Measuring Growth by Biomass - Online Tutor, Practice Problems & Exam Prep

Microbial growth can be measured by biomass using a spectrophotometer, which assesses light absorbance values (A) to determine turbidity, indicating cell concentration. High turbidity correlates with high biomass, resulting in lower light detection and higher absorbance values. Alternatively, total weight measurement involves centrifuging a liquid culture to form a cell pellet, allowing for mass determination. The greater the weight of the pellet, the more microbial growth is present. Both methods provide essential insights into microbial populations and their dynamics.

Measuring Growth by Biomass

In this video, we're going to begin our lesson on measuring microbial growth by biomass. Scientists can actually measure microbial growth by measuring its biomass, which is really just the mass of an organism. The biomass can be indirectly determined by using an instrument known as a spectrophotometer. A spectrophotometer acquires light absorbance values abbreviated with the capital letter A. These light absorbance values can be used to measure the turbidity of a liquid solution. Turbidity refers to the cloudiness of a liquid sample due to microbial growth.

It's important to note the relationship that high turbidity in the liquid culture is correlated with a high biomass of the organisms growing in it. More organisms lead to higher biomass and higher turbidity. However, high turbidity and high biomass correspond with a lower amount of detected light. High turbidity means high absorbance values.

There is a limitation to using spectrophotometers: the liquid culture must have a relatively high concentration of cells in order for it to be cloudy enough for the microbial biomass to be detected. If we take a look at the spectrophotometer, it is an instrument with a light source that shines a light at a liquid culture specimen. As the light shines through, some of it is scattered, absorbed by the specimen, and some is transmitted directly through. The light that is transmitted can be detected by a detector and turned into an absorbance value, between 0 and 1.

For example, an absorbance value of 0.08 indicates a relatively low biomass, as seen in the less turbid test tube. Conversely, a higher absorbance value of 0.41, like in the more turbid and cloudy test tube with lots of microbial growth, indicates higher biomass and more microbial growth, leading to higher turbidity.

Ultimately, absorbance values from a spectrophotometer can be used to determine the biomass, and higher absorbance corresponds with more biomass and microbial growth. This concludes our brief lesson on measuring microbial growth by biomass using a spectrophotometer. We'll be able to get some more practice on these concepts as we move forward. I'll see you all in our next video.

Measuring Total Weight

In this video, we're going to talk about yet another way to measure microbial growth, and that is through measuring the total weight. And so once again, another method to measure microbial growth is to use the total weight of the sample. And so in order to measure the weight, the cells in a liquid culture can be centrifuged. A centrifuge is really just an instrument that looks like this one down below that is going to spin the samples incredibly fast to create centrifugal forces that will pull the cells towards the bottom of the container that they are in and help to form what is known as a cell pellet or just a pellet at the very bottom of the tube that is being spun. Now the liquid media that remains above the cell pellet is going to be removed, and the cell pellet or the pelleted cells can then be weighed. Now the liquid that is removed here after centrifugation is called the supernatant.

And so if we take a look at our image down below, we can get a better understanding of measuring microbial growth by weight. And so notice over here on the far left-hand side we have a tube that contains a liquid culture with microbial growth. And so we need to obtain this liquid cell culture, and then take the container and place the container into an instrument known as a centrifuge, which is once again going to be an instrument that spins the sample incredibly fast to create centrifugal forces and to pull all of the cells towards the bottom of the tube to create a pellet. And so ultimately, what we are doing is we are spinning the cells by centrifugation, and this helps to create the cell pellet at the very bottom of the tube. And so once again, all of these cells that are free floating in the liquid culture in the beginning get pulled towards the bottom to create a cell pellet at the very bottom of the tube. And that's exactly what we see over here is the cell pellet.

The liquid that remains above the cell pellet is referred to as the supernatant, and it is carefully removed or decanted or poured out very carefully in order not to disrupt the cell pellet at the bottom of the tube. And then, of course, the cell pellet is going to remain inside the tube all by itself, and at that point, we can actually weigh the cell pellet to get its mass. And, of course, prior to weighing the cell pellet here, you would have had to have weighed the tube, prior, the empty tube previously so that you can subtract the weight of the pellet and the tube from the weight of the empty tube in order to get the weight of just the pellet. But ultimately, this process allows for obtaining the weight of the pellet, and the greater the weight, the more microbial growth there is. And so by weighing cell pellets, we're able to measure microbial growth.

And so this here concludes our brief lesson on measuring the total weight to measure microbial growth. And we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.

A scientist wants to determine the biomass of the bacterial populations in his experiment using a spectrophotometer. He finds that population A has a higher turbidity than population B. What does this tell us about the bacterial populations?

• A. More light was able to pass through population B than population A.
• B. The liquid population A is being grown in is more cloudy than the liquid population B is being grown in.
• C. Population A has more biomass than population B.
• D. All of the above.

If a microbiologist wishes to determine the growth of her bacterial sample without removing the bacteria from the liquid they are growing in, which method should she use?

• A. Spinning her population in a centrifuge and weighing the bacterial cell pellet.
• B. Using a spectrophotometer to determine the biomass of the population in the liquid.
• C. Passing the population through a membrane filter and counting the individual cells.
• D. None of the above, all methods require the scientist to remove the bacteria from the liquid solution.

Here’s what students ask on this topic:

What is the role of a spectrophotometer in measuring microbial growth by biomass?

A spectrophotometer measures microbial growth by assessing light absorbance values (A) to determine the turbidity of a liquid culture. Turbidity refers to the cloudiness caused by microbial growth. High turbidity indicates high biomass, as more organisms scatter and absorb more light, resulting in higher absorbance values. The spectrophotometer shines light through the sample, and the amount of light that passes through is detected and converted into an absorbance value. This value helps determine the concentration of cells in the culture, providing an indirect measure of microbial growth.

How does turbidity relate to microbial biomass in a liquid culture?

Turbidity is directly related to microbial biomass in a liquid culture. Turbidity refers to the cloudiness of the culture, which increases as the number of microorganisms grows. High turbidity indicates a high concentration of cells, leading to higher biomass. This is because more cells scatter and absorb light, reducing the amount of light detected by a spectrophotometer. Consequently, higher turbidity corresponds to higher absorbance values, which are used to estimate the biomass of the microbial culture.

What are the limitations of using a spectrophotometer to measure microbial growth?

One limitation of using a spectrophotometer to measure microbial growth is that the liquid culture must have a relatively high concentration of cells to be sufficiently turbid for accurate measurement. Low cell concentrations may not produce enough turbidity to be detected, leading to inaccurate biomass estimations. Additionally, the spectrophotometer only provides an indirect measure of biomass based on light absorbance, which may not account for variations in cell size or shape. Therefore, it is essential to use complementary methods for a comprehensive assessment of microbial growth.

How is microbial growth measured by total weight using centrifugation?

Microbial growth can be measured by total weight using centrifugation. In this method, a liquid culture is centrifuged to create a cell pellet at the bottom of the tube. The supernatant, or liquid above the pellet, is carefully removed. The cell pellet is then weighed to determine its mass. The greater the weight of the pellet, the more microbial growth is present. This method provides a direct measure of biomass by quantifying the total weight of the cells in the culture.

What is the supernatant in the context of measuring microbial growth by weight?

The supernatant is the liquid portion that remains above the cell pellet after centrifugation of a liquid culture. During centrifugation, cells are pulled to the bottom of the tube, forming a pellet. The supernatant is carefully removed to isolate the cell pellet, which is then weighed to determine microbial growth. The supernatant typically contains the liquid media and any dissolved substances that were not pelleted by the centrifugation process.