96 THE GROWTH AND DEATH OF BACTERIA 



explaining them. The problem still remains why it is that bacteria cease dividing 

 at maximal speed when a certain population density is reached. 



A suggestion which we put forward in 1936, and which has received some support 

 from the observations of Hershey and Bronfenbrenner (1937) and of Rahu and 

 Richardson (1942), is that the progressive retardation in growth may often be due to a 

 deficiency of oxygen in the culture. There are certain observations that seem to 

 support this view. We have already seen that the addition of 2 per cent, glucose to 

 a broth medium greatly increases the growth of Salm. typhi-murium. The mechanism 

 by which this increase is brought about is unknown, but it is not unreasonable to 

 suppose that the glucose provides a source of energy supply, which the organisms 

 are able to utilize in the absence of readily available oxygen. In support of this 

 we have found that the total number of organisms per ml. in a given culture can 

 be regulated by altering the available amount of oxygen. Thus, in a casein broth 

 culture oiSalm. typhi-murium incubated anaerobically, the total count after 24 hours 

 is about 500 million organisms ; in a culture incubated aerobically it is about 

 2,000 million organisms ; and in a culture through which pure oxygen is bubbled 

 continuously, it is about 8,000 million organisms (Wilson, 1930). There seems no 

 doubt that a liberal supply of oxygen enables growth to continue for some time after 

 it has ceased in a culture incubated anaerobically or under ordinary aerobic condi- 

 tions. The suggestion is that, in cultures incubated aerobically, growth continues 

 until the increasing density of the organisms renders it impossible for each individual 

 organism to obtain sufficient oxygen to meet its requirements. These findings 

 apply, of course, only to aerobic and facultatively anaerobic bacteria. 



This explanation, it will be observed, does not fit all of Bail's facts, though 

 it explains many of them. The main discrepancy is in the effect on the 

 M-concentration of adding certain substrates. Bail found that the addition of 

 glucose, for example, led to an increase in the total number of bacteria, while 

 leaving the M-concentration of viable bacteria unaltered. Our experience, both 

 with glucose and with increased aeration, entirely fails to support this finding. 

 The more favourable the conditions for growth are, the higher is the viable popu- 

 lation that the medium can support. It is true that in a medium containing a 

 fermentable sugar the viable population may fall rapidly after having reached 

 a height considerably greater than in the same medium without sugar (see Fig. 18), 

 but this appears to be due to the disinfectant action of the acid produced, and not 

 to any specific effect of bacterial density. That this is so is shown by the effect 

 of continuous oxygen passage in the experiments quoted above, in enabling a 

 broth medium to support four times its usual M-population. 



Broom (1929) and Gildemeister and Neustat (1935) have brought evidence to 

 show that the amount of growth of many organisms appears to depend to a con- 

 siderable extent on the presence in the medium of easily assimilable carbon com- 

 pounds, such as glucose. The addition of these to a culture in which gro'wth has 

 ceased may rapidly enable a fresh crop of organisms to be produced. The apparent 

 ability of some organisms to inhibit the growth of others in mixed cultures seems 

 to be due not only to their more rapid growth, but also to their more active fer- 

 mentative power, which enables them to break down certain compounds more 

 readily and so deprive the weaker organisms of their requisite nutritive materials. 



The fact that mere aeration of a culture may enable fresh bacterial multiplica- 

 tion to occur suggests that there are at least two factors leading to cessation of 

 growth. One is the exhaustion of easily assimilable food-stuffs ; the other is 



