Sterilization, one microwave at a time

[PandaE777]

Our project is to test the effects of the household microwaves, and see if the microwave could be used to reduce or sterilize the amount of bacteria on food. We will test this by seeing if the microwave can kill harmful bacteria by taking foods that normally have bacteria, and seeing if the food have less bacteria after certain increments of microwaving. We will see if food have less live bacteria after microwaving by growing bacteria cultures before and after we microwave the food. Keep in mind, we have a petri dishes, a microwave and a place to incubate the bacteria. We were wondering what is the best method to transfer bacteria from the food to petri dishes without carrying food along with the bacteria? In your opinion, what are the best foods to test for this project? Do you see any flaws in this project that can be corrected?

[deleted-143835]

Hi!

It looks like your independent/dependent variables and experiment plan is all set, so great job on that! Since bacteria literally exists everywhere, I don't think there is a type of food that would inherently harbor "more" bacteria than another. That being said, you may want to test not only solid foods (since they may become a bit difficult to control precisely), and test for bacterial contamination in liquids, such as water, as well. Swabbing the bacteria from the food onto the petri dish is probably the best idea I can think of - if any other experts know of another way that would be great as well - as the bacteria can be swabbed without carrying food particles over. Because any project with living things has a little bit of inbuilt inconsistency, my best advice would be to control all your factors as much as possible to ensure maximum accuracy in your results.

Good luck! Please post back with any more questions.

[deleted-132180]

Hello there,

I agree with the previous poster that the easiest types of food to test would probably be liquids (i.e. water, soda, soup, milk, coffee... etc.) if you do not what to carry the food along with the bacteria as you transfer to over to the petri dishes. For more solid foods, you can probably trying using something like a blender to homogenize the food as much as possible (hopefully without destroying any of the bacteria that are still alive in them) before plating it. Instead of just swabbing a sample of the food or liquid and plating that, you may want to plate a consistent volume for all of your samples just so they can be comparable. For example, if you plate a higher volume from your pre-sterilization food sample than your post-sterilization food sample and you see more bacteria on your pre-sterilization plate, how can you actually tell that it's because you plated more bacteria from the pre-sterilization sample or because the microwave actually did kill the bacteria?

One other question that I have is whether you would plate the bacteria on different types of nutrient agar plates? This is because different types of bacteria have different growth requirements. I think the most commonly used nutrient agar plate are Lb plates (are you using these?), and it does seem like a lot of bacteria can grow on them, but it is also possible that when you plate your samples on something like Lb, you may not see any bacteria at all before the microwaving and after the microwaving. This may make it difficult to say whether your food is sterile to begin with, or that there is bacteria in there, but the conditions at which you're growing them are not ideal and hence they cannot grow up. This may probably be beyond the scope of what you are trying to test, and you may just leave out the foods that don't give you bacterial colonies before the sterilization process, but this is something to keep in mind just in case! I would also like to give you a heads up that there is a rare chance that some foods may actually be contaminated with potentially harmful microbes, so just be very careful when you are handling your bacterial samples and wear gloves! Otherwise, I think your experimental ideas look good!

Let us know if you have anymore questions.

Best of luck,
Connie

[PandaE777]

Thank you for all the suggestions, we really appreciate all the help^^. But we have a few questions: 1. How would we transfer the bacteria from liquids to the petri dishes? would we use a cotton swab or something else? 2. Where would you recommend the best place to get agar and is LB the best choice or does it depend on the bacteria? 3. Any food/liquid suggestion to test? Are there specific things about food that will affect the results? ex. mixes of food like pizza vs the ingredients alone.

[deleted-140482]

Hi,

In terms of transferring bacteria from your liquids, there is no need to use a swab. All you would do is take a small, defined volume from your liquid sample and put it on your agar plate. Then you would spread it around evenly, using sterile technique so as not to introduce new bacteria. After that, you simply incubate your plates (usually for around 18-24 hours) and count the number of colonies you get. If you wanted to plate bacteria from solid foods you could swab a defined area (say 1cm^2) and then rub that swab around on the plate. Alternatively, as connief recommended, you could homogenize your solid food in a blender and then plate a defined volume of liquid just like your liquid samples. The blender is extremely unlikely to have any effect on your bacteria, but unless you used it before you microwaved your samples, you would need it to be sterile and cleaned in between each sample.

I'm not sure where high school students can order agar from, so maybe one of the other experts can chip in there. LB is a very good choice for a growth medium for bacteria. While it will not support bacterial growth from EVERY type of bacteria present, it is a fairly general medium, and it is good for growing E. coli, a common food contaminant. S. aureus can also grow on LB. I think you will just have to accept that you cannot find one medium that will grow all types of bacteria and you can mention that as a limitation of your experiment when you report your results. In the end, you are testing the effect of microwaves on bacteria that can be grown on LB.

