7.2 Population Growth Growth is defined as an increase in the number of microbial cells in a population. Growth rate is the change in cell number or cell mass per unit time. The interval for the formation of two cells from one is called a generation. The time required for this to occur is called the generation time. 7.2 Population Growth • Growth is defined as an increase in the number of microbial cells in a population. • Growth rate is the change in cell number or cell mass per unit time. • The interval for the formation of two cells from one is called a generation. • The time required for this to occur is called the generation time