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I have thrown a dirty spoon after eating some pasta into one of my desk drawers (doesn't do me much honor). It stayed there for around a year. My house is warm and I think there is enough humidity for bacteria to multiply, however I have found it today with seemly same amount of food on it when I left it.

I assume given enough time all food should get converted into bacteria (bacteria would eat and multiply until food is there and stop when it is all consumed). Is this assumption incorrect?

Why didn't bacteria decomposed the food on my spoon?

EDIT: This question has nothing to do with fried McDonald foods and is referring to bacteria consuming 'ordinary' food, plus I am asking if food should disappear completely while the other question is about it being decomposed, answers are completely different.

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    $\begingroup$ why would you assume that a bacterial colony looks "clean"? $\endgroup$ – rumtscho Oct 20 '15 at 11:05
  • $\begingroup$ @rumtscho I assumed given enough time all food should get converted into bacteria (bacteria would eat and multiply until food is there and stop when it is all consumed). Is this assumption incorrect? $\endgroup$ – Matas Vaitkevicius Oct 20 '15 at 11:12
  • $\begingroup$ The crucial point is water availability. The Antarctic is a big mass of water. Yet it is frozen and your spoonful would survive there too. The air may be humid in your place, but that doesn't mean organisms can deploy that moist and thrive. $\endgroup$ – AliceD Oct 20 '15 at 11:15
  • $\begingroup$ The assumption is indeed wrong. But even if it were right, the spoon would be covered in gunk consisting of dead bacteria bodies + their waste product. $\endgroup$ – rumtscho Oct 20 '15 at 11:16
  • $\begingroup$ @AliceD So had I placed spoon in conditions perfect for bacteria I would have found it 'eaten off' clean? $\endgroup$ – Matas Vaitkevicius Oct 20 '15 at 11:16
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First, your assumption is wrong. Bacteria multiplication is not limited by food availability only, it is one of many constraints.

Oxygen availability, water availability and the absence of toxins also play a role. You can easily start a thriving bacterial colony somewhere, but once one of these conditions changes sufficiently, the colony can die off. And it can be the growth itself which changes the conditions. A classic example is found in wine fermenting. The yeast there multiplies freely until it has produced so much alcohol as a waste product that it dies off. This is why wine is always in the 12 to 18% ABV range, anything above is "fortified wine" where the concentration was increased after the yeast's death. Yeast are not bacteria, but bacterial colonies' growth is limited in similar ways.

Second, even if the assumption were true, the spoon wouldn't be clean. Bacteria converting food in waste doesn't result in nothing, nor does it result in something gaseous which would literally disappear into thin air. OK, part of their byproducts are likely to be some gas. But you certainly get lots of bacterial bodies (alive, later dead) and also lots of waste product. Together, it forms some kind of sticky organic goo (frequently a biofilm).

So, about your spoon

  • it is entirely possible that it never got a thriving bacterial colony, because the conditions weren't right despite food availability
  • it is also possible that it got some kind of colony (pathogenic or not), and it died off before the food was consumed
  • in the case of a colony, it doesn't matter how much it got to consume before dying off. The spoon wouldn't have ended up clean under any circumstances, including the very unlikely "death by food going out" scenario
  • depending on the conditions in which the spoon is held, the colony which dominates the spoon can be bacterial, or fungal (mold, yeast), or you can end up with different species building their colonies in parallel. Still, the previous points hold under the "mold colony" or "multiple colonies" scenarios just as under the "single bacterial colony" scenario.
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    $\begingroup$ Thanks, I appreciate your patience with someone as ignorant as myself. I am working on a different angle - that they simply didn't like the pasta :) $\endgroup$ – Matas Vaitkevicius Oct 20 '15 at 11:33
  • $\begingroup$ Wouldn't you think fungi would be better candidates than bacteria? $\endgroup$ – AliceD Oct 20 '15 at 11:39
  • $\begingroup$ Heh, no problem - if I didn't have the patience, I wouldn't have written an answer. Also, you're raising an interesting philosophical question - can bacteria "like" something? $\endgroup$ – rumtscho Oct 20 '15 at 11:40
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    $\begingroup$ @AliceD both are possible, I suspect it's the circumstances which determine which ones will get the upper hand - I think it's mostly up to temperature, with bacteria being more likely at room temperature and fungi preferring it below 20 C. I can say from "standard food safety knowledge" (sorry that I can't point to a specific reference right now) that B. cereus is very fond of cooked starches, and can confirm from personal slob experience that you certainly can incubate a nice bacterial colony on pasta or rice - I didn't do anything to confirm that it's B. cereus, though. $\endgroup$ – rumtscho Oct 20 '15 at 11:43

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