I would think you might want to test foods that are known to have bacterial contamination as a problem some times. Leafy green vegetables have had many recent outbreaks of E. coli, for example, so they might be interesting to test. Meat is often contaminated. Also, dairy products that have been left out of the fridge for too long are a common source of staph poisoning. I would tend to use ingredients versus a total pizza, because it would be very difficult to get a good sample from an entire pizza. If you use the swabbing method you might not be able to readily swab all areas of your pizza. Plus, you won't know the contribution of the different parts of the pizza to the final bacterial counts. I would want to know whether the cheese, sauce, or pepperoni had the bacteria.

Hope this helps and please let us know if you have any more questions.

[deleted-132180]

Hello there,

1. I agree with JMP that you should take a small, defined volume from your liquid sample and just plate it. If you have access to laboratory equipment, you can use pipettes to isolate a sample of defined volume from the different liquids you are testing. Otherwise, another option would be to use measuring spoons. For example, you can plate 1/4 tsp of your liquids or something--however, if you are using the same measuring spoons over and over, make sure you clean them after plating each sample because you don't want to contaminate one liquid with microbes from another liquid!

2. I also agree that LB is probably the most commonly used medium used to grow microbes and should cover a wide spectrum of microorganisms. Carolina Biological Supply Company, which is a Science Buddies approved supplier, does sell nutrient agar that you can use to pour plates (http://www.carolina.com/prepared-biolog ... ?question=). They even sell plates that have already been poured (http://www.carolina.com/prepared-biolog ... ?question=)! The problem is that they don't mention what type of growth medium this "nutrient agar" actually is, so I'm not quite sure whether it's LB or not. It may be worth it to contact the supplier and ask them what kind of growth medium their "nutrient agar" is, but according to the website, they seem to support the growth of a wide range of microorganisms. Another thing you should think about is how you're going let the microbes grow after you plate them. Are you just going to leave them at room temperature? Many microbes that infect humans tend to grow best at 37 degrees, but if you don't have access to an incubator, letting the plates sit at room temperature is totally fine, since lots of food contaminated with bacteria are at room temperature for the most part anyway. Just note that it will probably take a longer while for the bacteria to show up on your plates compared to growing them at 37 degrees.

3. Meat is often contaminated with many microbes, but if you want to plate bacterial samples from meat, you will probably have to use a blender to homogenize your samples. Like JMP mentioned, remember to clean the blender every time you mash up a new sample just so you don't cross contaminate samples! Leafy greens is another good one. Some fruits have been associated with bacterial outbreaks before as well. Eggs are another good one.

Let us know if you have anymore questions!

Best of luck,
Connie

[PandaE777]

Thank you all for the replies, you have been a big help, we have a question though. What increments of time should we use to microwave? we were thinking no microwave, 1 min, 3 min and 5 min. Are these increments of time good or do we need a more wider range of times to get a better variety of results? Also, we do have access to a incubator and microwave but we do not have access to a lab so thank you connief for the suggestion with the tsp and others about the blender, we will most likely use those methods for our project.

[deleted-140482]

I would tend to go for a broader range of times in your experiment. I haven't done a literature search, and you might consider doing one to see if anyone else has done a similar experiment with certain times, but barring that, I would do something along the lines of no microwave, 5, 10, and 15 minutes. Those are arbitrary numbers and you could easily change that to something else, but I definitely think a broader range of times than the one you suggested would be beneficial. With 1, 3, and 5 minutes, I'm not sure you'll see much difference between the 3 and 5 minute timepoints, for example.

[deleted-143835]

I agree with the previous response that a wider range of times would be helpful. Remember that every good experiment has a control, or standard for comparison, so it's important that you have a time of 0 exposure to microwaves and build from there. If you start out with a few time intervals and see that the results aren't significant, remember an important part of science is revamping and improving your experiment before retesting! It's perfectly OK to increase your data set after one test, as long as you document it and present the results accordingly.

[deleted-132180]

I agree with the previous posters about the wider time intervals and to also include a 0 minute exposure as a control. Another thing to think about is how you're going to describe your observations. For example, are you going to count bacterial colonies to see whether you see a decrease in numbers as you expose them to the microwave for longer time points? Are you also going to take note whether certain bacterial colonies persist while some die off as you microwave them (it's often difficult to tell which strain of bacteria you have just by looking at the colony, but some bacteria definitely grow colonies of different sizes or even different colors)? It's great that you have the experimental setup mostly thought out, but recording your data is also very important and you want to think of the different ways that you can describe your observations, and definitely choose the methods that not only presents your data best but also makes it easier for others who didn't do your experiments to understand.