74 THE RISE AND FALL OF BACTERIAL POPULATIONS 



It is a curious and interesting fact that disinfection by sodium hydroxide follows 

 an entirely different law from that observed in other cases, K increasing progressively 

 with the time of exposure (Levine, Buchanan and Lease, 1927). 



It may be of interest, in spite of the various factors involved in the problem, to 

 consider some of the absolute values of K which have been observed in certain specific 

 instances and to note their significance in terms of percentage reduction. 



At one extreme stand such results as those obtained by Jordan (1926) for the 

 secondary reduction of Bad. coli in stored feces. These give (from the third to the 

 twenty-third day) a K per hour of .006. The values which I have cited for the reduc- 

 tion of the numbers of a certain strain of colon bacilli in water (Winslow and Cohen) 

 give a A' per hour of .01 for the third to the tenth day, corresponding to a reduction in 

 numbers of 2.5 per cent per hour. The reduction of Bad. typhosum in ice for the first 

 three days (Sedgwick and Winslow) is of the same order {K = .02) and involves a re- 

 duction of 5 per cent per hour. For Bad. coli dried in sand (Winslow and Abramson) 

 K = .o6 for the ninth to the forty-eighth hour (a reduction of 13 per cent per hour). 

 The mortality of anthrax spores exposed to 5 per cent phenol at 20° C. is about the same 

 (Chick, 1908). Falk and Winslow's studies give a A'- value of about .1 per hour for 

 dilute salt solutions (20 per cent per hour), and Cohen's data for acid (pH 8) give a K- 

 value of i.o (90 per cent reduction per hour). Chick's analysis of Clark and Gage's 

 results on the disinfectant action of sunlight give a A of 2.6, which would involve a 

 reduction of 95 per cent per hour. The death of paratyphoid bacilli exposed to 6 per 

 cent phenol is more rapid still, with a A of about 20, according to Chick's data (99.4 

 per cent reduction per hour) ; while hot water (54° C.) gives a A of over 60.0. This in- 

 volves a reduction of 99.75 per cent per hour. 



The factors which govern the rate of the mortality of bacteria are essentially the 

 same as those which govern the rate of their multiplication, though of course most of 

 them operate in an inverse sense. When bacteria die out in water or when stored in a 

 dried condition it is the normal katabolic processes of the cell (in the absence of com- 

 pensating anabolism) which must control the process. It seems doubtful whether 

 drying in itself exerts any specific harmful effect, and (if rapid and complete) it may 

 even slow down vital processes and thus prolong life. Ficker (1898), however, found 

 that alternate drying and moistening accelerated the lethal process. It is of interest to 

 note that Paul, Birstein, and Reuss (1910) found the disinfection constant in drying 

 proportional to the square root of the oxygen concentration of the atmosphere, fol- 

 lowing the law which obtains in the slow oxidation of phosphorus. The whole subject 

 of the longevity of micro-organisms under the influence of desiccation has been well 

 reviewed by Giltner and Langworthy (19 16). 



The presence of external toxic substances may of course be one important factor 

 in the death of bacteria in water or in soil. Thus Jordan, Russell, and Zeit (1904) found 

 that typhoid bacilli inclosed in collodion sacs survived much longer when the sacs 

 were suspended in pure water than when they were suspended in polluted water. In 

 the presence of known chemical disinfectants this factor of direct toxicity is of course 

 the determining feature. The value found for A may therefore vary within the widest 

 possible limits, depending on the toxicity of the particular disinfectant studied; and 

 the study of the relation between toxicity and chemical composition (as worked out, 



