How do you convert the OD reading of a bacterial growth to bacterial number

Question

How do you convert the OD reading of a bacterial growth to bacterial number

Answer 1

You need to create a standard curve of OD versus colony counts first. Make different dilutions of a culture. Take aliquots to do colony counts, and take the OD readings. Once you have all the data, plot the standard curve. From now on, all you have to read the OD of a culture, you will get the estimation of bacterial number, without waiting a couple of days for the colony to show up.  

Answer 2

Directly count the number of cells in the cultures by determining the CFU's (colony forming units) on solid medium.

Answer 3

In short, you can't.  Not without knowing some viable cell counts before hand sadly.  However you can figure out some useful information from OD readings.  You can figure out your doubling rate, and your mean growth rate constant.  Since every bacteria behaves differently, there is no set in stone way to figure out the bacterial number by OD.  Since OD only measures light intensity and not whether or not the cells are living or dead, a viable cell count must be done in order to correlate Optical Density to bacterial growth.

This might be a little complicated so bear with me for a bit

Let N of 0 = the initial population number

N of t = the population at time t

n = the number of generations in time t

N of t = N of 0 times 2^n

Solving for n, the number of generations, where all logarithms to the base 10.

logN of t = log N of 0 + n * log 2, where n= (logN of t- logN of 0)/log 2 = (logN of t -logN of 0)/0.301

The rate of growth during the exponential phase in a batch culture can be expressed in terms of mean growth rate constant (k).  This is the number of generations per unit of time, often expressed as generations per hour.

k= n/t = (logN of t - logN of 0)/0.301t

The time it takes a population to double in size--that is, the mean generation time or mean doubling time(g), can now be calculated.  If the population doubles (t=g), then

N of t=2N of 0

Subsitute 2N of 0 into the mean growth rate equation and solve for k.

k= ( log [2N of 0] -logN of 0)/0.301g = (log 2+ logN of 0 - logN of 0)/0.301g

The mean generation time is the reciprocal of the mean growth rate constant therefore

g= 1/k

Suppose your bacterial population increases from 10^3 cells to 10^9 cells in 10 hours.

k= (log 10^9 - log10^3)/(0.301)*(10hours)= (9-3)/(3.01hr) = about 2.0generations/hr

If by some off chance you know the viable cell count you can use this formula to correlate the viable cells from a specific OD:

# of Colonies/ Volume Plated [based on 1 = 1mL]*1 * the dilution factor

So if you count 150 cells on the 10^-6 plate, and you plated out 1mL of your dilution your formula would look like this

150/ (1mL * 1x10^-6), therefore your CFU per mL is 1.5 x 10^8.  So if you did a viable cell count at that specific OD your OD reading would correlate to an actual cell count.

Sorry if that was a bit long, but hopefully this might help you a bit more.