Question

When you walk into the lab your TA hands you a cuvette that contains a sample of a bacterial culture and asks you to read the optical density (OD) of the sample in the spectrophotometer. The reading you get is 0.85. Your TA then informs you that in the past the OD reading for this bacterium during its stationary phase has ALWAYS been 1.3. Just by the 0.85 OD reading, can you tell what growth phase the bacterium is in? Explain why or why not?
Appreciate it!!

Answer 1

Ans. The OD of a bacteria culture media is a measure of relative turbidity of the solution. Greater is the cell density in the media, greater is the turbidity of the solution and so is the OD of the sample. The OD of the sample is directly proportional to cell density, i.e. greater is the OD, greater is the cell density in the media.

Moreover, the turbidity gives total bacterial count i.e. include both living and dead cells. OD does not differentiate dead and living cells.

Now, given, the OD of a culture in stationary phase is always 1.3.

The OD obtained by the given sample = 0.85.

It means that the cell density of the media is less than stationary phase. The viable cell count lower than stationary phase is observed on either side of the stationary phase i.e. log phase on the left and decline phase on the right.

However, as mentioned above, the OD gives total (dead + live cells) cell count. The total cell count in decline phase is greater than stationary phase because it includes the living cells as well as the dead cells that were ever in stationary phase. So, it’s OD shall always be higher than that of stationary phase, i.e. OD higher than 1.3.

The culture in log phase (still growing in number) have relatively lesser cell density and very few dead cells (almost negligible) than stationary phase. Thus, it’s OD is always less than that of stationary phase. Hence, the given culture is in log phase.