THE LAG PHASE 85 



have apparently remained unaltered, and a large proportion that have divided ; 

 amongst this latter group considerable variation will be found in the number of 

 daughter cells produced, these varying in different micro-colonies from 2 to 

 about 16 or even more. Measurements of the size of these daughter cells 

 show that they are considerably larger than the original cells used for seeding the 

 preparation. Clark and Ruehl (1919) were the first to draw serious attention to 

 increase in size of the bacterial cell during growth. Using a micrometer, they 

 measured the size of seventy strains of organisms belonging to thirty-seven different 

 species at intervals during their growth. With the exception of members of the 

 diphtheria group, they found that the young organisms in cultures from 4 to 

 9 hours old were much larger than those in cultures 24 hours old. Henrici 

 (1926, 1928), who largely confirmed their work, found in a culture of B. 

 megatherium, that the average length of the original cells was 3-4 fx, but that during 

 the phase of maximum multiplication the average length of the constituent organ- 

 isms was about 15 /^. After this phase was over, the average length of the organisms 

 decreased till in 10 hours it approximated to that of the organisms used for the 

 original seeding. Similar observations on Salm. typhi-murium showed that the aver- 

 age size of the cells in a 4-hours' agar culture was 2-35 X 0-79 fi, whereas in the same 

 culture after 26 hours it was only 1-13 X 0-49 [x (Wilson 1926). Estimations 

 made from these measurements showed that the cells from the young culture 

 were nearly six times the volume of those from the older culture. By special 

 staining methods, appearances indicative of nuclear division may be demonstrated 

 in some bacteria before true cell division occurs (see Chapter 2). 



(2) Increase in Respiratory Activitij. — Observations by various workers (for 

 references see Walker et al. 1934, Winslow and Walker 1939) on the biochemical 

 activity of the cells during the lag phase have shown that this phase is by no means 

 a period of rest. Respiratory and metaboUc processes, such as an increased oxygen 

 uptake, a fall in oxidation-reduction potential, and a rise in the output of heat, of 

 NH3, and of CO2, become detectable soon after the organisms have been inoculated 

 into the medium. The rate at which these changes proceed increases till it reaches 

 its culminating point during the first hour of the logarithmic phase of growth. 

 These observations render it clear that, though the cells are not dividing, they are 

 metabolizing very actively during the so-called lag period. 



(3) Increase in Susceptibility to Disinfectants. — Further evidence of change in 

 the bacterial cells during the lag phase is afforded by a study of their resistance 

 to disinfecting agencies. Schultz and Ritz (1910) showed that in young cultures 

 of Bact. coli, 3 to 6 hours old, the organisms were heat-sensitive, whereas in 

 older cultures, 8 to 24 hours, they were nearly all resistant, when tested 

 at 53° C. for 25 minutes. Similarly Reichenbach (1911), working with Sahn. 

 paratyphi B, found that the younger the cultures were, the higher was the 

 proportion of heat-sensitive organisms. Sherman and Albus (1923) found that 

 in a culture of Bact. coli the freshly formed bacteria were more susceptible to the 

 action of inimical agents such as heat, cold, and disinfectants, than were the older 

 bacteria. Taking advantage of these differences, they attempted (Sherman and 

 Albus 1924) to study the relative numbers of the two kinds of bacteria present 

 during the lag phase of growth. A 1 per cent, peptone water medium was inocu- 

 lated from a culture of Bact. coli which had been grown in the same medium at room 

 temperature for a week ; the inoculum consisted therefore of old bacteria. The 

 culture was incubated at 37° C, and plate counts were made at intervals. At the 



