Growth phases

The growth of bacterial cultures involves several distinct phases

The six phases of a growth curve

Below, the different phases are illustrated for a simple batch culture like a shake flask.

In the below table, we draw inspiration from the classical Monod definitions for describing the different phases.

	Phase	Definition	Description	Synonyms
1	Lag	Growth rate is zero	A period of physiological adaptation where cells are adapting to the new growth conditions	Adaptation phase
2	Acceleration	Growth rate increases	Cells start to divide, but not at the same time because a proportion of the cells are still adapting	Transition phase; pre-exponential phase
3	Exponential	Growth rate is constant	Cell doublings occur at a constant, maximum growth rate	Log phase; logarithmic phase
4	Deceleration	Growth rate decreases	Cell division slows progressively. This is typically due to nutrient limitations and accumulation of growth-inhibitory substances	Retardation phase; pre-stationary phase; transition phase
5	Stationary	Culture has progressed through deceleration phase 2-4 and growth rate is zero	Cell concentration is constant. Usually due to severe nutrient limitation and accumulation of growth-inhibitory substances	Plateau phase; growth arrest phase; zero-growth phase
6	Decline	Growth rate is negative	The concentration of culturable cells declines over time. Essential nutrients are exhausted and the growth environment is hostile.	Death phase

Changes in growth rates for batch cultures

All definitions closely relate to the growth rate. This is clear when the growth rate is displayed on a separate plot above the concentration curve.

Growth rate determination in exponential stage

The exponential phase is often the most important region of the growth curve. The maximum growth rate defines how quickly cells can divide under the given cultivation conditions.

Exponential phase has a straight line in the log plot and a constant µ in the growth rate plot. This makes it straight-forward to do simple determinations of maximum growth rate.

Note that this only applies when using cell counting methods. Proxies like optical methods are somewhat difficult to interpret because the output depends on concentration as well as size. During the exponential phase, changes in cell size results in varied growth rates when measured optically, rather than a consistent rate.

Use cell counting methods to make determination of maximum growth rate easy

Plate counting, microscopy, and flow cytometry (including BactoBox®) display constant growth rate during the exponential phase. This makes it simple to determine the maximum growth rate, µ_max.

Growth rate determination may be tricky when using optical methods

Optical methods, such as optical density (OD) and backlight scattering, are influenced by both concentration and size. This makes it more complicated to calculate µ_max.