EFFECTS OF HEAT AND COLD ON BACTERIA 75 



organises are found in water at 60 C. and more. In the drainage from 

 the Carlsbader Strudel (54) there is a thick growth of Leptothrix with 

 green Oscillariae, and in other thermal springs Crustacea and insect larvae 

 are known to occur at 60. 



Bacteria may be roughly divided according to the optimum temperature 

 into two great groups, those which grow best at about 30 C. (B.fluorescens, 

 B. phosphor escens, B. prodigiostts, and many other metatrophic forms), and 

 those which need a higher temperature. For the maintenance of a regular 

 high temperature special incubators are used. These secure great uniformity 

 of any desired degree of warmth by the use of some form of thermostat, 

 and are subject to variations of 01 to 05 only. Still better is a whole room 

 maintained at a constant temperature. Practical details will be found in 

 the works cited in No. 3 of Notes (p. 169). 



When the temperature at which bacteria are grown approaches either 

 the maximum or the minimum, not only is growth arrested, but all the 

 functions of the cell are enfeebled. Continued cultivation under these 

 circumstances, particularly at temperatures near the maximum, effects 

 injuries from which the cell recovers only very slowly when the optimal 

 conditions are restored. (See Lects. III. & XVII.) 



Exposure to extreme cold has, apart from arresting growth, very little 

 ill-effect upon bacteria. In this respect they resemble other poikilothermic 

 organisms, falling into a lethargic state (hibernation}. Sporeless anthrax 

 bacilli can endure a temperature of 26-8 for twelve days without injury, 

 whilst the spores show no decrease of vitality or virulence after exposure 

 for twenty hours to 130. Long imprisonment in ice and even repeated 

 freezing and thawing can be endured for weeks and months, bacteria be- 

 having in this respect like algae and other water plants. The cold of our 

 winters is therefore quite inadequate to kill bacteria, and the application of 

 artificial low temperature is equally useless as a means of disinfection (49). 



In sharp contrast to this indifference to cold is the behaviour of bacteria 

 towards heat. If the maximum be exceeded death speedily follows (largely 

 as a result of the coagulation of the protoplasm), and consequently a tem- 

 perature of 50 or 60 C. for ten minutes is sufficient to kill the sporeless 

 vegetative cells of all but the thermophile bacteria. At 70 five minutes is 

 sufficient. It is on this principle that the preservation of tinned foods is 

 now so largely carried on and the ' Pasteurization ' of wine (heating for half 

 an hour to 70) is another instance of its application. In ' fractional sterili- 

 zation ' as applied to culture media like blood serum, which cannot be heated 

 to 100, the same result is aimed at, namely the destruction of the vegetative 

 cells. Spores which have resisted the first heating are then allowed to ger- 

 minate and are killed by a second heating, the process being repeated five or 

 six times if necessary until sterility is secured. At the present time hundreds 

 of such sterilizers are in daily use in the service of antiseptic surgery. 



