Microbiology, part 28: Growth - Measurement of Bacterial Growth

Hi. I'm Cathy with Level Up RN. In this video, we are going to be discussing plating methods, including the pour plate method and the spread plate method. At the end of the video, I'm going to give you guys a little quiz to test your understanding of some of the key points I'll be covering, so definitely stay tuned for that. And if you have our Level Up RN, microbiology flashcards, go ahead and pull out your flashcard on plating methods so you can follow along with me. And as always, pay close attention to the bold red text on the back of the cards because those are the things that you are likely to get tested on.

The pour plate method and spread plate method are techniques used in the laboratory to isolate, count, and grow microorganisms. In many cases, the sample will need to be diluted before using either of these methods in order to get a countable number of colonies after incubation. Of note, the steps involved in serial dilution are covered in a separate video in this video playlist.

With the pour plate method, the bacterial sample is mixed with liquid agar, and then the sample is poured onto a sterile petri dish, swirled to mix, and allowed to solidify. The plate is then incubated to allow for colony growth. The pour plate method is suitable for both aerobic bacteria, which are bacteria that require oxygen for growth, as well as anaerobic bacteria, which are bacteria that can survive and grow without oxygen. With this method, colonies may form on or beneath the agar surface. The colonies that form on the agar surface will be aerobic bacteria, whereas colonies that form below the surface will be anaerobic.

It's important to note that since we are mixing bacteria with the agar and pouring it onto a plate, if some of the aerobic bacteria end up underneath the agar, they're not going to be happy because there's no oxygen there. So they're not going to grow. And conversely, if we have obligate anaerobic bacteria that end up on the surface of the agar, they're also not going to be happy there because they're being exposed to oxygen, so they won't grow. So the bottom line is that the growth of these different types of organisms really depends on their location within the agar, which can absolutely skew results.

Moving on to the spread plate method now. With this method, the bacterial sample is pipetted onto solid agar and is spread evenly using a sterile spreader. The plate is then incubated to allow for colony growth. The spread plate method is only suitable for aerobic bacteria and facultative anaerobic bacteria. So as I mentioned before, aerobic bacteria are bacteria that require oxygen for growth. Facultative anaerobic bacteria are bacteria that grow best with oxygen but can survive without it as well. So with the spread plate method, colonies will form on the agar surface because that is where we spread the sample. We didn't mix it up with agar and pour it onto a plate like we did with the pour plate method.

All right. It's quiz time, and I have three questions for you. Question number one, with the blank plate method, colonies may form on and beneath the agar surface. The answer is, the pour plate method. Question number two, which plating method or methods would be appropriate for growing obligate anaerobic bacteria? The answer is, the pour plate method only. And question number three, which plating method or methods would be appropriate for growing aerobic bacteria? The answer is, both the pour plate and spread plate method.

All right. That's it for this video. I hope it was helpful. Thank you so much for watching, and good luck with studying.

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