High-throughput measurements of microbial growth rates have been made possible by using microtiter plate readers to measure the growth of cultures in the wells of microtiter plates. It is not necessary to use a very sophisticated or expensive plate reader to measure microbial growth. In most cases a fairly simple reader, even a used plate reader, will do just fine.
The plate reader just needs to have the following basic features:
- The ability to maintain a constant temperature
- The ability to shake the plate between readings in order to provide good aeration and to keep the cells in suspension
- The ability to set the wavelength to about 600 nanometers
- The ability to set the reading interval
- The ability to save the readings either to a text file or an Excel file
Turning the output of a plate reader into actual growth rate measurements requires the use of a growth rates computer program.
There are several such programs available, some of them sold by the manufacturers of plate readers themselves.
Keep These Five Tips in Mind:
- The software must provide the ability to determine the OD of a blank well (a well that contains the same volume of uninoculated medium as the culture volume in each well). It must be able to automatically correct each OD reading by subtracting the reading in the blank well. Rate determinations that use uncorrected ODs will simply be wrong – anywhere from a little wrong to vastly incorrect – because they will not be measuring the OD that comes from the cells themselves.
- The software must determine the growth rate based on the changes in the natural log of OD, not on the change of OD itself. The rate must be expressed in reciprocal time (per minute or per hour), NOT in OD per minute or OD per hour. Preferably, it should also report the doubling time, but that is not absolutely necessary. It should report the correlation coefficient of the fit to the time points. It should also report some statistical measure, such as the standard error, of the confidence in each growth rate reported.
- The software should automatically detect the maximum growth rate and report the time points used to determine that rate based on at least 5 time points.
- The software should require no programming skills or experience with programming languages such as Python, Perl, R or MATLAB. While some lab technicians may have these skills, it should not be required for technicians performing this type of work.
- The software should not require manual reformatting of the plate reader output. Ideally, formatting should be able to be applied retroactively to plate reader output and should be able to be applied to output from plate readers other than those for which it was originally designed.
One criterion we should not have to mention is the accuracy of program documentation. See my previous article, Don’t Make This Common Mistake When Calculating Growth Rates for a look at where some growth rates program documentation goes wrong.
One Program—Many Plate Readers
The GrowthRates program from www.bellinghamresearch.com meets all five of the above criteria. Unlike the limited tools from a singular plate reader manufacturer, it supports 16 different microtiter plate readers, can be licensed at reasonable rates and requires no programming skills to interpret the data. Visit the website to learn more about running growth rate experiments in your lab.
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