Most unicellular microorganisms divide by binary fission during which a single cell doubles in size and then splits into two nearly identical cells. Some cells, for example yeasts, divide by budding . In this process a bud forms on the mother cell, it grows until it is almost as large as the mother cell, then splits off. Budding also results in exponential population growth. The amount of time that it takes a cell to divide into two cells is called the generation time .
Generation time depends on the culture conditions.
For example, E.coli in culture can have a generation time as short as 18 min. When living in the large intestine, E. coli generally has a much longer generation time of about 12 hours. Associated with depletion of nutrients or build-up of toxic waste products. Period where cell death exceeds the amount of cell division. The microbial population is continuously maintained.