Serial Dilution in Microbiology: Calculation, Method & Technique

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  • 0:05 Huge Micro Problem
  • 1:11 Serial Dilution
  • 2:31 How to Perform a…
  • 6:45 Lesson Summary
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Lesson Transcript
Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Expert Contributor
Brenda Grewe

Brenda has 25 years of experience teaching college level introductory biology and genetics. She earned her PhD in Genetics from Indiana University.

Working with billions of tiny cells can pose a problem when you need to count the total number of cells in a sample. Fortunately, through precise serial dilution of a sample, it is possible to get down to a number that is much easier to work with.

Huge Micro Problem

As you know, bacteria are everywhere, invisible to the naked eye, yet influencing every environment on Earth. What happens when you need to know how many individual bacterial cells are contaminating a food, living in an environmental sample, or growing in a culture tube? You need some method for counting the bacteria accurately. But, it is not uncommon for a liquid culture of bacteria to have a billion cells in every milliliter of media. Think about that for one second. In your kitchen, you probably have a teaspoon. Every teaspoon has about 5 milliliters. That means that every teaspoon of liquid could potentially have 5 billion bacteria in it. Even if you counted one bacteria every second, it would take you over 150 years to get to 5 billion! Obviously, this is not a viable option. So, what can you do? You need fewer bacteria to count. Ideally, you want to only have to count between 30 and 300 bacteria, a range of numbers that takes only at most a few minutes to count. But, how do we get there?

Serial Dilution

The answer is through dilution. If you simply pull out a smaller, exact quantity of culture liquid, you could count those bacteria and, based on how much you pulled out of the total, you can determine how many bacteria are in your original sample. Sounds easy, right?

But first, one more analogy: you have billions of bacterial cells and need to get down to 30 to 300. In order to do that, you would have to dilute your sample about 10 million-fold. To do this, you would need to take about 15 milliliters of your sample, about 3 teaspoons, and dilute it into your swimming pool! I doubt this is a viable option, especially if you're working in a cramped lab space. So instead, let's not dilute just once. We can dilute once, then dilute this dilution, only to dilute this dilution, and so on until we get to the appropriate concentration of cells. This is called a serial dilution.

A serial dilution is a series of sequential dilutions used to reduce a dense culture of cells to a more usable concentration. Each dilution will reduce the concentration of bacteria by a specific amount. So, by calculating the total dilution over the entire series, it is possible to know how many bacteria you started with. The best way to fully grasp serial dilutions is to try out the procedure yourself.

How to Perform a Serial Dilution

I'm going to walk you through an example serial dilution using the easiest method, but, once you grasp the concept, you can change the actual numbers to whatever works best for you and do it the same way.

To start, we need 10 milliliters (10 ml) of your original bacterial culture (labeled OBC). Before we start diluting, we need to prepare several dilution blanks, which are tubes containing your diluting liquid in exact quantities. Your liquid could be growth media, saline, sterile water, or any other appropriate liquid. For this example, we need 5 dilution blanks, numbered 1-5. In each tube, we need exactly 9 ml of liquid media. The reason we need 9 ml will become apparent soon.

The tubes should be lined up like this:

How to line up tubes for serial dilution example
tubes lined up for serial dilution

The first step is to gently shake or swirl the tube. This will ensure that your cells are evenly distributed in the tube. If your cells settle to the bottom, and you remove liquid without swirling, you run the risk of not getting enough cells, invalidating your final count. Remember to always swirl the tube before removing liquid.

Once swirled, carefully transfer exactly 1 ml from your OBC Tube to Tube 1. Now, you should have 10 ml of liquid in Tube 1. Exactly one-tenth of your cells are now in a new tube with a final volume of 10 ml. You just performed a 1 in 10 dilution, or it could be written 1/10. 1 is the volume you transferred, and 10 is the final volume of the tube after the transfer.

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Additional Activities

Design a Serial Dilution

Your microbiology lab instructor taught you the basics of serial dilution and how to use it to determine the number of cells in an original bacterial culture. During the next laboratory session, you will determine the average number of cells in a single colony of baker's yeast.


  • Obtain a Petri plate of yeast colonies and pick an average-sized colony.
  • Remove the entire colony from the Petri plate and transfer it into a tube containing 1 ml of dilution medium. This will be your original culture (OC) for preparing three separate dilution series.
  • Assume that the average yeast colony has between 10^5 and 10^7 cells.
  • Design three separate serial dilutions with different assumptions about the starting number of cells. From that assumed starting number, design your dilution so that you spread 0.1 ml containing ~100 cells on a Petri plate to get ~100 colonies growing on the plate.
  • For each dilution series that you design, spread three plates. In the end, you will have 9 plates to incubate and count; three plates for each of three different serial dilutions.
  • After two days, terminate colony growth by moving the Petri plates to the refrigerator.
  • At the next lab, count colonies on plates that have 30-300 colonies.
  • Using the colony counts and the total dilution, which includes a 1/10 plating factor because only 0.1 ml was spread on the plate, determine the average number of cells in the original yeast colony.

Materials for Dilutions

  • sterile 1.5 ml tubes, pipet tips and micropipettors
  • sterile dilution medium
  • sterile Petri plates with growth medium

Example Dilution Scheme

A student designs a dilution with the assumption that an average yeast colony consists of 10^6 cells. She makes three 1/10 dilutions. She plates 0.1 ml of the final dilution on a Petri plate, and repeats this twice to obtain three plates total. She counts the colonies that grew on each plate.

  • 1: 131
  • 2: 138
  • 3: 122
  • Average: 130

The student determines that the number of cells in the colony she picked was 130 * 10^3 * 10 = 1.3 * 10^6 where 10^3 is the total dilution factor and 10 is the plating factor.

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