What's that taste?

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Sandra Porter

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.

I was wrong.

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.