Scientists use a number of different methods to determine the number of micro-organisms that are present in a given population. This can be accomplished by using the spectrophotometer to measure the optical density of the population, by directly counting the microorganisms using a haemocytometer, or by serial diluting the bacteria and plating the diluted bacteria on media that supports the growth of the micro-organisms. The latter method is somewhat more time consuming, but provides statistically accurate and repeatable results. This method is also the ideal method for enumerating microorganisms in a given population because it only identifies the living organisms in that population.
Microbial counting is useful in the basic sciences and is used determine the number of bacteria present for physiological or biochemical studies. For example, if one knows the number of bacteria present in a culture then one can calculate the amount of protein or DNA that can be isolated from that population. Microbial enumeration is also routinely used in the areas of public health. Food or water microbiologists test food, milk or water for the numbers of microbial pathogens to determine if these products are safe for human consumption.
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We will be using serial dilutions, plating and counting of live bacteria to determine the number of bacteria in a given population. To this end we will make serial dilutions of a solution containing an unknown number of bacteria, plate these bacteria and determine the total number of bacteria in the original solution by counting the number of colony forming units and comparing them to the dilution factor. Each colony forming unit represents a bacterium that was present in the diluted sample. The numbers of colony forming units (CFU's) are divided by the product of the dilution factor and the volume of the plated diluted suspension to determine the number of bacteria per mL that were present in the original solution.
Lab 1. Serial dilutions:
Students should obtain 10 small, sterile test tubes, label the tubes 1 through 10 and then add 4.5 mL of M9 salts to each test tube. M9 salts is a physiological buffered minimal medium that contains inorganic salts but no carbon source. Bacteria will not grow in this media but will remain in a state of stasis until the diluted cells are plated on media containing a carbon source.
The students should pipette 0.5 mL of the original solution into test tube 1. This bacterial suspension should be mixed thoroughly (using the vortexers on each bench) before proceeding to the next step. The students should obtain a clean pipette and withdraw 0.5 mL of the diluted bacterial suspension from the first test tube and pipette that into the second test tube. Continue in this fashion until you have serially diluted the original bacterial suspension into test tube 7. The instructor will show you how to perform this exercise. In test tube 1 you have diluted the bacteria 10 fold, a 1:10 or 1 x 10-1 dilution, in test tube 5 you have diluted the bacteria from the original tube to obtain a 1 x 10-5 dilution, in test tube 10 you have diluted the bacteria from the original tube to obtain a 1 x 10-10 dilution.
Below is the mathematical reasoning for performing the serial dilutions:
Tube 1 contains 4.5 mL of sterile media; you will add 0.5 mL of the undiluted bacterial suspension to yield a total volume of 5.0 mL.
1 x 10 -1
4.5 mL + 0.5 mL
Tube 2 contains 4.5 mL of sterile media; you will add 0.5 mL of the 1:10 diluted bacterial suspension to yield a total volume of 5.0 mL
1 x 10 -2
4.5 mL + 0.5 mL
Lab 1. Plating the serially diluted cells:
The students should obtain 10 TSA plates from the instructor; these plates should be labeled with your initials and the dilution factor. In this case we will plate the following dilutions: 1 x 10-1, 1 x 10-2, 1 x 10-3, 1 x 10-4, 1 x 10-5, 1 x 10-6, 1 x 10-7,
1 x 10-8, 1 x 10-9 and 1 x 10-10.
The students should also obtain a beaker containing a "hockey stick" and pour a small volume of alcohol into the beaker. The hockey stick is used to spread the diluted bacterial suspension evenly over the surface of the plate. The instructor will demonstrate this process.
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For the cell suspension that will be plated onto the TSA plate labeled 1 x 10-1, pipette 0.5 mL of the diluted suspension from the appropriately diluted test tube onto the surface of the plate. Dip the hockey stick into the alcohol solution and flame the stick until the alcohol has burned off. Do not heat the stick to long; you only need to flame the loop to burn off the alcohol-that will be sufficient for sterilizing the hockey stick. After sterilizing the stick, use the hockey stick to spread the bacterial suspension evenly over the entire surface of the plate. Allow the plate to dry. Continue this process with the remainder of the bacterial dilutions.
Tape all of your plates together and incubate your plates, upside down, at 37oC for 24 hours. During the next period you will count the number of colony forming units for each dilution and calculate the number of bacteria in the original suspension!!!!
Lab 2. Counting colony forming units and calculating the amount of bacteria in the original solution.
The instructor will provide a counter and your plates. For each dilution, count the number of colony forming units on your plates. Typically numbers between 30 and 800 are considered to be in the range where one's data is statistically accurate. If the number of CFUs on your plate are greater than 1000, you may record in your table TNTC (too numerous to count). Alternatively, if your numbers are greater than 1000 AND you have evenly distributed the diluted bacterial suspension on the surface of the plate AND you can discern individual colonies; divide the plate into 4 sectors, count the number of bacteria in one sector and multiply by four. If the number of CFUs on your plate is below 10, record the number of CFUs, but do not use this in your calculations. T=Trial
Number of bacterial colonies (CFUs)
Avg # CFU
Avg # bacteria/ml
Calculating the number of bacteria per mL of serially diluted bacteria:
To calculate the number of bacteria per mL of diluted sample one should use the following equation:
Number of CFU
Volume plated (mL) x total dilution used
Number of CFU
For example, if for the 1x 10-8 dilution plate you plated 0.1 mL of the diluted cell suspension and counted 200 bacteria, then the calculation would be: