Video: Effectiveness of disinfectants

﻿Hi, everyone, welcome back to cell biology with me Miss Wong. Today we are going to talk about how to test for the effectiveness of different disinfectants and we also want to elaborate that method into testing for the effectiveness of antibiotics. In this lesson, you will need a pen, paper or your book to write on and a calculator to complete the tasks. Please switch off your phones and close all other browser tabs. Pause the video if you need some time to get ready. Resume the video when you are. Let's have a look at the agenda for today. First of all we will talk about working scientifically as a process and then we will talk about how to test for the effectiveness of different types of disinfectants and then we will talk about antibiotic resistance and bacteria. Lastly, we'll look at some exam questions. Let's go through the process of working scientifically again. We've talked about this when we talked about the osmosis required practical. First of all, you will ask a research question, is a big question. And then you convert it into a testable statement, which is called the hypothesis. It is a prediction. It can be correct, it can be wrong. You need to test it out. Then with the thing that you want to test in mind, then you want to design the experimental setup, you want to think about, what do you want to change in this experiment? You want to think about, what to measure as the effect. And then you want to think about as well what you would like to control so that you can have fair comparisons. After the design has been done, you can start the test and collect some data. And after collecting the data, you will be able to respond to the research question and hypothesis. So you will want to go back to the statement, the prediction that you've made and see if your prediction is correct or wrong. And then you want to answer the research question. Just to remind you, what you would like to change in a practical is the independent variable. What you would like to measure is the dependent variable. And what you would like to control is the control variable. Here I would like you to pause the video and rearrange the statements into the correct order. These statements are now in the wrong order, but once they're in the correct order, they should show you the steps of how to carry out a research following the framework we have just discussed. Pause the video now and complete the task. Let's have a look at the answers. First of all, you will ask a research question and then you convert the research question into a testable hypothesis, a prediction that you can test if it's correct or wrong. And then you want to identify the experimental variables, the independent variable, the dependent variable and the control variable. Next, you will collect data. Lastly, you'll respond to the research question. Please mark your answers. I would like you to pause the video if you need more time. Resume the video when you're ready to move on. Before we move on to the next session of the lesson, I would like you to complete this task as well. I would like you to complete these three statements in full in your books. Pause the video now and resume the video when you're ready. The answers are, independent variable is the one you change. The dependent variable is the one you measure. And the control variable is the one you keep the same. A big well done to you if you've got all of them right. Please do remember these definitions. They are very important. You'll be asked to identify the dependent variable, the control variable and the independent variable in the exam. Now that we have gone through the process of working scientifically, now we would like to apply the same framework into this practical. In this practical we will test the effectiveness of different disinfectants. So let me hide my webcam here. Now we will want to talk about how to test the effectiveness of different disinfectants. On the board, there are four containers A, B, C, and D. Each of them contains a different type of disinfectants. Some are more effective than the others. We would like to test which one is the most effective. And we can use what we have learned before in a previous lesson. We can use the agar plate with bacteria to test which one is the most effective. So the idea is you want to put each type of disinfectant onto an agar plate with bacteria on it and see which disinfectant can stop the growth of the bacteria on the plate. To learn about how to test for the effectiveness of different disinfectants, we'll watch this video filmed by Miss Whittick. Let's have a look at this video in full screen, and then we'll come back to discuss further on the method of this practical. Before we press play, we will look at the equipment first. There are four beakers. Each of them contains a different type of disinfected and inside each of them, there are paper discs. They are soaked with the disinfectant of type A, B, C, and D respectively. There is a Petri dish. Inside the Petri dish there is an agar plate. The bacteria will grow into a bacteria colony if you leave it and let it grow under room or even warmer temperatures. So to test for the effectiveness of disinfectant A, B, C, and D, these are the steps that you will follow. First of all, you want to use a marker to mark on the bottom of the agar plate A, B, C, and D. This marks the location in which you will put the paper discs onto. Then you will use a pair of tweezers to pick up the paper discs and place them into the corresponding spots. Every time when you want to switch to a new disinfectant. Make sure you rinse off your tweezers using water. Again, you will want to repeat the steps I mentioned for disinfectant C and disinfectant D. Remember to rinse the tweezers every time. After that, you will need to get some sticky tape. Make sure you tape it tight so no other microorganisms can get in. Then you leave it on the side for a couple of days and then you will come back and check the results. Now we would like to look at how to analyse the results. After incubating the agar plate for 24 to 48 hours, this should be the results that we get. So you should get an agar plate with bacteria growing on it. So the yellow areas are the areas with bacterial growth. And the white areas are what we called the clear zones. The zones are clear because there is no bacteria on it at all. At this point, we can compare the sizes of the clear zones. The bigger the clear zone, the more effective the disinfected is. So let's assume this is A, B, C, and D. Because D has the biggest clear zone, we can conclude that disinfectant D, is the most effective disinfectant. And the second most is B. And number three is A, and for type C, it's not working at all, because there's no clear zone. Of course as scientists and in your exams, we will be asked to calculate the area of the clear zones. To find the area of clear zones, we will apply the equation that calculates the area of a circle. Which is pi R squared. In your exam, you'll be asked to use a ruler to measure the radius on the exam paper. But of course, we have different sizes of our screens, and we've got different resolutions, so we cannot do it here. But please remember you have a ruler and you can use a ruler to measure the radius when you are doing your exam. And then, now we would like to just draw these radii on the board and you can see them now on the left hand side and I'm going to give you the numbers. Now, we would like to calculate the area for disinfectant A. So I'm going to model the first one for you. And I would like you to take off your calculator and be ready, because I would like you to try to calculate the area for B and D. Let's have a look. You will apply the equation pi R squared. And pi is actually on your calculator. You can just key in pi on your calculator when you're doing the calculations. And then you want to do R squared. R squared means radius squared. So for A, the radius is 0.4 centimetres. So what you do is pi times 0.4 squared, you should get 0.50 cm squared. I would like you to correct your answers to three significant figures, so that you're radii of all these three clear zones. Now, we would like to calculate the area for disinfectant A. So I'm going to model the first one for you. And I would like you to take off your calculator and be ready, because I would like you to try to calculate the area for B and D. Let's have a look. You will apply the equation pi R squared. And pi is actually on your calculator. You can just key in pi on a calculator when you're doing the calculations. And then you want to do R squared. R squared means radius squared. So for A, the radius is 0.4 centimetres. So what you do is pi times 0.4 squared, you should get 0.50 cm squared. I would like you to correct your answers to three significant figures so that we can just, we don't end up with a long number. Now, I would like you to try to calculate the area for zone B. So for B, please calculate the area of the clear zone and we will come back and check the answers. Okay, let's see if you have applied these steps. So again, you apply the same equation pi R squared. So you do pi, which is the button on your calculator. And then you times it by R squared, which is one squared, you should get 3.14 cm squared. Please correct again your answer to three significant figures because that will help. If not, you end up with a very long number. What about D? I would like you to do D now on your calculators. Let's have a look if you've got this as the answer. Again, you will apply the same equation, pi R squared. So this is what you do. Pi times 1.4 squared gives you 4.40 centimetres squared, because you're correcting your answer to three significant figures so it's 4.40 cm squared. If you've got it right, really well done. Remember, correct your answers to three significant figures. We don't end up with a long number. Now I would like you to try to calculate the area for zone B. So for B, please calculate the area of the clear zone. And we will come back and check the answers. Okay, let's see if you have applied these steps. So again, you apply the same equation pi R squared. So you do, pi, which is the button on your calculator, and then you times it by R squared, which is one squared. You should get 3.14 cm squared. Please correct again your answers to three significant figures because that will help. If not, you end up with a very long number. What about D? I would like you to do D now on your calculators. Let's have a look if you have got this as the answer. Again, you will apply the same equation, pi R squared. So this is what you do. Pi times 1.4 squared gives you 4.40 centimetres squared, because you're correcting your answer to three significant figures. So it's 4.40 cm squared. If you've got it right, really well done. Remember, correct your answers to three significant figures. I would like you to pause the video now and complete these four questions here. We have discussed the method for this practical. And I would like you to think about the method again and answer these four questions. Pause the video now, complete these questions and resume the video when you're ready. Let's have a look at the answers. So the research question was, which disinfectant was the most effective? And then we want to think about what was the independent variable? The independent variable was the type of disinfectant used. The dependent variable was the sizes of the clear zones because you want to measure how big those clear zones are. And lastly, the control variable was the size of the paper discs. So they should have the same size and they can absorb the same amount of disinfectant. Please mark your answers here and a big well done to you if you have got them correct. Moving on, we will talk about bacteria and antibiotic resistance in bacteria. First of all, we can use the same setup as discussed before to test for the effectiveness of antibiotics. Antibiotics are produced by bacterial cells. Antibiotics attack and destroy bacteria as well. So we can use the same logic and the same method to test which antibiotic can be used against a certain type of bacteria. Let's look at the board here. This is a Petri dish with agar jelly. And you can spread the bacteria on this dish. Again, you will soak some filter paper discs with antibiotic A, antibiotic B, and antibiotic C. You place these discs onto the Petri dish, and then you incubate the dish for 48 hours. Now, we want to think about the temperature of incubation. We want to be sensible. We don't want a temperature too high or too low. A temperature too high, will cause the denaturing of enzymes and if the temperature is too low, then the growth will be so slow that you will need a very long time to see the results. After 24 to 48 hours, you'll be able to see the results like this. We'll use the same logic to analyse the results. The bigger the clear zone, the more effective that antibiotic is against that bacteria. So we can say that antibiotic A is not helpful at all because the bacteria is totally resistant to antibiotic A. There is no clear zone at all. And for B, B is quite effective. You can see quite a clear zone around B and C is the most effective because the clear zone is the biggest. So if you want to treat a disease caused by this bacteria on the dish, you would want to use antibiotics C. So where does antibiotic resistance come from? This resistance is usually from a mutation in the plasmid of the bacterial cell. If you remember, the plasmid is an extra ring of DNA in the cytoplasm of bacterial cells. It is not essential for survival because the very, very important DNA or genetic information required for survival will be here in the cytoplasm we have the free floating DNA. But if there is a mutation or if there is a useful gene in the plasmid, the gene will also be expressed. So if there is a mutation that provide the bacteria with antibiotic resistance, and it happened in a plasmid, then this bacteria will become more resistant to these antibiotics. Also, it depends on which type of antibiotic we're talking about. So, some bacteria can be resistant to several types of antibiotics, but not one or two, and some bacteria will be resistant to just one or two types of antibiotics. So, it really depends. Now, we would like to analyse the results, let's assume that I have got this results here. And the results show you different sizes of the clear zones. So, these clear zones will tell you how effective the antibiotic is, which antibiotic is not effective at all? A, B, C, or D? The answer is A, because there is no clear zone so if you use antibiotic A, you cannot kill the bacteria. What about, which antibiotic is the most effective? What do you think? Is it A, B, C or D and why? The answer is C because it has the biggest clear zone. Well done. Now we will look at some exam questions because we've gone through the most important steps of the practical and how to analyse the results. Let me hide my webcam here so you can have a better look. Let's look at the question together. Probash is ill and is having tests in the hospital. His doctors want to check that the bacteria causing his disease are not resistant to antibiotic. They set up an experiment to test this. This is the method they use. A Petri dish is made that has bacteria growing evenly over the surface. A disc of filter paper is soaked in the antibiotic concerned. The disc is placed on the agar in the centre of the Petri dish. The lid of the dish is fixed on with a piece of tape. The dish is then incubated. I have three questions for you. Number one, what is an antibiotic? What equipment should be used to spread the bacteria across the agar plates? And number three, suggest suitable temperatures for incubation of the agar plate. Explain your choice. I would like you to pause the video now and actively think about what we have learned in previous lesson and in today's lesson to answer these three questions. Let's have a look at the answers. An antibiotic is a chemical that attacks and destroys bacteria. You should use an inoculating loop to spread the bacteria evenly across the agar plate. And you will want to use a temperature of 25 to 30 degrees Celsius. If the temperature is too high, it will denature the enzymes. But if the temperature is too low, the growth of the bacteria will be too slow. Let's have a look at the next slide. Now we continue with the same question. The diagram shows the doctor's results. You can see the Petri dish in the diagram and you can also see the clear zone with no living bacteria. It says use a ruler to measure the diameter of the clear area in millimetres. And use the diameter to calculate the area of the circle where there are no living bacteria. Again, because we cannot do measurements using our rulers at the moment, so I'm going to give you the radius. The radius of the clear zone five millimetres. I would like you to calculate the area of the clear zone. Pause the video, complete the calculation and continue for the answers. Great, let's have a look at the answers. So this is what you need to do. You do pi R squared, you do 3.14 times radius five millimetres squared, and you should get 78.5 millimetres squared. I corrected my answer to three significant figures. Well done if you've got this one correct. Lastly, we would like to look at this table and continue with the question. This table is used to analyse the results of the tests. So remember, the doctor has done a test on the antibiotic resistance of the bacteria of Probash. I would like you to think about what you've got in the previous question. So the answer of the previous question is going to help you in this one. What is the extent of antibiotic resistance of the bacteria found in Probash? Explain your answer. You will need to refer back to the data table and explain how resistant that bacteria is in Probash. Pause the video now and complete the task. Let's have a look at the answers. The area was 78.5 millimetres squared. Therefore it is antibiotic resistant. Because the data says that, if it's less than 133, it is a resistant bacteria. Thank you everyone for joining me today. Today we've talked through the method of how to test for the effectiveness of disinfectants and also you can apply the same practical to test for the effectiveness of different antibiotics. I hope you found this lesson helpful. And now I would like you to proceed to the independent practise and the exit quiz. Thank you again for joining me. I will see you in the next lesson.

Worksheet: Effectiveness of disinfectants

Exit Quiz: Effectiveness of disinfectants

Completed: Effectiveness of disinfectants

This is another practical lesson on comparing the size of clear zones when discs of different disinfectant is applied onto an agar plate with bacteria.

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Effectiveness of disinfectants

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