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.
Measuring Growth by Plate Counts - Online Tutor, Practice Problems & Exam Prep
Measuring Growth by Plate Counts
Video transcript
Viable cells have what characteristics?
Measuring Growth by Plate Counts
Video transcript
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?
What must be assumed when using the plate counting method for measuring microbial growth?