Plating Methods - Online Tutor, Practice Problems & Exam Prep

The streak plate method is a fundamental technique for isolating specific microbial colonies from a mixed culture. It involves sterilizing an inoculating loop, dipping it into a culture, and streaking it across a petri dish in a series of steps to dilute the microbes. In contrast, the spread plate method distributes a diluted liquid culture evenly across an agar plate using a sterile glass rod, while the pour plate method mixes the culture with liquid agar before solidification. Both methods are essential for transferring microbes but do not isolate specific species like the streak plate method does.

Streak-Plate Method

In this video, we're going to begin our lesson on plating methods, starting specifically with the streak plate method. The streak plate method is actually the most common and the simplest way of isolating microbes in a laboratory. By isolating, we mean that if we have a mixture of a bunch of different types of microbes, we can actually isolate one single type of microbe by performing the streak plate method. In the streak plate method, a mixed culture of cells is transferred to a petri dish and is then streaked across the plate using what's known as an inoculator.

An inoculator can be defined as either a metal or glass loop that is used to isolate and transfer colonies of bacteria between growth media. The method of the streak plate method is outlined in a series of 8 steps that we have numbered down below in our image. This image focuses on the streak plate method. Let's say over here on the far left in step number 1, we are going to take our inoculator, which is a metal loop, and we are going to sterilize our inoculator or our loop with a direct flame from a Bunsen burner as you can see here. This flame is heating up our metal rod to help sterilize it and make sure that there's nothing alive on that metal loop.

Once the loop has been sterilized, and once the loop has cooled down, you can take your loop and dip it into the culture. The glass or the metal loop is dipped into a culture media, which contains a bunch of microbes. This media will have a mixture of different types of microbes. You dip the inoculator or this loop into this media, and you get some of those microbes on the tip of the loop. In step number 3, you are going to streak the first area of the plate. Notice here we are streaking area number 1 and basically spreading those microbes in just one specific area on the plate. This is going to be a mixture of a bunch of microbes.

Then, in step number 4, we are going to sterilize the loop once again. We take our inoculating loop and sterilize it by using the Bunsen burner flame. After the loop has cooled down, we will streak area number 2 on the plate. So basically, what you do is pull a little bit of microbes from area number 1, and you use that to streak area number 2. Area number 2 will have fewer microbes than area number 1. Then, in step 6, you sterilize the loop once again. In step 7, you streak area number 3. Here, you pull a little of microbes from area number 2, and you spread them in area number 3. Area number 3 is going to have even fewer microbes than area number 2. Area number 1 will have the most amount of microbes, area number 2 will have the second most, and area number 3 will have the least amount of microbes.

Ultimately, what you end up getting is your final streak plate. Your final streak plate here is going to have area number 1 which has the most amount of microbes, area number 2 over here which is going to have the second most amount of microbes, and then area number 3 notice has the least amount of microbes. These microbes are in so little amount that they actually form individual colonies. These individual colonies can be selected and picked in order to isolate them from the remainder of the group.

This is really the basics of the streak plate method, and it is a common method that is used in many microbiology labs. If you take a microbiology lab, you're very likely to use this streak plate method. This here concludes our introduction to plating methods and the streak plate method specifically. As we move forward in our course, we'll be able to apply these concepts and learn about other plating methods as well. So, I'll see you in our next video.

The streak-plate method is useful for:

• A. Quantify the number of cells in a culture.
• B. Measuring the metabolism of a cell.
• C. Isolating a specific species of bacteria.
• D. Determine cell shape of bacteria.

In the streak-plate method, what is done with the inoculating loop after each streak?

• A. It is discarded and another one is used.
• B. It is re-introduced to the culture stock.
• C. It is disinfected with an antiseptic.
• D. It is flamed until the loop is red with heat.

Spread-Plate Method vs. Pour-Plate Method

In this video, we're going to continue to talk about plating methods by talking specifically about the spread plate method and the pour plate method. When transferring a liquid culture to a solid agar plate, the liquid cultures can be added to that solid agar plate in one of 2 methods that we have numbered down below, 12. The first is the spread plate method, where 0.1 to 0.2 milliliters of a diluted liquid culture is added onto the solid agar plate, and then it is spread with a sterile glass rod. If we take a look at our image down below at the spread plate method, notice that in this first tube what we have is a liquid culture that will have microbes growing in a liquid broth. We can take some of this liquid culture and the microbes growing in the liquid broth, and transfer about 0.1 to 0.2 milliliters over to a solid agar media. We are inoculating the culture onto the agar media, this is the inoculum right here. Then we can spread the inoculum with a glass rod. Here we have the glass rod which can be used to spread the inoculum across the entire plate. If the liquid culture is diluted enough, then eventually you will get colonies growing on the solid agar media. Ultimately, we've taken microbes growing in a liquid broth and transferred those microbes over to a solid agar plate through the spread plate method.

The other method is the pour plate method. You can take anywhere between 0.1 and 1 milliliters of diluted liquid culture, and pour it onto an agar plate before the pre-warmed liquid agar is poured onto the plate. The warmed liquid agar media that's added afterwards will solidify but only after it is mixed with the liquid culture sample. If we take a look at our image down below we can get a better understanding of this pour plate method, which is in the bottom half of the image here. Once again, we're starting with a test tube here that is containing a liquid broth with microbes growing. We can take anywhere between 0.1 and 1 ml of this diluted liquid broth and pour it onto an empty plate. Notice that this is an empty petri dish and there is no agar added yet. Notice that the microbes are added first. After the inoculum is added, you can pour the pre-warmed media over the inoculum. This is the pre-warmed liquid agar media, and it is being poured on top of the inoculum that contains the microbes. Instead of spreading using a glass rod, you pick up the plate and physically swirl it around. You mix by swirling. Over time, if the correct dilution of the initial liquid culture is achieved, you will get colonies growing on a solid plate. Once again, we've taken microbes growing in a liquid broth and transferred them onto a solid agar plate through the pour plate method. These methods, spread plate and pour plate, allow for transferring microbes from liquid broths over to solid agar plates.

One thing to note about both of these methods is that, unlike the streak plate method that we covered in our previous lesson videos, these two methods, the spread plate method and the pour plate method, cannot be used to isolate specific species of bacteria from a mixed culture. That is a unique feature of the streak plate method. The spread plate and pour plate methods are not capable of doing that. This concludes our introduction to the spread plate method and the pour plate method, and we'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.

The __________ method is used to isolate microbial colonies while the __________ method is used to count the number of viable microbial cells or colonies.

• A. Spread-plate method; Pour-plate method.
• B. Streak-plate method; Spread-plate method.
• C. Pour-plate method; Steak-plate method.
• D. Pour-plate method; Spread-plate method.

Which plating method can be used to isolate a species of bacteria from a mixed culture of cells?

• A. Pour-plate method.
• B. Streak-plate method.
• C. Spread-plate method.
• D. All methods can be used.

Here’s what students ask on this topic:

What is the streak plate method and how is it used to isolate microbes?

The streak plate method is a fundamental microbiological technique used to isolate specific microbial colonies from a mixed culture. It involves sterilizing an inoculating loop, dipping it into a culture containing various microbes, and streaking it across a petri dish in a series of steps. The process dilutes the microbes progressively, resulting in isolated colonies in the final section of the plate. This method is essential for obtaining pure cultures of a single type of microbe from a mixture, which is crucial for further study and experimentation.

What are the main differences between the spread plate method and the pour plate method?

The spread plate method involves adding 0.1 to 0.2 milliliters of a diluted liquid culture onto a solid agar plate and spreading it evenly using a sterile glass rod. In contrast, the pour plate method involves adding 0.1 to 1 milliliter of a diluted liquid culture onto an empty petri dish, followed by pouring pre-warmed liquid agar over the culture. The plate is then swirled to mix the culture with the agar before it solidifies. While both methods are used to transfer microbes from liquid to solid media, they do not isolate specific species like the streak plate method.

Why is the streak plate method preferred for isolating specific species of bacteria?

The streak plate method is preferred for isolating specific species of bacteria because it progressively dilutes the microbial mixture across the plate, resulting in isolated colonies. By sterilizing the inoculating loop between streaks and spreading fewer microbes in each subsequent section, individual colonies can be obtained. These isolated colonies represent pure cultures of a single type of microbe, which is essential for accurate identification and further study. In contrast, the spread plate and pour plate methods do not provide the same level of isolation.

How do you perform the spread plate method in microbiology?

To perform the spread plate method, start by adding 0.1 to 0.2 milliliters of a diluted liquid culture onto the surface of a solid agar plate. Next, use a sterile glass rod to spread the inoculum evenly across the entire surface of the agar. Ensure the rod is sterilized before use to prevent contamination. After spreading, incubate the plate to allow colonies to grow. This method is useful for quantifying microbial populations and transferring microbes from liquid to solid media, but it does not isolate specific species like the streak plate method.

What are the steps involved in the pour plate method?

The pour plate method involves several steps: (1) Add 0.1 to 1 milliliter of a diluted liquid culture onto an empty petri dish. (2) Pour pre-warmed liquid agar over the inoculum. (3) Swirl the plate gently to mix the culture with the agar before it solidifies. (4) Allow the agar to solidify, then incubate the plate to let colonies grow. This method is useful for transferring microbes from liquid to solid media and for quantifying microbial populations, but it does not isolate specific species like the streak plate method.