94 THE GROWTH AND DEATH OF BACTERIA 



after which a progressive fall occurred. It must be pointed out, however, that 

 this might equally well be explained by assuming that the type of metabolism 

 had changed during a period of constant growth. 



Care must, of course, be exercised in drawing any conclusions on the rate of 

 multiplication from the rate of growth, since it has already been pointed out that, 

 owing to variations in cell size, increase in bacterial numbers may give a very 

 fallacious idea of the true rate of growth, and the converse is equally true. Further 

 observations at frequent intervals on the numerical increase occurring during the 

 logarithmic phase, made with due attention to multiple small factors that may 

 influence the result, are required before concluding that the rate of cell-division 

 during this phase is discontinuous. 



Stationary Phase 



After multiplying at a maximum rate for a variable length of time during the 

 logarithmic phase, the organisms become less active, and divide less frequently, 

 till finally their numbers remain practically constant. What is responsible for 

 this decrease in the reproduction rate 1 The natural suggestion is that it results 

 from an exhaustion of the food supply. Against this view, however, is the fact 

 that if a culture that has reached the decline phase is sterilized by boiling, and 

 then reinoculated with the same organism, growth occurs in the usual way, though 

 the actual numbers attained may not be so great as in the primary culture (Graham- 

 Smith 1921). The same result is obtained if the culture is sterilized by filtration 

 instead of by boiling, provided that the organism has not produced some volatile 

 substance which would be removed during boiling, as in the special case of the 

 pneumococcus. Penfold (1914) found that if a 24:-hours' culture of Bad. 

 coli was centrifuged, and the supernatant fluid was incubated at 37° C, fresh 

 growth took place. It is probable, therefore, that some other factor than exhaus- 

 tion of the food supply is responsible for the cessation of maximal growth. 



We have already mentioned Chesney's hypothesis, that toxic substances are 

 produced during the phase of multiplication,»and that these so injure the bacteria 

 as to delay their subsequent division when introduced into a fresh medium. 

 Although this explanation may hold good for the pneumococcus, which produces 

 H2O2 in considerable amount, it does not seem to be of more general appUcability. 

 We may similarly regard as special cases those instances in which the reaction of 

 the medium is rendered acid during growth owing to the inclusion of a fermentable 

 carbohydrate. In such cases neutralization of the acid by the addition of alkali 

 often suffices to enable growth to occur (Kojima 1923). 



Working with the fruit-fly Drosophila melanogaster Pearl and Parker (1922) 

 found that the effective reproduction rate decreased as the population density 

 became greater. 



Following on these observations Bail (1929) carried out a number of experi- 

 ments on different bacteria, from the results of which he concluded that in any 

 fluid culture there was a limiting population density that could not be exceeded. 

 This he referred to as the M-concentration. A few of his findings may be briefly 

 recorded. 



(1) Any given species of bacterium reaches in a fluid medium a particular and 

 constant M-concentration of living organisms, the value of which differs with 

 different species. 



