When I was a kid, my parents told me that everyday items are always filled with bacteria, such as banknotes, tabletops, keyboards, smartphones, floor (pick up your fallen food within 5 seconds or it will be infected with bacteria), I grew up told there are millions of bacteria under the fingernails all the time, is this really true? How can they be always there and survive that long if they are on the floor, banknotes etc.? They are living organisms, need to eat something, right? For my thinking there is nothing to eat on the banknotes normally. Can anyone bust this myth or confirm? Thanks in advance.
In: Biology
While my answer may touch some domains that are far-removed from ELI5, I hope it serves to spark inspiration and dispell concerns. I truly hope you, OP, go and learn more about this outside of Reddit. Because the microscopic world is just as incredibly beautiful and majestic as the heavens I frequently study!
One of my intellectual hobbies is something called “Artificial Life”. It’s the notion that with a few simple mathematical transforms at each time step, we can simulate the most basic behaviors of “life” instead of just the inert material it is composed of. Though there are a lot of different models (like Conway’s Game of Life, Emergent Gardens, and Lenia), there are a few conjectures that seem applicable to all of them. One of those being that *Life* **needs** *an environment filled with resources to flourish; once it reaches a “vacuum”, it eventually converges/terminates/halts/dies.* I know that seems round about, but this isn’t just the individual that dies. This means that ALL life ceases to flourish if there isn’t variability and resources available at every scale of existence.
So, why do I bring this up in this context? Let’s look at trees and corals. They are individually **MASSIVE** organisms, but they have tons of pockets with branches inside of them. They make the perfect home for smaller creatures like birds, fish, and invertibrates. But when you zoom in even closer, you find that some processes *need* other organisms who are specialized to process some raw materials into other things that are useful. Specifically with trees, they form a nearly symbiotic relationship with various mycilia (fungi): The fungi break down soil and other organic matter for the roots to absorb, and get excess sugars & water and a low-oxygen environment from the tree. But the thing is, mycilia are dumb; they just grow into whatever gaps they can find. And since they can be individual cells (though typically a culture), they can literally squeeze between two plant cells and make a nice home for itself without disrupting the tree. Just like how birds make nests! The key thing to note is that the fungi **is not** consuming the tree while the tree is still alive — That’d be like being a little kid eating the gingerbread house you live in!
In terms of magnitude, the difference in size between you and the sun is the same as between you and an organic molecule. And we’ve found living things so small that they even live inside of other cells! For example, I recently learned from [Kurzgesagt](https://youtu.be/GFLb5h2O2Ww?si=XjaGFyCcCD9Ap7v2) that TB bacteria are smaller than our macrophages (immune cells), take up residence inside of them, and replicate! As scary as that may sound, this is also a mechanism for evolution: Plant chloroplasts and all mitochondria were once bacteria that took up residence *inside* eucaryotic cells.
I like to think that all multi-cellular living things should be thought of like trees: Yes, lots of things live inside of us. But they are either doing no harm (like birds in trees, or the [little mites](https://youtu.be/YW2eGaUzq7E?si=pfvfSMzTFb6WJu4a) that live in our eyelashes) or doing something useful (like mycilia in roots, or [anaerobic bacteria](https://my.clevelandclinic.org/health/body/25201-gut-microbiome) in our colon). When things are out of balance, just like loss of habitat, that’s when the whole ecosystem (or in this case, your body) gets ill.
But you should take this message to all levels: ***We live on a giant wet bubble that can also be modeled as an unintelligent macro-organism! If we don’t start living benignly or beneficially in our environment, everything is going to wither & die.***
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