48 VETERINARY BACTERIOLOGY 



TEMPERATURE RELATIONSHIPS OF MICROORGANISMS 



Optimum Temperature. The optimum growth tempera- 

 ture is that which most favors the development of the micro- 

 organism. The optimum varies with the species. A few organisms 

 found in the ocean, in cold waters, alpine regions, etc., prefer a 

 low temperature, from to. 15. These are called psychrophilic. 

 Those which prefer a somewhat higher temperature are called 

 mesophilic. These latter may be again subdivided into those that 

 prefer a " room " temperature of 18 to 25, and those that 

 prefer blood heat (man 37.5) for the most parasitic forms. Tem- 

 peratures such as are found in hot springs, interior of compost 

 heaps (50 to 70) favor the development of thermophilic bacteria. 



Minimum Temperature. The lowest temperature at which 

 an organism will continue growth is said to be its minimum. 

 This temperature varies for different species. Some organisms 

 will multiply in brine held at temperatures lower than the freez- 

 ing-point of water. 



Maximum Growth Temperature. The highest temperature 

 at which an organism will multiply is called its maximum. This 

 must not be confused with the thermal death point (see below). 

 The majority of bacteria cannot grow above 45. 



Growth Temperature Range. The differences between the 

 minimum and maximum growth temperatures vary within rather 

 wide limits. Those organisms which exhibit considerable adap- 

 tability and are able to grow through a wide range of temperature 

 are called eurythermic. Most of the saprophytic organisms belong 

 here. The parasitic types which have minima and maxima 

 varying but little from the optima are stenothermic. 



Thermal Death Point. The thermal death point of an organism 

 is that temperature which under given conditions will certainly 

 destroy all the cells. The following factors must be taken into 

 consideration in the determination of any thermal death point: 



1. The Absence or Presence of Spores. Spores are much more 

 resistant to high temperatures than the vegetative cells. Forms 

 having spores, therefore, have two thermal doath points, one for 

 the vegetative cells and the other for the spon >. 



2. Presence or Absence of Moisture. Bacteria are more resistant 

 to dry than to moist heat. The thermal death point is probably 



