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.

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.

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I.....I cant help it, I have to quote Cowboy Bebop: "Never open the fridge."

How long did it take to get the smell out of the fridge itself ?
It would have spoiled my breakfast too !

By Elizabeth (not verified) on 29 Nov 2006 #permalink

Really, I don't mean any offense, but wasn't this a pretty stupid thing to do? One of the most basic lab safety SOPs we were taught (and which I taught in turn, when I was an instructor) was that you don't store food and biological specimens in the same places. We had separate refrigerators and sinks for those things. And weren't the agar plates parafilmed?

Ah well, I guess these things are more obvious in hindsight. It's a good thing nobody was poisoned.

We cleaned the refrigerator pretty well after that incident!

You're right about storing specimens and food in different areas. That is a good practice. I thought Streptomycetes would be okay in our refrigerator because they do not cause human diseases and we had the plates were wrapped with parafilm and stored in a plastic bag. It just didn't occur to us that they would make volatile compounds with a nasty taste.

I should point out, sometimes there isn't a great distinction between a "biological specimen" and the stuff that we eat. Just last week, we brought a dead turkey home (biological specimen?) and stored it in our refrigerator until we cooked it on Thanksgiving. You can see pictures of it under the microscope.

The fact is, everything in a refrigerator contains microbes - either on the surface of the container, or inside the food. It's even part of product marketing. The people who sell Acidophilus milk, some kinds of yogurt, and Hefeweisen beer, all make a big deal out of including live cultures. So it's impossible to completely separate food from bacteria unless you sterilize the food, and we usually don't do that unless we're canning vegetables. So, I'm not overly paranoid where bacteria are concerned unless they're bacteria that are known to be a problem.

Ack, that sucks. Maybe if you'd sprayed it down with ethanol before sticking it in the fridge it would've been okay...but yeah, ask any yeast lab how paranoid you have to be about bacteria. Those guys can grow anywhere.

I once had a set of petri dishes w/ agar from Dad the doctor sent to me in West Africa to use for a middle school science project 2 weeks later. I didn't have access to refrigeration, so I put them in the closet. They were inside the original sealed package (sterile, I assumed), inside a box, inside another two loose plastic bags, inside a duffle bag, inside the closet, for 2 weeks.

On the morning of, I pulled them out to discover the most amazing variety of colors, shapes, fuzzy stuff, slimy stuff. I was totally fascinated, and could'nt believe that the stuff would get in there. Every plate was totally covered with stuff, devoured.

I never know if I should (a) respect west africa and it's mutant super clever unstoppable bacteria or (b) be amazed that the earth isn't taken over entirely, or (c) be confused that commercial U.S. agar packaging and sterilization is pathetic.

That reminds me of the bachelor's rule about ... erm ... antique food in the fridge: If the green fuzz ain't moving it's OK to eat. :)

Dear Sandra, I liked your post but I believe there is a little inaccuracy in the information you gave. Humic acids are really among several other substances that confer the typical soil smell to "dirt", as you said, and it is not one substance but an assembly of substances, originated from the incomplete decomposition of organic matter, with similar chemical behavior. Humic acids are not "made" by Streptomyces, but they originate from the decomposing activity of a myriad of soil microorganisms on soil organic matter. I find it very unlikely that the smell on your refigerator was from humic acids, perhaps from some volatile precursor.

It sounds like maybe somebody is going to be hoping for a bacteria fridge under the Christmas tree next year! LOL! Dirt is a developed taste! LOL!
Dave Briggs :~)

hi, madem
Good day,
I m doing PhD. on streptomyces spp., but i face a problem during isolation. After incubation colony of streptomyces grow with Bacillus spp. there is a mix contamination there on my bennets agar plate.
so, pls tell me what should i do?
Yours Faithfull,

Hmm, could you try making a series of dilutions in broth and plating them?