Petri dish use to find bacteria

Joined Dec 7, 2009
Someone answered one of my posts about the dangers of chicken juice and said they put some in a petri dish and it just went wild.

Could you use a petri dish to do random sampling of products in your refrigerator just out of curiosity?

Where do you find petri dishes and are they all alike?

It just sounds kind of interesting to me.

Thanks for tolerating my interests.


Staff member
Joined Oct 7, 2001
You need to remember a couple of things though. First off there are bacteria virtually everywhere. Give them the right conditions and they all will multiply rapidly. Once cultured will you even be able to tell the difference between non harmful bacteria and harmful bacteria? If you cant' then it is just a fruitless excerise as there is guarneteed to be bacteria living in your fridge. It might be a fun experiment but be careful that you don't freak yourself out as you might be surprised, and disgusted, by what you find.
Joined Feb 1, 2007
Kevin, keep in mind, too, that the term "petri dish" is being used as shorthand for the entire process.

A petri dish, of itself, is just a two piece glass container that is sterile and air-proof. As such, nothing will grow in it. You have to add a growing media (agar agar was the usual choice when I was involved (back before rocks were invented) but that may have changed.

Then, to be sure there is no cross-contamination, everything you use must be sterile. That is, if you're testing an apple in your fridge, you need to wear sterile gloves, and the knife used to cut the sample must be sterile, etc.

And then, as Pete says, if you don't know what you're looking at once a culture grows, what's the point?

One other thing, related to that. Many of the microbes in your fridge are perfectly safe as they are. But once you culture them, you are concentrating both the bacterium and any toxins they may produce. You then have a problems: how are you going to safely dispose of that culture?
Joined Mar 21, 2008
We used a premade sample sheet that is an easy form of petri dish, I can't remember the name though. Disposal involved soaking in pure bleach and then double bagging and into the trash. Refrigerators slow bacterial growth but they don't stop it. I have something in the back of the fridge that is starting to move around on its own :lol: time for the gloves and empty the fridge and clean.
Joined Feb 26, 2007
Hmmm that's why I love plastic storage containers - new forms of life evolve if you leave them long enough....And they mutate. You can't recognise what they started life as. A bit like caterpillars and butterflies, but in reverse.
Joined Feb 16, 2010
Don't do it. I'm a trained researcher and in my lab, I wouldn't dream of swabbing something as dirty as a meat -- or heck, an air-conditioning duct -- and culturing it. Here's why:

Most microorganisms are harmless. The ones that aren't are pretty rare... which is why we're alive. But, if you put one of those nasty bugs on a petri dish then you suddenly find yourself with a few trillion. A single colony on a petri dish is enough to kill you if it's E. coli O157:H7... you and about a hundred thousand people. The process magnifies the harm presented by single organisms, and if food safety is your concern then it's antithetical.

And aside from concentrating the biohazards, what do you accomplish? Almost nothing. As has been said, you cannot distinguish between good and bad bacteria based on characteristics of the dots on the plate (also called 'colony morphology'), much less the number. For some organisms, like the bug that goes by "anthrax" -- it's still virtually impossible to tell it apart from Bacillus cereus when you have sophisticated molecular biology equipment at your disposal.

And really, it takes lots of practice with sterile technique to use a nutrient agar properly... so that the only possible inoculations are the ones you introduce on purpose.

At the end of the day you wasted time and possibly concentrated a hazardous organism. Definitely not worth the effort.
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