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According to a recent paper from the university of Edinburgh, the ground of Mars contains a lot of perchlorate, which combined with the high level of ultraviolet radiation, makes a very bacteria hostile environment.

Let's imagine that we are going to colonize Mars. Bacteria which would go out of our spaceships would die because of the environment. Let's now suppose that our spaceship is a sterile environment, that the water is treated against bacteria, and the food is only dry food contained in sterile aluminium cans (no bacteria in it).

It would be reasonable to say that the only possible bacteria in such environment would come from the human body of our astronauts.

Now let's suppose that our astronauts would clean themselves a few times per day with betadyne, and would be under heavy dose of wide-spectrum antibiotics.

How likely a human could survive in such a bactericide environment?

EDIT: I am aware that good bacteria play an important part in our diggestive and immune system, but could we live even without these? The same situation could also be imagined on earth in a sterile bunker.

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    $\begingroup$ There is such a thing as good bacteria, and they are necessary to human survival. I don't think your astronauts would last very long. $\endgroup$
    – DisturbedNeo
    Jul 18, 2017 at 14:07
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    $\begingroup$ That is only hostile to some bacteria, bacteria is a wide category. And NASA is having problems with super-robust bacteria colonizing their spacecraft so there are other sources. $\endgroup$
    – John
    Jul 18, 2017 at 14:28
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    $\begingroup$ Why do you put them on antibiotics? Also please be aware that not all astronaut food is sterilized and even the food that is still contains bacteria. It would also be nice if you could tell us if you plan on having those guys return to earth and if there are any children present. Mostly we need bacteria because they produce substances we need and occupy space - and similar things. All of this isn't really necessary given the right environment and food, but when you return to earth, this might kill everyone $\endgroup$
    – Raditz_35
    Jul 18, 2017 at 14:36
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    $\begingroup$ Check Bruce Sterling's Mechanist/Shaper stories, collected in Schismatrix Plus, in which the Shaper faction maintain a sterile environment, and older Shapers without biological adaptations to such an environment have to eat medicinal yogurts to aid their digestion. $\endgroup$
    – Mike Scott
    Jul 18, 2017 at 16:46
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    $\begingroup$ Perhaps relevant: the Martians in H, G, Wells' The War of the Worlds come from a bacteria free Mars and succumb to Earth's bacteria when they get here. $\endgroup$
    – Ethan Bolker
    Apr 23, 2018 at 12:29

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You could in theory have germ free astronauts. There are germ free lab animals.

https://en.wikipedia.org/wiki/Germ-free_animal

They get by ok.

The problem is getting from here (loaded and coated with germs; at least I am) to germ free. Our bacterial ecosystem is like a healthy lawn: vigorous grass keeps out weeds. Kill the grass and the weeds might not die. Then they thrive. Humans getting lots of antibiotics can get sick from deranged and imbalanced bacterial ecosystems: an example is clostridium difficile diarrhea which afflicts people who have been on broad spectrum antibiotics.

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  • $\begingroup$ are you sure? Humans are very symbiotic creatures. $\endgroup$
    – Doomed Mind
    Jul 18, 2017 at 16:29
  • $\begingroup$ Umm, how about vitamins that we extract using bacteria like K? how about skin flora? $\endgroup$
    – Aus
    Jul 18, 2017 at 17:02
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    $\begingroup$ @Aus you are right about vitamin K deficiency. The germ free mice need supplements. ncbi.nlm.nih.gov/pubmed/17660681 I recommend that the germ free astronauts take vitamins. And exercise regularly and get enough sleep. Not sure they will need to brush teeth, though... $\endgroup$
    – Willk
    Jul 18, 2017 at 17:34
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    $\begingroup$ @Will "Note sure they will need to bruth teeth" is enough motive for all children to become astronauts :) $\endgroup$
    – Aus
    Jul 18, 2017 at 17:57
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    $\begingroup$ @Aus back in my day, being an astronaut was enough motive for all children to become astronauts $\endgroup$
    – David Starkey
    Jul 18, 2017 at 20:01
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We are Aliens

You are only 10% human. That is, the number of cells with human DNA in your body are outnumbered by "foreign" cells by around 9:1 or 10:1. One of the reasons the Human Genome Project found such a small number of genes is because many of the genes we use to survive are not encoded in our genome. This is quite apparent if you own a pet dog. It's very important to give dogs a consistent diet and to only change it slowly. That's because dogs have a very limited digestive capability compared to humans.

We are Factories

The reality is that humans have a fantastic ability to metabolize all kinds of chemical compounds, but not because we have amazing DNA. Just ask someone with Crohn's disease, colitis, IBS, or just diarrhea. The vast majority of our digestive capability comes from our bacterial symbionts. That's why Fecal Matter Transplant (FMT) is so effective, and even why vaginal birth is better than Caesarean section (because believe it or not, the baby is likely to get some crap on them as they are delivered, and this initial exposure to the mother's gut biota helps colonize the baby's gut).

Conclusion

Even if it is technically possible to make a bacteria-free human, I don't think that human would be happy for very long. They would likely have to eat an extremely restricted diet, and would probably be susceptible to far more negative conditions. Note that bacteria not only exchange nutrients and energy with us, they can also signal changes in the body by emitting hormone-like compounds. That is, they can control us to some extent (again, why FMT can sometimes make dramatic changes in a person).

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    $\begingroup$ maybe the word you are looking for is not "Aliens" but rather "Hybrids"... ;-) $\endgroup$
    – JJJ
    Jul 18, 2017 at 18:35
  • $\begingroup$ 10 to 1 ? Are you sure about that? how could police figure out someone's DNA from skin cells samples, fecal matter or hair if there was so little cells containing our own DNA? $\endgroup$
    – Vincent Teyssier
    Jul 19, 2017 at 7:07
  • $\begingroup$ Well, apparently science has advanced since the 70's. :/ en.wikipedia.org/wiki/Human_microbiota Closer to 2:1, maybe. Anyway, the secret is that most of the "foreigners" live in your gut and on your skin, and that your cells are generally much, much larger. So they may be more numerous, but still only make a fraction of your mass. $\endgroup$
    – Lawnmower Man
    Jul 19, 2017 at 18:26
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A human wouldn't survive without bacteria.

A quick Google search indicates that the amount of bacteria in a human is huge, and that certain amount of these are useful for the body to function normally.

For example E. Coli are found in the intestines of humans and aid in digestion.

Now an interesting question would be how sterile (and bacteria free) can the environment be for a human to survive?

However, this is a field in which research isn't quite as developed as it could be, and it's going to be complicated to give a precise answer for the next several years.

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    $\begingroup$ Adding to your answer, If we accedentaly keep some of our microbiome (on the skin or inside us), such hostile environment (too clean plus radiation) may cause evolutionary pressure and change them into something we don't want to have in our bodies. No reference, just a thought. $\endgroup$
    – Aus
    Jul 18, 2017 at 17:05

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