Spectrophotometer, OD, and yeast growth

As you recall one of our new toys is a spectrophotometer. How does it work? You can check the entry on Wikipedia, but idea is very simple. A light beam of a known wavelength is passed through a small cuvette (fancy name for a container) that holds a fixed volume of sample. On the other side it hits a detector that measures the amount of light that has passed through the sample.

The amount of light that has been absorbed is informative on the content of the sample. Certain wavelengths for example are absorbed by poly-nucleotide and thus can report on DNA/RNA content. In our case, we use 600nm wavelength which is absorbed by cells. Thus, the amount of "lost" light intensity reflects the amount of cells.

As you might guess once you have sufficiently "dense" liquid, no light will pass through, and you cannot really learn much from the measurement. Thus, it is important to understand at what values the measurement is linear in the number of cells.

Today Avital and Ayelet tested the new spectrophotometer by performing serial dilution of dense culture of yeast cells to see how they vary. If we are in a linear range, than a dilution of 1:1 should reduce the loss of light (reported value) by 1/2. They report that they get a linear range from about 0.65 OD (units of Optical Density) and downwards.

On the cool side, we found out that the spectrophotometer is based on batteries and can be moved to the bench you are working on, which is really bonus.

In addition, they used the new incubator to perform a growth curve experiment. They seeded yeast cells in different media and grew them in the incubator.

Then they measured the OD every hour using the spectrophotometer. During exponential grow we expect that every XX minutes the OD will double. The value of XX is the doubling time of the yeast in that media and tells us how "happy" the yeast is there. Hopefully I will load some results in one of the next entries.