**4. Cell proliferation**

Cell growth (proliferation) can be evaluated by the time-dependent changes of the total number of proliferating cells, as well as the ratio of cells among individual phases of the cell cycle. The cell growth curve was plotted to show the time-dependent increase in cell numbers. Depending on the cell type and growth condition, the required cell density to enter the exponential phase and the rate of cell growth measured by the doubling time (Tpot) could be very different. For example, most microbials (e.g., *E. coli*) enter the exponential growth phase with low cell numbers and show fast growth rate with a doubling time less than an hour [45],

whereas *M. tuberculosis* and mammalian cells have longer doubling time of several hours or a day. The growth and reproduction of cells can be further altered by changes in temperature, nutrition, viral infection, and/or the presence or absence of inhibitors [46]. Therefore, the growth curve is important in guiding clinical drug usage, investigating gene functions, and understanding drug mechanism of action. Various methods have been developed to measure the absolute number of cells or the changes in cell number, as shown in **Figure 4**.
