Box 56 Calculating an increase in microbial numbers Example

An inoculum of 107 bacterial cells was introduced into a flask of culture medium and growth monitored. No change was seen for 18 minutes (the lag phase), then growth occurred rapidly. After a further 76 minutes, the population had increased to 4.32 x 108 cells. What is the doubling time (Td) of the culture? (To obtain values of log2, multiply log 10 values by 3.322)

1 _ log2 Nt - log2 No T d = T - L 1 _ log2 (4.32 x 108) - log2107

3 Stationary phase. As discussed above, the exponential phase is limited by environmental factors, and as the rate of growth slows down, the culture enters the next phase. The levelling out of the growth curve does not mean that cell division has ceased completely, but rather that the increase due to newly formed cells is cancelled out by a similar number of cell deaths. Eventually, however, as the death rate increases, the overall numbers fall and we enter the final phase of growth.

4 Death phase. As cells die off and the culture is unable to replace them, the total population of viable cells falls. This is the death (or decline) phase.