Measuring Growth by Plate Counts: Study with Video Lessons, Practice Problems & Examples

Measuring growth through plate counts involves counting viable cells on solid growth media, offering an advantage over direct cell counting, which includes nonviable cells. The process assumes each colony arises from a single viable cell, allowing for the determination of colony-forming units (CFUs). Serial dilutions are essential to achieve a countable range of CFUs, typically between 33-100. Accurate counting enables scientists to estimate the number of viable cells in the original culture, facilitating studies in microbiology and cell viability.

Measuring Growth by Plate Counts

In this video, we're going to begin our lesson on measuring growth by plate counts. And so, recall from our previous lesson videos that viable cells are really just living cells that are capable of multiplying on growth media. Now, counting viable cells or living cells can have an advantage over direct cell counting because recall that direct cell counting includes both viable cells and nonviable or dead cells as well. And so it may be the case that the scientist is only interested in the viable cells. And so, in those cases, scientists need to use other methods, other than direct cell counting. And so, plate counts will require the use of solid growth media in a petri dish or in a plate. And, the plate or the petri dish is where the cells are grown and counted on. And once again, these cells, that are counted with these plate counts are going to represent viable or living cells. And so, in our next lesson video, we'll be able to talk more about the method and process for plate counts. But for now, this here concludes our brief introduction to measuring growth by plate counts, and we'll be able to get some practice and learn more as we move forward. So I'll see you all in our next video.

Viable cells have what characteristics?

• A. Viable cells are able to grow and multiply on growth media.
• B. Viable cells are able to multiply and form a colony.
• C. Viable cells are also known as CFU's.
• D. All of the above are characteristics of viable cells.

Measuring Growth by Plate Counts

In this video, we're going to talk more about the process of plate counts. Plate counts refer to the process of counting the number of viable or living cells in a culture after those cells have been plated onto a petri dish with solid growth media. It is important to emphasize that plate counts will only allow for the determination of the number of viable cells, which is an advantage over direct cell counting that includes nonviable or dead cells as well. The process of plate counts has a critical assumption: it assumes that each individual colony of cells that forms on the petri dish or plate is formed from a single individual cell. This assumption is crucial for the plate count process to work. Initially, cells are grown in a liquid culture or broth and then transferred to solid growth media on a petri dish or plate, which is then incubated to determine the number of colony-forming units (CFUs).

A colony forming unit (CFU) represents a single viable cell that is capable of multiplying to form a colony of cells. The number of CFUs represents the number of viable cells initially added to the plate. By counting the colonies, one can determine the number of colony-forming units, and thus the number of viable cells. Scientists must ensure the liquid culture is diluted appropriately using serial dilutions, a set of consecutive dilutions, to achieve a countable range of CFUs. A reasonable counting range is usually between 30 and 100 CFUs.

It is crucial for scientists to know which dilution was used to accurately calculate the number of CFUs and viable cells. For example, starting with an original liquid culture, transferring cells directly to a petri dish usually results in too many CFUs to count them accurately. Serial dilutions are performed to manage this, such as a 1 to 10 dilution—1 milliliter of the original liquid culture is diluted into 9 milliliters of broth. This dilution process is repeated until a countable amount of CFUs, typically between 30 and 100 colonies, is achieved. Once the scientists count the number of viable cells in a particular dilution, they can calculate how many viable cells were present in the original liquid culture. However, if the dilution results in too few colonies, as might occur with a 1 to 1,000 dilution resulting in only three colonies, it is not considered reliable.

This process allows for the determination of the number of viable cells, and this concludes our brief lesson on plate counts. We will practice applying these concepts as we move forward. I'll see you all in our next video.

Why is a culture diluted during plate counts?

• A. Because it would be incredibly tedious to count all of the viable cells in an undiluted sample.
• B. To give the microbiologist an easy number of viable cells to count.
• C. To remove the dead or non-viable cells from the microbial culture.
• D. A and B.
• E. A and C.
• F. B and C.

What must be assumed when using the plate counting method for measuring microbial growth?

• A. A single cell is formed from a colony on each plate.
• B. Each colony that you count on a plate is formed from a group of cells that you must average after.
• C. Each colony that you count on a plate is formed from a single cell.
• D. Each colony forming unit (CFU) you count is formed from multiple cells.

Here’s what students ask on this topic:

What is the advantage of using plate counts over direct cell counting?

Plate counts offer a significant advantage over direct cell counting because they specifically count viable or living cells. Direct cell counting includes both viable and nonviable (dead) cells, which can lead to inaccurate assessments if the goal is to measure only the living cells. By using plate counts, scientists can ensure that each colony counted represents a viable cell capable of multiplying, providing a more accurate measure of cell viability and growth. This method is particularly useful in microbiology for studying cell viability and the effectiveness of antimicrobial treatments.

What is a colony-forming unit (CFU) in the context of plate counts?

A colony-forming unit (CFU) is a term used in microbiology to describe a single viable cell that has the potential to multiply and form a colony on solid growth media. In the context of plate counts, CFUs are used to estimate the number of viable cells in a sample. The assumption is that each colony observed on the plate originates from a single viable cell. By counting the number of colonies, scientists can determine the number of CFUs, which directly correlates to the number of viable cells in the original culture.

Why are serial dilutions necessary in the plate count method?

Serial dilutions are essential in the plate count method to achieve a countable range of colony-forming units (CFUs). The original liquid culture often contains too many cells, making it difficult to count the colonies accurately. By performing serial dilutions, which involve a series of stepwise dilutions, scientists can reduce the cell concentration to a manageable level. This ensures that the number of CFUs on the plate falls within the optimal range of 33-100, making it easier to count and providing more reliable results. Accurate dilutions are crucial for calculating the number of viable cells in the original culture.

How do you calculate the number of viable cells in the original culture using plate counts?

To calculate the number of viable cells in the original culture using plate counts, follow these steps: First, count the number of colonies (CFUs) on a plate that falls within the optimal range of 33-100 CFUs. Next, determine the dilution factor used for that plate. Multiply the number of CFUs by the reciprocal of the dilution factor to estimate the number of viable cells in the original culture. For example, if you counted 50 CFUs on a plate with a 1:1000 dilution, the calculation would be 50 × 1000, resulting in 50,000 viable cells in the original culture.

What is the critical assumption made in the plate count method?

The critical assumption made in the plate count method is that each colony observed on the plate originates from a single viable cell. This assumption is crucial because it allows scientists to equate the number of colonies (CFUs) directly to the number of viable cells in the original sample. If this assumption is not met, the accuracy of the plate count method would be compromised, leading to incorrect estimations of cell viability and growth. Ensuring that each colony forms from a single cell is fundamental to the reliability of this technique.