OD600 GUIDE
OD600 and CFU/mL: why the relationship needs validation
Why OD600 cannot be converted to CFU/mL with a universal constant, what shifts the ratio, and how to build a paired calibration for your own system.
OD600 and CFU/mL are related — denser cultures scatter more light and produce more colonies — but the relationship is empirical and system-specific. There is no universal constant that converts one to the other, and published factors are best treated as hypotheses to test on your own bench.
The core reason
OD600 measures light scattering by every particle in the beam: live cells, dead cells, cell debris. CFU/mL counts only the units that form colonies on your medium under your incubation conditions. Any factor connecting them absorbs, into a single number, the ratio of scattering particles to culturable units for the specific culture that was measured. That ratio is not a physical constant; it is a property of the culture’s physiological state.
What shifts the ratio
- Growth phase. Stationary-phase cultures accumulate dead and damaged cells that still scatter light, raising OD relative to CFU. Fresh log-phase cultures have a higher culturable fraction, so the same OD corresponds to more CFU.
- Strain and morphology. Cell size, shape, and chaining change both scattering efficiency and how many cells make one colony. A filamenting mutant scatters more per unit mass; a clustering species counts clusters as units.
- Medium. Particles, precipitates, and coloured components contribute to the reading unless properly blanked. Serum-containing media are a common offender.
- Instrument and cuvette. Path length and optical geometry differ between devices; a factor measured on one spectrophotometer does not transfer cleanly to a plate reader.
- Sample handling. Vigorous mixing, freeze-thaw, or surfactants change clumping and viability without changing OD proportionally — the numerator and denominator move independently.
Building a defensible pairing
- Take one culture and prepare a dilution series covering your working OD range — the same series design as in building an OD600 to cells/mL calibration, with plate counts as the paired method.
- Measure OD600 and CFU/mL on each dilution, in replicate, from the same well-mixed preparation.
- Fit CFU/mL against corrected OD. A line through the origin is the usual first model; check the residuals for curvature before keeping it, and restrict the fit to the range where the data are proportional.
- Report the calibration with its context: strain, medium, growth phase, instrument, date, and the OD range over which the fit was made.
- Re-validate periodically, and immediately after any change to strain, medium, or instrument.
One culture, one day, gives a calibration for that day. If your application spans weeks, build the pairing across several days and check that the scatter between days is acceptable for your purpose.
What about RLU and other signal-to-CFU conversions?
The same logic applies to any instrument signal — relative light units, fluorescence, qPCR Ct values. None of them converts to CFU/mL by a universal constant; each needs an instrument-specific, assay-specific calibration built from paired measurements. Treat any published “RLU to CFU” factor as a rough orientation, not a transferable number, and apply the same validation discipline described here.
A worked pairing
A concrete mini-example of the procedure, with numbers. One E. coli culture is diluted into five points, and each point is read for OD600 and plated in triplicate:
| Diluted OD600 | CFU/mL (pooled plates) | Implied ratio (CFU per OD) |
|---|---|---|
| 0.12 | 0.98 × 10⁸ | 8.2 × 10⁸ |
| 0.25 | 2.05 × 10⁸ | 8.2 × 10⁸ |
| 0.41 | 3.20 × 10⁸ | 7.8 × 10⁸ |
| 0.62 | 4.85 × 10⁸ | 7.8 × 10⁸ |
| 0.79 | 6.30 × 10⁸ | 8.0 × 10⁸ |
The ratio is stable near 8.0 × 10⁸ CFU per OD unit across the range, so a line through the origin is defensible here, and the calibration would be entered as K = 8.0 × 10⁸ with a valid range of roughly 0.1–0.8 corrected OD. Had the last point come in at 5.5 × 10⁸ — ratio 7.0 × 10⁸ — the curvature would say the top of the range is leaving linearity, and the fit should be truncated below it. Checking the implied ratio column point by point is the simplest residual diagnostic there is: flat means proportional, drifting means rethink the model or the range.
Using the result
Enter your fitted factor or curve into the OD600 calculator and keep the valid-range check on. For monitoring relative growth within a single experiment — comparing induced vs uninduced flasks, for instance — OD600 alone is usually sufficient, and no CFU conversion is required at all. Reserve the paired calibration for the moments when you need an absolute viable concentration from an optical reading.
What to avoid
Do not apply a conversion factor published for a different organism, medium, or instrument as though it were universal, and do not treat any calculator as able to convert OD600 to CFU/mL without a calibration you supplied. The tool performs arithmetic on the values and model you provide; the biology lives in the calibration.
Frequently asked questions
Can OD600 be converted directly to CFU/mL?
Not with a universal constant. OD600 measures light scattering by every particle — live, dead, and debris — while CFU/mL counts only units that form colonies on your medium. The ratio between them shifts with strain, growth phase, medium, and instrument, so any conversion needs a calibration built from paired OD600 and plate-count data for your own system.
How do I validate an OD600 to CFU/mL conversion?
Take one culture, prepare a dilution series, and measure OD600 and plate counts on every dilution in replicate. Fit CFU/mL against corrected OD over the range you will use, keep only if the fit is proportional across that range, and record the strain, medium, phase, and instrument alongside it. See the full validation procedure.
Does the OD600 to CFU ratio change during growth?
Yes. Stationary-phase cultures accumulate dead and damaged cells that still scatter light, so OD stays high while CFU plateaus or falls. A ratio measured in log phase will overestimate CFU if applied to a stationary culture — which is why a single conversion factor is only trustworthy near the conditions where it was measured.
More od600 guides
- OD600 during bacterial growth: growth curves and log phaseHow to read lag, exponential, and stationary phases from an OD600 growth curve, calculate doubling time, and know when OD stops tracking viability.
- OD600 to cells/mL: build and use a calibrationHow to build a calibration from paired OD600 and cell-count data, choose between a factor, linear fit, or log model, and apply it with the OD600 calculator.
- What OD600 measures and how to take a readingWhat optical density at 600 nm represents, how spectrophotometer settings affect the reading, and practical steps for reproducible OD600 measurements.