Isolation of bacteria from soil by serial dilution

Requirement

Test tube, petri plate, alcohol, burner, sample (Soil), etc.

The sample/culture is taken in a test tube and six test tubes, each with 9 ml of sterile diluent, which can either be distilled water or 0.9% saline, are taken.
A sterile pipette is taken.
1 ml of properly mixed sample/culture is drawn into the pipette.
The sample is then added to the first tube to make the total volume of 10 ml. This provides an initial dilution of 10-1.
The dilution is thoroughly mixed by emptying and filling the pipette several times.
The pipette tip is discarded, and a new pipette tip is attached to the pipette.
Now, 1 ml of mixture is taken from the 10-1 dilution and is emptied into the second tube. The second tube now has a total dilution factor of 10-2.
The same process is then repeated for the remaining tube, taking 1 ml from the previous tube and adding it to the next 9 ml diluents.
As six tubes are used, the final dilution for the bacteria/cells will be 10-6 (1 in 1,000,000).

Serial dilution is performed in a number of experimental sciences like biochemistry, pharmacology, physics, and homeopathy.

Serial dilution is used in microbiology to estimate the concentration or number of cells/organisms in a sample to obtain an incubated plate with an easily countable number of colonies.
In biochemistry, serial dilution is used to obtain the desired concentration of reagents and chemicals from a higher concentration.
In pharmaceutical laboratories, serial dilution is performed to receive the necessary concentration of chemicals and compounds as this method is more effective than individual dilutions.
In homeopathy, homeopathic dilutions are used where a substance is diluted in distilled water or alcohol. It is believed than dilution increases the potency of the diluted substance by activating its vital energy.

Even though serial dilution is a useful technique in laboratories, it faces some challenges. Some of which are:

An error might occur during the propagation of the sample, and the transfer inaccuracies lead to less accurate and less precise transfer. This results in the highest dilution to have the most inaccuracies and the least accuracy.
Because serial dilution is performed in a stepwise manner, it requires a more extended period of time which limits the efficiency of the method.
Serial dilution only allows the reduction of bacteria/cells but not the separation of bacteria/cells like in other techniques like flow cytometry.
This technique also requires highly trained microbiologists and experts in aseptic techniques.

A simple example of serial dilution performed in our daily life is tea or coffee. In coffee, we add a certain amount of cold press coffee and add water over it so obtain a desired concentration of coffee.
Another example of serial dilution is the dilution of acids and bases in chemistry to obtain a required concentration.
Serial dilution of culture to determine the number of bacteria in a given sample through a plating technique is also an essential example of serial dilution.

Basic Practical Microbiology-Manual. The society of General Microbiology. Retrieved from https://microbiologyonline.org/file/7926d7789d8a2f7b2075109f68c3175e.pdf
Avishai Ben-David, Charles E. Davidson, Estimation method for serial dilution experiments, Journal of Microbiological Methods, Volume 107, 2014, Pages 214-221, ISSN 0167-7012, https://doi.org/10.1016/j.mimet.2014.08.023.\
Cullen, J. J., & MacIntyre, H. L. (2016). On the use of the serial dilution culture method to enumerate viable phytoplankton in natural communities of plankton subjected to ballast water treatment. Journal of applied phycology, 28(1), 279–298. https://doi.org/10.1007/s10811-015-0601-x
https://www.biomol.com/dateien/Bethyl–Serial-Dilutions.pdf
https://bio.libretexts.org/Bookshelves/Ancillary_Materials/Laboratory_Experiments/Microbiology_Labs/Microbiology_Labs_I/04%3A_Dilution_Worksheet_and_Problems

Serial Dilution Analyser Available online