Is it a forgone conclusion that any exo-bacteria discovered on an alien, albeit Earth-like planet will be deadly to human life?
Would the discovery of alien bacteria found on even an Earth analog planet preclude the possibility of colonization due to the potentially deadly nature of bacterial/viral infections? Put differently, would the fact of our evolution in Earth's biosphere alongside its (albeit rapidly mutating) microscopic life prove to be deadly in the face of new microorganisms that our immune systems have no defense against, not having evolved on said planet, or is there a possibility of developing more robust antibodies over time either artificially or through eventual natural selection? Put differently again, would the discovery of ANY life at all spell doom for potential colonists, leaving only lifeless worlds as desirable colonizing destinations?
We can assume the planet has reached at least a Pre-Cambrian stage (as an Earth analog) in its development of multicellular life forms, but perhaps no further.
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Humans are basically warm bags of slightly salty water. If an exo-bacteria can grow on your skin, there isn't much stopping it from growing inside your body.
Your immune system could give it problems, but if the bacteria is foreign enough your body might not be able to do anything about it.
The immune system fights bacteria 3 ways: Via complement-mediated lysis, phagocytosis, or cell-mediated immunity.
These mainly operate by attaching proteins to the bacteria that either destroy the bacteria or flag it for destruction.
But if the structure of the exo-bacteria is such that it doesn't have the weaknesses that our immune systems can exploit, then it may be possible for the bacteria to reproduce without fear of destruction.
However, if the organism is more like a virus that needs cells to reproduce, then we're probably safe.
Edit:
Condensing some of the comments.
Things that will work in our favor:
- If the organism needs a mineral or substance that isn't present in humans, such as Nitrobacter.
- Any virus analog that needs a specific type of cell to replicate.
- Any organism that finds the conditions inside us hostile: too wet, too warm, too cold, too salty, not salty enough, too much oxygen, not enough cyanide, etc.
- Any organism that is similar enough to earth organisms that our immune systems can strongly react to it.
Things that could cause us trouble:
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An organisms that are so alien that our immune systems can't handle them. Earth organisms have certain characteristics that our bodies have developed to exploit.
A "vulnerability" is any vector that can be attacked. In one case the protein that makes up the bacteria's membrane is a vulnerability, since one defense we have is for antibodies to bond with the membrane and allow a place for a C1 protein complex to attach, which then makes holes that lysozyme can enter to cause further damage and kill the bacteria. If the antibodies were unable to bond with the bacteria then we'd lose one attack vector, making the bacteria harder to kill.
Phagocytosis would still work, but only if the alien organism is smaller than a phagocyte, and if they can be opsonised with complement. If they resist opsonisation then they are less likely to be destroyed by phagocytes. Check out http://bitesized.immunology.org/pathogens-and-disease/immune-responses-to-bacteria/ for a little more info in how they operate.
An organisms that work faster than our immune systems can build an immunity. When Europe began colonizing the Americas many natives died when exposed to new diseases which the Europeans had aquired an immunity to. Now, years later we've eradicated many of those, and so as a population would be vulnerable to them if we were visited by a time traveling colonial era European. If an organism is able to effect us at all, it's likely to be harmful/deadly until we can develop a acquired immunity to it. Thankfully you can't get smallpox twice.
An organism that is naturally toxic. There are several of these that exist on earth: Clostridium botulinum produces botulism toxin which is the deadliest neurotoxin known. Stachybotrys chartarum (toxic black mold) produces toxins called mycotoxins. Arsenic based organisms could potentially poison us even as our bodies disposed of them, releasing the poison when the organism dies. Since it is a completely alien world you have to take a lot of possible bases for life into account.
Lastly, though I generally think it's a dumb theory, if panspermia is a thing then we're in more trouble, since then that life would be much closer to us, and not really alien.
So do we need to worry? Maybe not. First there has to be some form of life in the first place.
Is there a valid risk? Yes.
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