Growth curves
A flask culture followed by taking samples at intervals and reading OD600 in a cuvette, from inoculation to stationary phase. The classic way to see the phases of batch growth and measure a doubling time, with dense samples diluted into the photometer's linear range.
Our ruleSimulations prepare you for the bench but don't replace hands-on practice.
Lag, log & stationary phases
A flask culture followed by taking samples at intervals and reading OD600 in a cuvette, from inoculation to stationary phase. The classic way to see the phases of batch growth and measure a doubling time, with dense samples diluted into the photometer's linear range.
How it differs from the standard method
- Fill the flask to no more than a fifth of its volume, baffled if possible, and shake at 200–250 rpm.
- Read every 10–20 min through log phase for a fast-growing organism; less often in lag and stationary phase.
- Blank against sterile medium; dilute any sample above ~0.4 into range in medium and multiply back.
- Plot ln(OD) or OD on a log axis, find the straight stretch, and fit the growth rate to it alone.
1Four phases of a batch culture
Put a few bacteria into fresh medium and follow them, and the population goes through a recognisable sequence. First comes a lag, while the cells repair, make the enzymes the new medium calls for and build up ribosomes; little division happens.
Adjusting
Length depends on the inoculum's history: short from an exponential pre-culture, longer from a stationary overnight, long and erratic from an old cold plate.
Constant rate
A straight line on a log axis. E. coli in LB at 37 °C doubles in about 20 minutes, in minimal glucose medium in about an hour.
Something ran out
The plateau density tells you about the medium and the aeration, not about the growth rate.
Slow decline
Viable counts fall over days; OD may barely move, because dead cells still scatter light.
2What OD600 actually measures
Optical density at 600 nm is not absorbance by the cells but light scattered out of the beam. While the suspension is dilute, each photon is scattered at most once and the reading is proportional to the number of cells.
3Growth curves in a plate reader
A shaking plate reader can follow dozens of strains or conditions in parallel for a day, reading every few minutes. The trade-offs are physical.
- Specific growth rate (μ)
- The slope of ln(N) or ln(OD) against time in exponential phase, per hour. Doubling time = ln 2 / μ.
- Linear range
- The OD range over which the reading is proportional to cell density for a given instrument and path, typically up to ~0.4 in a 1 cm cuvette.
- Path length
- The distance the light travels through the sample: 1 cm in a cuvette, set by the volume in a microplate well.
Key terms
Practise these as flashcards →The period after inoculation before cells divide at their full rate; often quoted as where the exponential line meets the starting density.
Growth at a constant specific rate, a straight line of ln(OD) against time.
The plateau reached when a nutrient or oxygen runs out; cells change physiology and become more stress-resistant.
Time for the population to double in exponential phase, ln 2 / μ.
Light scattered out of a 600 nm beam by a cell suspension; a proxy for biomass, linear only at low density.
Sterile medium the photometer is zeroed against, so the medium's own absorbance is not counted as cells.
When to use it
- Comparing how fast strains, mutants or conditions grow: doubling time and lag from the exponential and lag phases.
- Timing a harvest: inducing protein expression in mid-log phase, making competent cells at OD 0.3–0.5, or collecting stationary-phase cells.
- Screening many strains, drugs or media at once in a shaking plate reader.
- Checking a medium: the stationary density shows which nutrient or how much aeration limits growth.
Compared with alternatives
Counts viable cells rather than biomass, so it sees death that OD misses and calibrates OD against cells, at the cost of a day's wait and many plates.
The instrument behind OD600; its linear range, blanking and stray light are the same issues, met here as scattering rather than absorbance.
The same reader can follow a fluorescent reporter alongside OD, but the path-length and evaporation issues apply to both.
Where the inoculum comes from: the state of the overnight sets the lag of the curve that follows.
Sub-techniques
Choosing one here switches the whole page, including the protocol and the lab, to it.
A flask culture followed by taking samples at intervals and reading OD600 in a cuvette, from inoculation to stationary phase. The classic way to see the phases of batch growth and measure a doubling time, with dense samples diluted into the photometer's linear range.
- Fill the flask to no more than a fifth of its volume, baffled if possible, and shake at 200–250 rpm.
- Read every 10–20 min through log phase for a fast-growing organism; less often in lag and stationary phase.
- Blank against sterile medium; dilute any sample above ~0.4 into range in medium and multiply back.
- Plot ln(OD) or OD on a log axis, find the straight stretch, and fit the growth rate to it alone.