Measuring Growth by Plate Counts: Videos & Practice Problems

Measuring growth through plate counts is a crucial technique in microbiology, focusing on viable cells—those capable of multiplying on growth media. Unlike direct cell counting, which includes both living and dead cells, plate counts specifically target living cells, providing a more accurate representation of microbial populations when viability is the primary concern.

To perform plate counts, scientists utilize solid growth media contained in petri dishes or plates. These environments allow for the growth and subsequent counting of viable cells. The process involves inoculating the media with a sample and incubating it, leading to the formation of colonies that can be counted. Each colony typically originates from a single viable cell, making it possible to estimate the number of living cells in the original sample.

Understanding the significance of viable cell counts is essential for various applications, including assessing microbial contamination, evaluating the effectiveness of antimicrobial agents, and studying microbial ecology. As we delve deeper into the methods and processes for conducting plate counts, we will gain practical insights into this fundamental microbiological technique.

Plate counts are a crucial method for determining the number of viable or living cells in a culture. This process involves plating cells onto a petri dish containing solid growth media, allowing for the growth of colonies that can be counted. Unlike direct cell counting, which may include nonviable cells, plate counts focus solely on living cells, providing a more accurate representation of cell viability.

One key assumption in plate counting is that each colony formed on the petri dish originates from a single viable cell. This assumption is essential for accurately determining the number of colony-forming units (CFUs), which represent the viable cells capable of multiplying into colonies. By counting the number of colonies, scientists can infer the number of CFUs, thus estimating the number of viable cells initially plated.

The process begins with growing cells in a liquid culture, which must be appropriately diluted using serial dilutions. Serial dilutions involve a series of back-to-back dilutions to achieve a countable number of CFUs. Ideally, the target range for CFUs on a plate is between 30 and 300, as this range allows for reliable counting. If the number of CFUs is too high, it becomes difficult to count accurately, while too few CFUs may not provide reliable data.

For example, a common dilution method is a 1:10 dilution, where 1 milliliter of the original culture is mixed with 9 milliliters of broth. This process can be repeated to achieve further dilutions, such as 1:100 or 1:1,000. After plating these dilutions, scientists can count the colonies formed and use this data to calculate the number of viable cells in the original culture. If a dilution results in too few colonies, it may not yield reliable results, emphasizing the importance of achieving an appropriate dilution for accurate counting.

In summary, plate counts are a vital technique for assessing cell viability, relying on the assumption that each colony arises from a single viable cell. Through careful dilution and counting of CFUs, scientists can effectively estimate the number of living cells in a culture, facilitating further research and applications in microbiology.