42 GENERAL PHYSIOLOGY OF BACTERIA 



The saprophytic bacteria, as for example B. subtilis, which develop 

 through a relatively wide range of temperature are also called 

 Eurythermic bacteria. The pathogenic bacteria, as for example the 

 tubercle bacillus, which exhibit but little latitude in this respect, are 

 called Stenothermic bacteria. 



2. Cold. All bacteria grow best and most rapidly in an environ- 

 ment which is maintained at the optimum temperature for the organism. 

 If this temperature is lowered even a few degrees, the rate of reproduc- 

 tion is proportionately reduced. As the temperature approaches C., 

 there is complete or nearly complete cessation of growth with a corre- 

 sponding complete or nearly complete restriction of chemical inter- 

 change between the organism and its environment. The viability, 

 and in the pathogenic bacteria the virulence, is not seriously impaired 

 even by exposure to these low temperatures for considerable periods 

 of time. Practical advantage is taken of this restriction of bacterial 

 development by cold in the preservation of food by refrigeration or 

 by cold storage, and also for the preservation of laboratory cultures 

 of many non-spore-forming bacteria by placing them in the ice-box 

 at 5-10 C. So resistant are bacteria to low temperatures that they 

 may be actually frozen solid and kept in this state for days and even 

 weeks without killing all the individuals of the cultures. Alternate 

 freezing and thawing is much more disastrous to them than simple 

 freezing. Thus, typhoid bacilli may be suspended in water and 

 exposed to a freezing mixture of ice and salt at 18 C. for several 

 weeks without killing all the organisms, although the majority of them 

 are killed within a few hours. At the end of a week fully 90 per cent, 

 are dead; over 95 per cent, succumb by the end of four weeks' continual 

 freezing; but from four to six months' continuous freezing is required 

 to kill all of the typhoid bacilli. The survivors appear to be no more 

 resistant to subsequent freezing than similar organisms which have 

 not been frozen. It is a noteworthy fact that bacteria suspended in 

 colloidal substances, as egg albumen, are much more resistant to freez- 

 ing than similar organisms frozen in water. Alternate freezing and 

 thawing in colloids is much less disastrous to bacteria, in other words, 

 than the same freezing in aqueous solutions. It is probable that the 

 mechanical factor of crystallization which takes place when water is 

 frozen actually crushes many of the bacteria, thus accounting, in part 

 at least, for the greater death rate in aqueous solutions than that 

 observed in colloids. When bacteria are once frozen, further lowering 

 of the temperature has surprisingly little influence upon the death 

 rate. Typhoid and colon bacilli will survive freezing, in moderate 



