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What’s that taste?

This wasn’t in the lab, but it was an accident, and it was funny later on.

Normally, I wouldn’t think twice about storing bacterial cultures in a refrigerator. After all, bacteria on a petri plate, inside of a plastic bag, are kind of stuck. They can’t get out of the plates, and even if they did, they certainly can’t crawl out of a plastic bag.

I thought soil bacteria, on agar plates, were mostly harmless.

Reposted from DigitalBio’s greatest hits.
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i-7cd09d5b6e6f7c07647b901c7633f30f-streptomyces_web.jpgWhen my husband was finishing graduate school, he brought home some agar plates that he had streaked, with different Streptomyces species, so that he could photograph them for his thesis defence. Streptomcyetes grow fairly slow, but when you store plates at room temperature, the agar tends to dry out and the cultures don’t look as nice. So, he put the agar plates, with the Streptomyces cultures, in a plastic bag and stuck them in our refrigerator.

You can see one of his agar plates here with five different species of Streptomycetes. All of these bacteria normally live in the soil and make antibiotics. Some, like Streptomyces azureus, also make colorful pigments as you can see on this plate. They’re lovely bacteria.

There they were were sitting in our refrigerator at home, waiting to be photographed.

One morning I came downstairs for breakfast and made myself some hot buttered toast.

A few minutes later I was frantically spitting toast, crumbs, and butter out of my mouth.

Yuck! It was dirt! The butter tasted like dirt!

I yelled at Todd to come downstairs and told him about the butter. We agreed that we would not store bacterial cultures in our refrigerator anymore and immediately started sniffing other items in the fridge.

Cream cheese? Smells like dirt.

Cream? Dirt.

Peanut Butter? Dirt

Cheese? Dirt, again.

Yogurt? More dirt.

I don’t remember how many things we tossed in the garbage that day. Any substance with any kind of fat had picked up the smell and taste of soil.

We were both puzzled, though. What was this stuff that made everything taste like dirt? It must have been volatile and soluble in lipids, but what was it?

It was humic acid or perhaps, geosmin. Dirt smells like dirt because of humic acid, and humic acid is made by, you guessed it, Streptomyces.


  1. #1 arby
    November 23, 2008

    I love it when there are a series of deep freeze/thaw cycles in the soil, followed by a rain. The freezing opens up spaces in the soil to fill with the scent, the rain forces it above ground. Second only to wet sheets hung out to dry on a bitter cold sunny winter day in my book. Thank you Streptomyces. So I imagine that, unlike winter sheets, soil scent can be bottled. Or even buttered. Might be good on garlic bread. rb

  2. #2 chat
    November 23, 2008


  3. #3 ...tom...
    November 24, 2008

    Hmmm. I scanned the wiki article looking for any reference to: the effects of humic acid on stored food but did not find one. That update must still be pending…


  4. #4 Sandra Porter
    November 24, 2008

    I referenced the wikipedia article because it has a description of humic acid, which is one the molecules that gives dirt it’s distinctive smell, not because of any reference to food.

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