Is it a forgone conclusion that any exo-bacteria discovered on an alien, albeit Earth-like planet will be deadly to human life?

Question

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.

Answer 1

Exo-bacteria is more likely to be harmless than harmful

There are a couple assumptions that you're making that really aren't the case. To start with,

microorganisms that our immune systems have no defense against

Our bodies are really good at recognizing anything foreign in them. In order to evade our immune system, bacteria (and viruses) need to have evolved mechanisms specifically to do so. The serious plagues through history have been diseases that already are or have become adapted to people encountering a new group of people that haven't encountered the disease before. It would actually be more accurate to assume that the microorganisms we encounter will have no defense against us.

Second, most bacterial infections aren't trying to kill you - they want to survive and reproduce, and that is done better by not killing their host. The ones that are really successful are those that actually benefit their host - you have a microcosm of bacteria in your gut that aids in digestion and keeping you healthy.

Related to that, in addition to your immune system alien bacteria also has to deal with the bacteria already in your body. Being already adapted to living in our bodies, our helpful bacteria would almost certainly out-compete non-adapted alien bacteria.

Answer 2

I think the question is less, "Would we be able to survive alien bacteria?" and more, "Would alien bacteria be able to harm us?" Bacteria and viruses typically harm people because, to put it simply, they are eating us. This is a broad blanket statement, which isn't one hundred percent true one hundred percent of the time, but most microbe-based illnesses are caused by either:

1. Microbials harming our cells. Typically, these microbes are trying to either directly consume our cells, consume things near our cells (gut bacteria), or, in the case of many viruses, co-opt our cells for reproductive purposes.
2. Our cells response to microbials in our body. Fevers, for instance, are our bodies' defense mechanism against microbials, and not a "hazard" directly caused by the microbes.

Given that there's some doubt cross-planetary flora would be able to derive nutrition by consuming life from another planet, I think it's likely microbes from another planet would have the same restrictions. Most especially, viruses that co-opt cells for reproductive purposes would find it very difficult to convince an alien cell to replicate its DNA/RNA/whatever pattern since it likely won't have the same genetic chemical structure.

This doesn't mean there wouldn't be microbial hazards. Bacteria colonies could still grow on our skin, even if they weren't deriving nutrition from us, which could cause all manner of problems. But anything that tried to live inside of us (where it could do the most damage) would find itself starved out relatively quickly.

Answer 3

Earthly germs have a hard time infecting anything. They have specific tricks to get past the defenses of specific types of cells in specific species. Most bacteria is harmless as it is quickly rejected by the immune system. Germs from other species occasionally cross over due to mutation, as it may stumble on a trick that works for a different species. Even with animals, vertebrates, mammals, etc. being so closely related, the “trick” relies on very specific vulnerabilities so doesn’t work on a cell with slightly different coding for the same receptors and membranes.

An alien microbe will have no clue. It may be toxic but not pathogenic.

Answer 4

Microbes and virii evolved in parallel with their hosts and are highly tailored to the environment they live and breed in. Even on Earth, there are huge classes of microorganisms which cannot affect human beings at all. To put it in its simplest form, humans don't get Dutch elm disease and trees don't catch colds.

Even with a very similar environment (Earth like planet) with a totally different evolutionary history, it is extremely unlikely that any of the organisms will be able to interact with Earthly life forms in any manner. If we can see creatures as dissimilar as octopi and giraffes which evolved ultimately from the same common answer, then any being from another planet will be far more alien.

The most common interaction imaginable would be that Earth organisms develop violent allergic reactions as the immune system interacts with alien protein analogues. Of course whatever alien equivalent of the immune system will probably have a similar reaction to our proteins as well, so alien tigers, lions and bears won't have much incentive to eat us, while our internal microbiomes will not take too kindly to having alien organisms attempting to move in. Alien microbes will have the same reaction to Earthly microbes as well.

One other interesting conjecture is that alien life has never developed the symbiotic relationship that mitochondria developed with Earthy cellular organisms. Mitochondria allow high energy reactions to take place in cells, providing for the high energy lifestyles that all Earthly life enjoys. if alien life never picked up this trick, then it is probably caught at the evolutionary stage of pond scum (or maybe even les than that) which might explain the Fermi Paradox (Where is everyone?)

Answer 5

Much that has been written here is correct as far as it goes, but none of it goes far enough. A planet that has life might have exactly one life form, a very few life forms or many life forms.

A single life form might exist if it's the very first to exist on the planet in which case it might not be adapted for competition yet. It might be extremely simple from our perspective. But then prions are simple, and we appear to have no useful defense against at least some of them. (From link,"All known prion diseases in mammals affect the structure of the brain or other neural tissue; all are currently untreatable and universally fatal.") If proteins or protein analogs are involved in foreign life, there's a risk that every foreign 'protein' is a potential prion.

