C.-E. A. WINSLOW 73 



the bacterium." Later Miss Chick (1910) showed that the death of bacteria when 

 dried or exposed to sunhght, or even when killed by moderate heat in water, proceeded 

 in general in accord with the logarithmic law. Cohen (1922) confirmed these general 

 conclusions for the much more gradual death-curve of colon bacilli in water. In all of 

 these latter cases, of course, the reacting agents which cause death must be within 

 the bacterial cells themselves. 



This simple concept of the disinfection process has been vigorously challenged by 

 a considerable group of workers such as Loeb and Northrop (1917), Brooks (1918), 

 Peters (1920), and Smith (1921) who attribute the form of the mortality curve to bio- 

 logical differences in the resistance of the individual bacterial cells. 



Miss Chick herself invoked this conception of individual-cell variation to explain 

 irregularities in her mortality curves for non-spore-forming organisms such as para- 

 t>phoid bacilli and staphylococci. When very young cultures of these organisms were 

 used she obtained constant values of K, but with older cultures the rate of disinfection 

 fell off in the later stages of the process. Even Cohen's curves in many instances ex- 

 hibit a tendency to flatten out toward the end of the periods studied, and Falk and 

 Winslow (1926) present results on the death of Bad. coli in dilute salt solutions 

 which suggest an initial rise in the value of K with the passage of time, followed by a 

 subsequent gradual fall. 



It appears certain, however, from a review of all the available literature that when 

 a bacterial population of a reasonably homogeneous character (spores or young vege- 

 tative cells) is subjected to an unfavorable environment — whether the unfavorable 

 condition be a chemical disinfectant, heat, sunlight, or merely storage in a dried con- 

 dition or in an aqueous medium where growth cannot occur — there is a period during 

 which the death of the bacterial cells follows a fairly regular logarithmic rate. This 

 period of regular decrease may be preceded by a brief period of slower decline, repre- 

 senting a sort of lag or adjustment to the unfavorable environment, and seems general- 

 ly to be followed by a final period of still slower decline. The latter may not occur 

 when death is due to strong disinfectants such as were used by Miss Chick and 

 did not appear in Cohen's studies because they were not prolonged for a sufficient 

 period. 



It seems, however, entirely unnecessary to postulate biological variations in the 

 cells of the bacteria to account for these deviations from the logarithmic curve. As 

 Falk and Winslow (1926) have pointed out, it is much simpler to assume that the 

 lethal reactions which go on in bacterial cells dying in an unfavorable environment 

 proceed in accord with a bimolecular reaction or reactions of a still higher order rather 

 than in accord with the monomolecular formula. There is no reason to assume that 

 the decomposition of a single chemical compound is always the determining cause of 

 death. It is shown in the paper cited that the values of K may often be best explained 

 on such an assumption. Just as the lag period in bacterial growth involves the as- 

 sumption of catenary reactions, so do the variations at the beginning and end of the 

 curve of bacterial mortality. To obtain the values which most nearly approximate a 

 logarithmic curve and represent the period when the lethal process most nearly sim- 

 ulates a monomolecular reaction, Phelps has suggested that K should be determined 

 for the middle portion of the curve — say from a reduction of 75 to one of 25 per cent. 



