RELATION TO HEAT 



97 



increase in number near the freezing-point. Some bacteria of this 

 type probably play important roles in soil fertility and in the decay 

 of foods in cold storage. 



3. Mesophilic bacteria are those whose optimum temperature is 

 between these two extremes. They comprise the great group of 

 pathogenic organisms occurring in the bodies of men and animals. 

 To this class also belong many of the decay and putrefying organisms 

 found in the soil. All of the more important bacteria belong to 

 this group. 



Three temperature limits may be distinguished for bacterial 

 growth: (a) Minimum, the lowest temperature at which bacterial 

 growth will occur. This for the true thermophiles is about 40 C., 

 for some pathogenic 29 C., and for the mesophilic as low as C v 

 or in solutions which do not solidify it may be even lower than this. 

 (b) Optimum, that of most luxuriant growth. This, like the minimum 

 temperature, varies greatly with the species, (c) Maximum, the 

 highest temperature at which growth and multiplication can take, 

 place. This may be a few or many degrees above the optimum. 

 For the thermophilic it may be as high as 89 C., whereas for the 

 pathogenic bacteria it lies between 40 and 50 C. The growth of 

 some pathogenic organisms at a high temperature for some time 

 causes them to lose their virulence or disease-producing power, and 

 is, therefore, made use of in the preparation of vaccines. 



The temperature relations are seen from the following table 

 reported by Fischer: 



The growth temperature range of an organism is the number of 

 degrees difference between the minimum and maximum. This is 

 very small with some bacteria like thegonococcus, thepneumococcus, 

 the tubercle bacillus, and others which are highly susceptible to 

 temperature changes and have the power of growing only within 

 limits varying but a few degrees from the optimum. However, 

 most pathogenic bacteria may grow at temperatures ranging 

 between 20 C. and 40 C. Others, like the colon bacilli group, the 

 Bacillus anthracis, and the Spirillum cholera asiaticce, may develop 

 at temperatures as low as 10 C. and as high as 40 C. or over. The 

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