Bacterial Enumeration: Definition, Methods & Example

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  • 0:04 Bacterial Enumeration
  • 1:09 Viable vs Total Cell Counts
  • 1:39 Methods
  • 5:36 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll learn about bacterial enumeration. We will discuss what this process is and different enumeration methods such as standard plate count, direct microscopic counts, cell mass determination, measurement of cellular activity, and much more.

Bacterial Enumeration

When you have a party, it's important to ensure you have enough chairs and cups for all of your guests. Enumerating, a fancy word for counting, these items is simple. They're large enough to see with your naked eye. However, sometimes things aren't so easily visible. Bacteria are microscopic single-celled organisms that can't be seen without a high-powered microscope.

However, bacteria are incredibly important for human health. Some bacteria live in harmony with our body, helping us carry out biological processes like digestion. Other bacteria are pathogenic and cause disease in humans.

Either way, it's important to keep track of who's around in our environment and in what quantities. For example, scientists need to know how many bacteria are present in certain food and drink items to determine if they're sanitary or not.

One way to do this is through bacterial enumeration, or, well, counting bacteria. However, since bacteria are so small, we can't just look around to do the count. Scientists use a variety of methods to do this, which we'll be learning about here.

Viable vs. Total Cell Counts

Before we dive into the methods, it's important to check in about two types of enumerations that scientists might use. First, sometimes scientists just want to know about the viable cell count. This is the number of cells that are alive at the time of counting. These cells are able to grow, divide, and carry out their functions.

However, other times scientists want to use a total cell count, which counts all cells in a sample, whether they're living or dead. Different methods use viable versus total cell counts, and we'll look at the advantages of each.


Okay, now that all that is out of the way, let's take a closer look at the different enumeration methods that are used by scientists, one at a time.

1. Standard Plate Count

Bacteria are microscopic, so it can be difficult to simply count them by looking at a sample. However, luckily bacteria form colonies when grown on solid agar media. These colonies are collections of cloned bacterial cells, all originating from one single cell. The standard plate count takes advantage of this situation. In the standard plate count, a sample of bacteria is diluted and then plated on agar media. The plates are incubated and then colonies form.

The colonies can be counted with the naked eye and then the total number of bacteria present in the sample can be calculated from the dilution factor. Since this count only takes into consideration bacteria capable of cell division, it produces a viable cell count.

2. Turbidimetric Measurements

The standard plate count, however, can take some time since the bacteria need to grow to form colonies. A faster way to enumerate bacteria is to use turbidimetric measurements. This method of enumeration measures the amount of light that can pass through a suspension of bacteria. The more bacteria, the less light that will pass through. The amount of light that passes through is measured by a spectrophotometer. Since this method only measures the amount of light passing through the sample, it produces a total cell count, rather than a viable cell count.

However, the spectrophotometer only measures how much light passes through the sample. How do we correlate that with the amount of bacteria present? Prior to measuring an unknown sample, known controls must be used to create a standard curve. Using either direct counts or a standard plate count, samples with known numbers of bacteria are used in the spectrophotometer, and a standard curve is created. Then the amount of light that passes through unknown samples can be fitted on the standard curve and the bacteria enumerated.

3. Direct Microscopic Counts

Although bacteria can't be counted visually with the naked eye, you can see them using a microscope. Direct microscopic counts are great for looking at total cell counts. This is because it involves using a known volume of liquid and, thus, being able to count the total number of bacteria in 10-20 microscope fields. Using the average number of cells per field, the total number of cells can be calculated using the total volume of the sample. Although this method allows scientists to directly count the cells present in a sample, it can be time-consuming.

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