LOGARITHMIC PHASE 93 



various species of bacteria growing under optimal conditions. Among the most 

 rapidly growing organisms are the members of the coli-aerogenes group, with a 

 generation time of less than 20 minutes. Some of the large spore-bearing bacilli 

 grow at about the same rate, while the thermophilic species may grow even faster. 

 Organisms of the Salmonella, Proteus, and probably Vibrio, groups have a genera- 

 tion time of 20-30 minutes, Staphylococcus and Streptococcus of 25-35 minutes, 

 Pseudomonas of 30-40 minutes, Corynebacterium of 35-40 minutes, Clostridmm of 

 35-50 minutes, Lactobacillus of 40-80 minutes, Rhizobimn of 100 minutes, Azolo- 

 bacter of 30-240 minutes, and Phytomonas of 75-165 minutes. Mycobacterium 

 multiplies very slowly ; the slowest of all appear to be Nitrobacter and Nitro- 

 sococcus. 



Within certain limits definable for each organism, the duration of the logarithmic 

 phase varies — 



(1) inversely with the temperature. For example, Lane-Claypon (1909), 

 and Jennison (1935), working with Bad. coli, made the following observations on 

 the duration of the logarithmic phase at different temperatures : 



6-7| hours at 37° C. 



8-8| hours at 30° C. 

 11-15 hours at 25° C. 

 14-24 hours at 20° C. 



(2) inversely with the size of the inoculum. 



(3) directly with the quantity of culture medium. 



If a flask of 50 ml. of broth is seeded with one million living Bad. coli, then 

 the logarithmic phase at 37° C. will last, generally, between 3 and 4 hours. 



At the beginning of this section the logarithmic phase was defined as that 

 period during which regular and maximal multiplication was occurring. Kecent 

 work, however, has cast some doubt on the accuracy of this definition. Rogers 

 and Greenbank (1930) and Hirsch (1933), for example, have obtained evidence 

 that the rate of growth during this phase is intermittent. Whether measured 

 by the increase in numbers (F. metchnikovi) or by the oxygen consumption (mouse 

 typhoid bacillus), Hirsch found that. 3 or 4 peaks of growth occurred every hour. 

 It is conceivable that these results may have been partly influenced by the periodic 

 shaking of the culture, since the effect of aeration in stimulating growth is very 

 marked. But this criticism does not apply to the results of Rogers and Green- 

 bank, who inoculated their organisms {Bad. coli and Str. ladis) at one end of a tube 

 15 metres long and measured the rate of advancing growth by observing the 

 decolorization of a dye in the medium. The rate of advance was intermittent, 

 and the intermissions showed a striking degree of periodicity. When plotted 

 against time, the curves depicting the rate of growth showed a moderate peak 

 every 4-5 hours with a much higher peak every 15-20 hours throughout the 8 days 

 required for the organisms to reach the distal end of the tube. It will be noted 

 that the technique used by Rogers and Greenbank was very different from that 

 of Hirsch, and it is by no means certain that the two sets of observers were dealing 

 with the same phenomenon. 



Further evidence that growth is not uniform throughout the logarithmic phase 

 is suggested by the observations of Walker and his colleagues (1934) on the rate 

 of CO2 production by Bact. coli in a peptone water medium. It was found that 

 the maximum output of COjj per unit volume of bacterial substance occurred 

 during the third hour of growth, i.e. the first hour or so of the logarithmic phase, 



