Microbiology, part 53: Control of Growth - Use-Dilution Test & Disk Diffusion Method

Hi, it's Cathy with Level Up RN. In this video, I will be discussing two key methods for evaluating the effectiveness of an antimicrobial agent. This includes the use-dilution test and the disk diffusion 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 facts I'll be covering. So be sure to stay for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull out your flashcards so you can follow along with me.

The use-dilution test is used to evaluate the effectiveness of a disinfectant on an inanimate surface. During this test, stainless steel cylinders are dipped into a culture containing the microorganism and then allowed to dry. Then each cylinder is dipped into decreasing concentrations of the disinfectant and then transferred to a new test tube containing fresh sterile media. The test tubes are then incubated, and after incubation, they are assessed for the presence of turbidity, which indicates bacterial growth is present. So as you can see in the image, the right two tubes, which contain cylinders that were dipped into lower concentrations of the disinfectant, are showing turbidity. Therefore, lower concentrations of the disinfectant are not sufficient for inhibiting microbial growth.

Next, we have the disk diffusion method, which is also known as the Kirby Bauer test. During this test, we are evaluating the susceptibility of specific microorganisms to chemicals or antimicrobial agents. So in other words, we are testing to see if a specific chemical or antimicrobial agent can inhibit the growth of a specific microbe. During this test, filter paper disks, which are labeled A, B, and C in this image, are soaked in known concentrations of different chemicals or antimicrobial agents. Next, these disks are transferred to an agar plate, which has been inoculated with a known concentration of the bacteria that we're trying to inhibit or kill. The agar plate is then incubated and assessed. If a chemical is effective, then a large clear area will be observed around the disk. This is the zone of inhibition. And the larger the zone, the more effective the agent is. So as you can see in the image, antibiotic C was the most effective at inhibiting this particular bacteria. Antibiotic B was somewhat effective, and antibiotic A was not at all effective.

All right. It's quiz time, and I have three questions for you. Question number 1. The use-dilution test is used to determine the effectiveness of an antimicrobial drug on living tissue. True or false.

The answer is...false. It's used to evaluate the effectiveness of a disinfectant on inanimate surfaces.

Question number 2. With the disk diffusion method, what is the zone of inhibition?

The answer is...it's the clear area around the disk where bacterial growth has been inhibited.

Question number 3. With the disk diffusion method, the larger the zone of inhibition, the more effective the antimicrobial agent is. True or false?

The answer is...true.

All right. That's it for this video. I hope it was helpful. Take care and good luck with studying.

[BLOOPERS]

Or antimicrobial drug can do something.