Alternatively, a single life form might also have out-competed everything else to death leaving the raw material of the planet to itself. Hard to guess if that's better or worse for us.

A "few" life forms might be an early extrapolation from "single life form", i.e., the earliest variants or alternatively the few survivors of competition. Either way, the dangers are still potentially great.

Most likely, after life begins to evolve, it becomes varied. If a planet is habitable for humans and has life, we can expect many (millions?) of different forms. Sure, our "immune" systems might easily handle 99.9999% of them, but it's that one-in-a-million that can do all of us in. Our "immune" systems don't work too well against things they've never been exposed to before, especially things that the whole species has never seen. History is full of epidemics from unfamiliar infectious agents even if it only rarely happens.

Further, even on earth, our "immune" systems have trouble with things that they don't actually interact with. A fairly common example is amoebic dysentery. Living in us doesn't necessarily mean "in our blood stream". We can be inhabited in ways that avoid most immunity protections. It seems likely that at least one-in-a-million could find a comfortable living in some warm, moist niche.

The trouble comes in numbers. There can be so many possibilities in millions of life forms. We regularly run into new ones here on Earth. Fortunately, most of them are harmless to us. But in an absolutely foreign ecosystem? It won't be a rare organism that we run into, but every single one that is foreign to humans. We still might expect an extreme majority to be benign or even helpful. It'd be a pretty serious mistake to think that there won't any that would kill us, though. There's just too many possibilities, and it only takes one. And we can expect them (as a massed group) to evolve/adapt faster than we do, just as they do here.

Answer 6

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:

• 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.

Answer 7

Lets take a moment and consider the following recurring argument:

We are from very different places, and microbes have co-evolved with their hosts. It is unlikely that microbes would be well adapted to our physiology, and thus unlikely they would be effective at harming us.

I completely disagree with this argument, and here is why:

Our immune system has co-evolved with the microbes in our locality, and our immune system should therefore be considered ineffective vs microbes from different locations.

The immune system has a single objective vs foreign microbes: elimination. Foreign microbes have 2 objectives vs hosts: their own survival and the survival of the host. It is almost always the case, that as microbes evolve, they become less harmful to their host, not the other way around (a few notable cases could be argued, when the spread vector of the microbe is due to the death of the host, but that is very rare in comparison).

The most deadly microbes for humans, are those that are actually niched for other species or have recently migrated from another species to humans. It can also be the case that a local group of humans have microbes locally adapted to their immune systems, and as those humans migrate, we get catastrophic consequences to other groups of humans.

An illustrative example would be native americans exposed to European settlers/conquerors: https://en.wikipedia.org/wiki/Native_American_disease_and_epidemics

This trend of microbes is part of a larger concept that we find in evolution: finding equilibrium is simply the better/more common strategy. It is when genetic carriers (organisms, viruses etc) move out of their niche, that catastrophes happen most often. On a continental scale, we see this also when larger animals are moved over boundaries previously impassable (often with humans assisting transport). Rats can devastate small islands, foreign toads wreak havoc in Australia.

Of course, we could argue that these examples are from planet earth, and we are considering exo-bacteria here. Arguments based on science fiction and not actual observations are fun too.

Answer 8

Well. Considering the Earth sustained life for millions of years longer then humans existed on this particular planet. Could I not interject that WE are the result of alien bacteria that was introduced to Earth after the former inhabitants were "evicted"?

It shouldn't therefore be a foregone conclusion that we could be most likely threatened by an alien bacteria. It would rather be a more legitimate argument of our ability to adapt and develop either immunity to possible attacks, or harmonize with it as is the bacteria we symbiotically live with in our very own bellies.

The evolution of the alien bacteria and our own would certainly play a part, but so would the current environmental compatibility of the bacteria. The fact it is in an early evolutionary stages from the home planet may give it an adaptation advantage allowing it to survive the often sterilizing environment of space during its journey to the new host Earth.

Discovering it already exists on the planet would prove the bacteria's ability to resist extinction, and it becomes a question upon us as to whether we benefit or lose due to its presence, and if so, can we control it before spreading rampant and consuming threatened human life?

Answer 9

Similar circumstance produces similar outcome. It's likely that an earth like planet would have earth like creatures, flora, fauna, and other similarities. This is the principal behind lots of science fiction and science fantasy on how aliens are predominantly predatory bipeds not unlike humanity. There's actually a strong reason to believe that their exo bacteria would be so similar to ours that differences would be negligible enough as to have no significant effect on us.

Answer 10

Wipe out the microbes

Another way of considering the situation is that if you have a spaceship large enough to move hundreds of humans and going at say 0.01c then the energy released by the retro rockets slowing down is easily enough to cook the surface, killing off any life in the process. (If aimed at the planet) Even without this there are other ways to wipe out unfriendly microbes. Places in the universe where you can survive without a spacesuit (or at least breathing equipment) will be few and far between.