Science Buddies: "Ask an Expert"

I am doing an experiment to see want type of cutting board will contain the most bacteria. I will use a swab and then place it on the Petri dish (Tryptic Soy Agar). I have some questions:
1. I need a catchy title does, "Bacteria Back on your Cutting Board" work? If not, any suggestions?
2. When swabbing for bacteria does the swab need to be wet, if so what should I wet it in?
3. Do you know any certified (trusted) websites to get PREPARED tryptic soy agar Petri dishes?

Hi,

Tryptic soy agar plates can be purchased from Carolina Biologicals: https://www.carolina.com/prepared-biolo ... c+soy+agar

The swab should be used from an unopened package and moistened with distilled water from a freshly opened container. Be sure to swab thoroughly and don't touch the tip to anything before you apply it to the agar surface. Label the Petri dishes with the date and description on the bottom of the dish (not on the lid which can get misplaced, and also because you need to take photos through the lid).

How are you making sure that each cutting board is treated in exactly the same way as the others so that the only difference will be what the cutting board is made of? Are they being used normally in a kitchen or are you deliberately applying something to each board?

Also, in order to make your results more accurate I would suggest doing the experiment three times so that you can average the results and get standard deviations. This means that after you do one experiment and swab the boards, you wash them and do the experiment again, then repeat it a third time. You will have three results for each board and your conclusions will be more accurate by comparing the average of three results than with only one.

Be sure to incubate the agar plates in a warm location (22-30 C) in the dark and let them go for at least a couple of days to allow good colony growth. Take photos every day through the lid. DO NOT remove the lid! In fact, it is a good idea to put some tape from the edge of the lid to the bottom so that it cannot come off accidentally. Some of the bacteria that grow under these conditions are human pathogens and you do not want to expose yourself or anyone else to them. When handling bacteria and plates always wear safety glasses and disposable gloves.

When you are done with the experiment, get a pail and fill it with a dilute solution (1:10) of Clorox--13 oz of bleach plus 115 oz of water (3 qts plus 19 oz). Immerse the Petri dishes with lids on in this and let them soak overnight. Wear gloves and eye protection when handling bleach as it is quite irritating to eyes and skin. After the dishes are sanitized drain the bleach and dispose of them in regular trash with the lids still on.

If you have more questions, let me know.

Sybee

Hi, daeng!
I think that the name, "Bacteria Back on your Cutting Board" is a great idea! But I am sorry that I do not know any trusted websites for preparing tryptic soy agar Petri dishes. Maybe try researching it more.
I hope you find your answer,
Tayla

Is distilled water the same as vapor distilled water or is there a difference?

I have never seen distilled water described that way, but distilling is the condensation of vapor, so it has to be OK. Try to get a few small bottles of distilled water because what you need is water that is sterile--that contains no bacteria or fungal spores. Once you open a container of water, unless you do it in a sterile area using aseptic technique, it is no longer sterile.

So, several pint or quart bottles would be better to get than one gallon container. You can, of course, sterilize water by boiling it 15 minutes.

Hope this helps. If you have more questions let me know.

Sybee

Hi, I would like to know if an object is considered sterile if it was just cleaned and washed w/ water and soap?

No, the object is not sterile. Sterile specifically means NO bacteria or viruses or fungi. Washing with soap and water well can greatly reduce the microbe count and can protect you from infection with influenza or bacteria, but it is not a 100% guarantee. There are some bacteria that are even resistant to boiling, though these are relatively uncommon.

In order to sterilize an object it should be put into a 350F oven for an hour: https://en.wikipedia.org/wiki/Dry_heat_sterilization

You can't sterilize liquids in the oven, however. These need to be autoclaved, which is heating under pressure in a controlled chamber: https://en.wikipedia.org/wiki/Autoclave

If you have more questions, please ask away.

Sybee

Can I throw away the used Petri Dishes by putting it in a Ziploc bag and putting it in a trash can?

Hi,
After you have soaked the SEALED dishes in 10% Clorox for several hours or overnight you can drain out the liquid, put them in bags and dispose as regular trash.

Be sure you wear safety glasses and disposable gloves when you do this.

If you have more questions, be sure to check with us first.

Sybee