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BioAtlas

Growth and generation time

Bacteria do not grow by getting bigger. They grow by dividing, and dividing doubles — which is why a population plotted on a normal axis looks like nothing at all until suddenly it is everything.

The growth curve

Plotted on a logarithmic axis, because only a log scale makes the exponential phase look straight — which is what the phase is named for.

LagLog (exponential)StationaryDeath (decline)10⁹10⁶10³Log of cell numberTimeOne generation — the population doubles
No increase in number. Cells are making enzymes and building blocks, getting ready to divide.
Doubling at a constant rate. Conditions are ideal and nothing is limiting.
Number holds steady. Division and death balance out as nutrients run low and waste builds up.
Numbers fall. Death outpaces division.
Fig. 1A closed culture runs through all four phases in order: lag, log, stationary, death.

Solve it

Fill in three of the four boxes and leave the fourth blank. N = N₀ × 2ⁿ, where n is the number of generations.

cells

N₀ — what you inoculated.

cells

N — what you ended up with.

min

How long the culture grew.

min

How long one doubling takes.

Why the numbers get absurd

Doubling is unforgiving. One E. coli with a 20-minute generation time becomes over 16 million cells in eight hours, and in a day the mass would be measured in kilograms. Nothing grows like that for long, which is exactly what the stationary phase is.

Final number

4.096e+6

cells

Generations

12

12 doublings — the population multiplied by 2^12.

Working

  1. 01Each generation doubles the population: N = N₀ × 2ⁿ, where n is the number of generations.
  2. 02Generations: n = time ÷ generation time = 240 ÷ 2012
  3. 03N = 1.00e+3 × 2^124.096e+6 cells
  4. 04In 12 generations one cell becomes 4.10e+3. Doubling is deceptive — twenty generations is over a million cells.

The four phases

A closed culture — a flask nobody adds to — always runs through all four in this order.

01

Lag

No increase in number

Cells are making enzymes and building blocks, getting ready to divide. Metabolically busy, numerically flat.

02

Log (exponential)

Doubling at a constant rate

Conditions are ideal and nothing is limiting. This is where generation time is measured, and where cells are most sensitive to antibiotics.

03

Stationary

Number holds steady

Division and death balance out as nutrients run low and waste builds up.

04

Death (decline)

Numbers fall

Death outpaces division. Some cells form endospores here rather than die.

Lag phase is not idle

Nothing is dividing, but everything is working — enzymes are being made, and the cell is adjusting to a new medium. A culture moved between very different media has a long lag; one moved into the same medium barely has one.

Log phase is when drugs work

Most antibiotics attack processes that only happen during active growth — building a wall, copying DNA. A cell sitting in stationary phase is doing far less of that, which is one reason chronic infections are hard to clear.

Generation times

The spread here explains a great deal about how the diseases behave.

OrganismGeneration timeNote
Escherichia coli20 minutesThe textbook fast grower
Staphylococcus aureus30 minutes—
Pseudomonas aeruginosa30 minutes—
Lactobacillus acidophilus75 minutes—
Mycobacterium tuberculosis12–24 hoursWhy TB cultures take weeks and treatment takes months
Mycobacterium leprae≈ 14 daysAmong the slowest known

Why TB takes months to treat

Mycobacterium tuberculosis doubles roughly once a day rather than three times an hour. Drugs that act on growing cells therefore get far fewer opportunities, and the course of treatment is measured in months rather than days.

Binary fission, not mitosis

The chromosome is copied, the cell elongates, a septum forms, and one cell becomes two identical daughters. There is no spindle and no nuclear membrane to break down, which is part of why it is so fast.