6o 



THE RISE AND FALL OF BACTERIAL POPULATIONS 



steadily to 2,341,000 after 8 days. Whipple (1901) noted the same effect — an initial 

 fall, followed by a rise, as a universal phenomenon in portions of natural waters stored 

 in sample bottles under various conditions. This type of reaction, indicated by the 

 dotted line AB in Figure i, does not appear to be substantially different from the lag 

 phase in a richer culture medium and, indeed, it seems probable that whether the ob- 

 served net effect be an increase or a decrease there is going on during this period a 

 multiplication of some cells and a death of others, the relative rate of these two proc- 

 esses determining the end-result. The fact that the bacterial count of milk shows an 

 initial decrease was pointed out by Fokker (1890), and a long controversy has been 

 carried on in regard to its cause (excellently summarized by Heinemann, 1919). Many 

 investigators claim that freshly drawn cow's milk possesses special germicidal proper- 

 ties due to the presence of agglutinins and other bactericidal substances, the presence 

 of living phagocytes, or the restraining action of lecithin. Heating the milk destroys 

 this "bactericidal power," but it must always be remembered that heating also changes 

 the nutritive qualities of a medium and may therefore make it more favorable. In 

 general, the phenomenon seems to be a special case of the lag period in a medium 



more or less unfavorable to certain types of bacteria which have gained access to the 

 milk. 



Ledingham and Tenfold (1914) and Slator (1917) have attempted to formulate 

 mathematical expressions for the multiplication rate even in the highly variable lag 

 period of the population cycle; but if the conclusion be justified that the phase of ad- 

 justment may, according to circumstances, be characterized by a decrease followed by 

 an increase or by a gradually accelerating increase, the futility of any attempt at 

 mathematical analysis will be apparent. 



The inflection and the slope of the curve of the bacterial population will vary with 

 three general factors: the type of bacteria involved, the medium into which they are 

 introduced, and the temperature. The first two factors may in a sense be reduced to 

 one — the suitability of the particular medium for the particular bacteria in question. 

 If the medium be entirely inadequate, the bacteria simply die off and the phase of ad- 

 justment merges into the phase of decrease. On the other hand, as I shall point out, 

 if the medium be ideal and the bacteria in the right condition, the phase of adjustment 

 may be reduced to proportions which are not measurable, and the logarithmic in- 

 crease will begin at once. There may be an infinite number of gradations between 

 these two extremes, giving a longer or a shorter phase of adjustment. Coplans (1909), 

 for example, found that in transferring from peptone water to peptone water the lag 

 period lasted about i hour while in transfer from one dulcitol medium to another it 



