Serial dilution is a technique used in microbiology to obtain manageable concentrations of a desired organism. It is a step-wise series of dilutions, where the dilution factor stays the same for each step. The purpose of serial dilution is to estimate the concentration of a sample, or to obtain the desired concentration of a reagent, chemical, or compound. The process involves the systematic reduction of a known or unknown entity (a solute, organism, etc.) through successive re-suspension of an initial solution into fixed volumes of a liquid diluent. These blanks usually consist of 0.45% saline, though the composition can be varied. While an experimenter can choose any volume for each diluent, it is most often a multiple of 10, facilitating logarithmic reduction of the sample. Serial dilution is complemented by petri dish streaking and spreading, just two of many plating techniques used by microbiologists. The following steps are involved in performing a serial dilution:
- Prepare several dilution blanks, which are tubes that contain a certain volume of an appropriate solution.
- Transfer a known volume of the sample to the first dilution blank and mix well.
- Transfer a known volume of the first dilution to the second dilution blank and mix well.
- Repeat step 3 for each subsequent dilution blank until the desired dilution is achieved.
- Plate each dilution onto an appropriate agar medium and incubate.
- Count the number of colonies on each plate and calculate the concentration of the original sample.
Serial dilution is used to reduce a dense culture of cells to a usable concentration level that allows for the quantification of cell populations that are easier for both practical and research efforts. It is a common practice to determine microbial counts for both liquid and solid specimens, such as suspensions of E. coli in nutrient broth all the way to soil samples and hamburger meat.
