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On the physics StackExchange site people are trying to figure how and why these branching structures formed. The two questions can be found here and here. The latter had no cooling fans and was situated in an unventilated cupboard. There are many processes that could explain such structures without them being a bacterial colony, for example by Diffusion-limited-aggregation as explained in my answer here. To me it looks like this could be a bacterial colony. Is there any way to rule this out or say with precision that this is a bacterial colony?

I think some organic material and humidity could latch on to the irregular edges upon which bacteria could then grow in a warm environment.

  • What ways are there to tell from just a picture if this is a bacterial colony or not?
  • What ways are there to tell if this is a bacterial colony or not if you have the structure at hand?
  • What ways are there to rule out or say that this is some kind of fungus or living thing if you have the structure at hand?

enter image description here enter image description here

The second pic was taken by Wossname:

"the equipment is in a temperate climate in a non-aircon area of an office building near the external door. In high summer it would get to probably 40 Celcius and 85% or more relative humidity in my estimation. It's an area of the building that people most definitely don't loiter in during July/August. :)"

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    $\begingroup$ Unlikely to be living bacteria or other microorganisms as the environment is dusty plastic with moving air, so moisture would be at very low levels, probably too low to support bacteria. $\endgroup$ – mgkrebbs May 13 '17 at 18:26
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    $\begingroup$ Wow so similar to dendritic -pseudofossils $\endgroup$ – Always Confused May 13 '17 at 19:32
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    $\begingroup$ @mgkrebbs In the image below there was no cooling fan or real airflow. It could have been humid and warm. Some organic material and humidity could have accumulated along the irregularities upon which bacteria could grow. $\endgroup$ – macco May 14 '17 at 8:24
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    $\begingroup$ If it is a microbial colony indeed, I'd say it looks like a fungus rather than a bacterial colony. Not sure it's really a living thing though $\endgroup$ – Flo May 14 '17 at 12:43
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    $\begingroup$ Microbiologist here, I agree with @Flo's comment. These do not look like bacterial colonies to me, the morphology is way too unusual. If anything they more resemble fungal colonies. However, I have my doubts as to whether these structures are caused by living organisms. Have you got some microbiology guys at your institution? You could inoculate some agar plates with a small sample and see if something grows? $\endgroup$ – Johnny May 22 '17 at 9:58
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These look like crystallization structures when liquids dry up. But if it were to be a living thing, I'd go with fungal colonies. In my opinion, the best methods to tell is to scrape off a small piece, flood with KOH, and observe under a microscope, if you see filaments then it is definitely a fungus, and alternatively you can inoculate an agar (Sabouraud agar could work, or LB agar) to see if something grows. If its a fungus, it can take from 4 days up to 3 weeks to grow depending on the species and the growth conditions provided.

Good luck.

Here's a ref that might help you.

Microscopic potassium hydroxide preparation: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3994805/

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