Introduction to microbial growth

The growth of a microbial population in a limiting fluid medium (one in which no additional nutrients are added after inoculation) can be divided into a number of different phases.
The first phase is a variable period of time during which no increase in numbers occurs and an actual decrease in viable cells may occur. Cell volume, total protoplasm and RNA content increases, i.e. metabolic changes occur, including an increase in respiration. This phase is called the LAG phase. It is the period when the bacteria are adapting to a new environment. Different enzymes are being synthesized and intermediary metabolites for constitutive enzymes have to be produced.
The second phase of growth is the transition from lag to logarithmic growth. The rate of cell division is increasing during this period.
The third phase is the exponential or LOGARITHMIC phase of growth. This phase represents the maximal reproduction capacity of the organism under the given conditions. All cells produced in this phase are viable, and hence there is a direct correlation between turbidity and viable count at this time. True logarithmic growth in a limiting medium occurs for a relatively short period of time (8-10 generations) but this can result in high numbers within 3-4 hours for a rapidly dividing organism such as E. coli (which has a generation time of about 20 minutes). As a result of extreme metabolic activity nutrients become depleted and toxic products start to accumulate (particularly acid).
When the log of numbers of viable organisms is plotted against time, a straight line is obtained. The slope of this line gives a measure of growth rate and the mean generation time can be calculated. Different slopes are obtained under different conditions, allowing direct comparisons.
The fourth phase of growth begins when one of the limiting factors comes into play (lack of an essential nutrient or accumulation of a toxic byproduct). The viable count continues to rise but at a reduced rate, which continues to slow as nutrients are consumed and toxins build up.
The fifth stage is the stationary phase where the rate of cell division matches the rate of cell death. Thus there is no further increase in the numbers of viable cells but the total number of cells increases.
At the end of the stationary phase there is an acceleration in the numbers of cells dying, representing the accelerated death phase. This leads rapidly into the logarithmic death phase where cells are dying at an exponential rate.
The final phase occurs when the rate of death slows (decelerated death phase) until eventually only a few individuals survive (sometimes for weeks or months, depending on conditions).
The general shape of the growth curve is similar for most bacteria, but the details of time intervals and slope will vary for different organisms under different conditions.

In summary:

1. lag phase
2. accelerated growth phase
3. logarithmic growth phase
4. decelerated growth phase
5. stationary phase
6. accelerated death phase
7. logarithmic death phase
8. decelerated death phase
9. lag death phase

Each phase is represented on the graph: