteria imder the influence of heat is due to a protein 

 coagulation ; that this phenomenon occurs not at one definite point on the tem- 

 perature scale, but over a considerable range of temperature ; and that therefore 

 the death of bacteria within a given range is mainly a fimction of time. To take 

 for example Sahn. typhi : the thermal death point of this organism is usually 

 given as 55° C. In experiments carried out between 49° and 59° C. the tempera- 

 ture coefiicient, i.e. the rise in the velocity of disinfection, was found to be 1'635 

 for 1' C. Given a value for k (see p. 137) of 0-111 at 49^ C, it can be calculated 

 that a suspension containing 100,000 bacilU per ml. would be sterilized in about 

 2 hours at 47° C, in 48 minutes at 49° C, in 18 minutes at 51° C, in 7 minutes 

 at 53° C, in 2|- minutes at 55° C, and in 21 seconds at 59° C. If therefore a suspen- 

 sion was gradually heated, death might apparently take place suddenly at 55° C. 

 But it is clear that this cannot be regarded as a point possessed of any special 

 significance ; it is merely a point near the upper end of a series of temperatures, 

 each of which in itself can legitimately be regarded as a thermal death point. It 

 follows that for purposes of comparison of the heat susceptibiUty of organisms of 

 different species, it is essential to use suspensions of equal numbers of bacteria, and 

 to ascertain at what temperature complete sterilization is produced within a given 

 time. Even with these precautions, as we shall see later, there is a certain inac- 

 curacy, due to the apparent variation in susceptibility of organisms of the same 

 species in the same suspension, resulting in the survival of some long after the 

 majority have been killed. 



