OD600 is a standard laboratory measurement that estimates how many bacterial cells are in a liquid culture. It stands for “optical density at 600 nanometers,” meaning a spectrophotometer shines orange-yellow light (600 nm wavelength) through a sample of bacteria suspended in liquid and measures how much light makes it to the other side. The more cells in the sample, the less light gets through, and the higher the OD600 reading.
How OD600 Actually Works
Despite the name “optical density,” OD600 doesn’t measure true light absorption the way you’d measure, say, the concentration of a dye. Bacterial cells are physical objects suspended in liquid, and they scatter light in all directions rather than absorbing it. When light hits a bacterium, it bounces off at various angles, deflecting it away from the detector on the far side of the sample. The spectrophotometer interprets this lost light as “density,” even though it’s really measuring scattering.
This distinction matters because scattering behaves differently than absorption. At high cell concentrations, light scattered away by one cell can be scattered back into the beam by another cell, making the reading lower than it should be. The relationship between OD600 and actual cell count is only reliably linear up to about 0.1 to 0.4 OD units, depending on the instrument. Above that range, you need to dilute the sample and multiply back to get an accurate number.
Why 600 Nanometers Specifically
The choice of 600 nm isn’t arbitrary. Bacteria scatter light most efficiently at wavelengths near 600 nm, which gives a strong, sensitive signal. Equally important, bacterial cells don’t significantly absorb light at this wavelength, so you’re measuring cell density without interference from the cells’ own pigments or internal molecules.
The growth media also plays a role. Common broths like LB (Luria-Bertani) have a yellowish color, which means they absorb light strongly in the 430 to 480 nm range. At 600 nm, the medium itself is essentially transparent, so it doesn’t add background noise to the reading. This combination of strong scattering, minimal cellular absorption, and minimal media interference is what made 600 nm the standard.
How to Take an OD600 Reading
The basic setup uses a cuvette, a small transparent container with a standard 1 cm path length. You fill it with your bacterial culture, place it in the spectrophotometer, and the machine reports an OD value. Before measuring your sample, you “blank” the instrument by running it with sterile growth medium (no bacteria) so the machine subtracts the medium’s own light interactions from the final number. This way, the reading reflects only the cells.
Path length matters because it’s built into the underlying math. The Beer-Lambert law says that the signal scales with both concentration and the distance light travels through the sample. A standard cuvette is 1 cm, but microplate wells have variable and shorter path lengths, so readings taken in 96-well or 384-well plates need a correction factor to be comparable to cuvette measurements. Most modern plate readers can apply this correction automatically.
Converting OD600 to Cell Counts
One of the most common questions is: how many cells does a given OD600 actually represent? The short answer is that it depends heavily on the species, the strain, and even the growth medium. For E. coli, the most widely used lab bacterium, an OD600 of 0.1 corresponds to roughly 100 million cells per milliliter. However, one study noted that the same strain grown in LB broth gave only about 20 million cells per mL at OD600 of 0.1, a fivefold difference from other conditions.
Cell size is the main reason for this variability. Smaller cells scatter less light per cell, so you need more of them to reach the same OD600. Data from the Harvard BioNumbers database illustrates this clearly: at an OD of 1.0, small species like Staphylococcus epidermidis (cell volume around 1 cubic micrometer) pack in roughly 34 billion cells per mL, while the much larger Pseudomonas putida (about 12 cubic micrometers per cell) reaches the same OD with only around 500 million cells per mL. If you need precise cell counts, you have to build a calibration curve for your specific organism by plating serial dilutions and counting colonies.
Where OD600 Fits in Everyday Lab Work
OD600’s biggest advantage is speed. Counting colonies on plates takes overnight incubation. Counting cells under a microscope is tedious. OD600 takes seconds and doesn’t destroy or disturb the culture, so you can track growth in real time by pulling small samples at regular intervals.
Researchers use these readings to identify which growth phase their bacteria are in. A culture typically starts with a lag phase where OD barely changes as cells adjust to fresh medium. Then comes exponential (log) phase, where cells divide rapidly and OD climbs steeply. Eventually, nutrients run low and waste builds up, pushing the culture into stationary phase where OD plateaus. Knowing where you are on this curve is critical for many experiments.
Protein expression is a good example. When scientists want bacteria to produce a specific protein, they grow the culture to mid-log phase, typically an OD600 between 0.6 and 1.0, before adding a chemical trigger to switch on the gene. Starting induction at this density ensures there are enough healthy, actively dividing cells to produce large quantities of protein, but the culture isn’t yet crowded enough to slow down metabolism.
OD600 Compared to Other Wavelengths
You’ll sometimes see measurements at nearby wavelengths like 595 nm or 450 nm, but these serve different purposes. OD595 is commonly used in biofilm assays, where bacteria are stained with crystal violet dye and the dye’s absorbance (not cell scattering) is what’s being quantified. Researchers sometimes normalize biofilm data by dividing OD595 by OD600 of the total culture, separating how much biofilm formed from how much the bacteria grew overall. OD450 appears in enzyme-linked assays and viability tests that rely on colored chemical reactions. These wavelengths measure the products of specific assays, not raw cell density.
Limitations Worth Knowing
OD600 counts everything in the light path, not just living cells. Dead cells, cell debris, and even small particles in the medium all scatter light. A culture that has been heavily stressed or treated with antibiotics might show a stable OD600 even though most cells are dead. For that reason, OD600 is best understood as a measure of total biomass, not viability.
Cell size also introduces complications beyond simple conversion factors. E. coli cells are roughly the same size as the 600 nm wavelength itself, so they scatter light without casting a “shadow.” Larger organisms like the yeast Pichia pastoris, with cells 10 to 20 times the wavelength, can block light in a shadow-like pattern. Cells sitting in another cell’s shadow scatter less light than they otherwise would, pulling the OD reading lower than expected. This means OD600 systematically underestimates the density of cultures with large cells at high concentrations.
Despite these quirks, OD600 remains the default because nothing else matches its combination of speed, simplicity, and non-destructiveness. For most routine microbiology, it provides exactly the level of precision needed: a fast, reliable snapshot of how dense your culture is right now.