16 LOGARITHMIC ORDER OF DEATH 



found that bacteria die in quite a different order. This 

 seemed so unusual and so significant that much experi- 

 mental work and many theoretical discussions have been 

 published to explain this interesting phenomenon. If the 

 culture for an experiment on the order of death is care- 

 fully chosen, not too young and not too old, the bacteria 

 die at a constant rate. This means that the same per- 

 centage of all bacteria alive at any given time will die iu 

 the next time unit. If, for instance, one-half of the bacteria 

 die in the first minute of exposure, one-half of the remain- 

 ing bacteria will die in the second minute. This leaves one- 

 fourth alive, of which again one-half will die during the 

 third minute and so forth. The survivors represent a geo- 

 metrical progression. If a is the original number, and s the 

 percentage of survivors, then the actual number of sur- 

 vivors in successive time units will be 



a a- 



100 



vm) \-m) 



If s = 10 (that is, if the survival rate is 10%) and a = 

 1,000,000 bacteria, the successive numbers of survivors 

 will be 



1,000,000 100,000 10,000 1,000 100 10 1 0.1 0.01 



A geometrical progression can be recognized graph- 

 ically by the straight line that one obtains by plotting 

 the logarithms of its members against their exponents. 

 Since, in a disinfection experiment, the number of sur- 

 viving bacteria plotted logarithmically against the time 

 of exposure, furnish a straight line, it has become cus- 

 tomary to speak of a ''logarithmic order of death" of 

 bacteria. 



Figure 1 shows mortality curves and survivor curves. 

 The left side represents multicellular organisms, the 

 right side, bacteria. Figure 2 gives the corresponding 

 survivor curves in semi-logarithmic plotting. With bac- 

 teria, they are almost rectilinear ; with higher organisms, 

 they are concave downwards. 



