Mars probes find more frozen water -
Northern ice boosts search for ancient life
This article made me wonder what would happen in real world terms if some Martian bacteria from a soil sample does escape. Would it even be possible for it to survive on earth given it’s radically different home environment?
Microbes can be hardy creatures, so it’s possible that some could survive on Earth. As to what danger they could pose to us, it’s really impossible to say. After all, we don’t know how much alike they would be (or are) to our own natures. The microbes might not have anything like DNA at all.
Hrm. BABB is down right now. 
Well, worst case scenario I can think of right now, is they’re made up of right handed amino acids, they take over and we starve. Oh! And the last things left to eat for decades are mushrooms and lima beans. 
You might wanna read this article on sample containment. As another aside, I remember reading someplace that Armstrong or Aldrin were (I think touring) in some lunar containment facility, and noticed ants inside someplace. I guess they knew then the “containment” wasn’t really gonna contain anything that didn’t wanna be. 
What does “inches (centimeters)” - mean. Are they pretending that they are the same ?
That’s weird.
Obviously inches aren’t the same as centimeters, but the phrase “several inches” (a fairly nonspecific expression) is for all practical purposes equivalent to the equally nonspecific “several centimeters” (whereas the more precise “1 foot” is approximately equal to “30 centimeters”).
I like mushrooms. You guys can eat my lima beans, though.
There is some evidence that earth life has made & survived a trek that is worse than the Mars to Earth scenario. In a camera the Apollo 12 astronauts brought back to Earth from the U.S. Surveyor 3 robotic Moon lander (in November 1969) there was a terrestrial bacterium. Scientists reported that the biology survived the not only the unpressurized trip to the Moon, but then a 2.5 yr stay on the lunar surface.
Especially if they were microbes that lived in the Ice (like in lake volstock) & didn’t interact the atmosphere at all, only gravity and radiation would be different on earth-- so it is possible.
Knowing that Tuckerfan gave the right answer as to what might happen, let me talk now out my bottom:
Worst case I can truly think might even have an outside chance of happening in real life: maybe they kill off another type of earth microbe which triggers somekind of ecological disaster somewhere.
The best case scenario is that nothing would happen. No big deal.
The worst case scenario is that it would be some weird bacterium or virus to which human beings have no immunity, which would then end life on earth as we know it.
Read “The Andromeda Strain” by Michael Chrighton.
It might include the line, “Now we know what wiped out the Martians.” 
I’d like to think that a microbe for which earth-based physiology has no way to fight off an infection, maybe the microbe isn’t able to infect in the first place.
I’m assuming you’re being facetious here. I’d recommend reading “Travels” by Mr. Crichton to find out what a complete flake the man is. A medical degree does not an expert in microbiology make.
-Apoptosis
Due to the huge and highly competative ecosystem of Earth compared to the very much more limited (if existing at all) ecosystem on Mars, I’d have to say the Martian bacteria wouldn’t stand a chance. But I am no expert in these fields.
On Earth I am aware of many cases where a small seperated ecosystem has been invaded by outside organisms from a larger ecosystem and been decimated by them (toads, rats imported into Australia for example). But are there any cases where the species from the smaller ecosystem has moved to a larger ecosystem and decimated an established species in that larger ecosystem?
I can think only of the grey squirrel displacing european red squirells, but I don’t think the Americas ecosystem is really much smaller than the euyro-asian ecosystem.
I would give 99-1 odds that any martian microbes would be completely harmless. Either they would be similar enough to already existing Earth microbes that we wouldn’t notice the difference, or they would be outcompeted by microbes specially adapted to Earth conditions. Everything I know about bacteria leads me to believe that there would be no real problem.
However.
The truth is, we really have no idea. Maybe martian microbes would metabolize every organic molecule on earth and turn the biosphere into gray goo. It strikes me as highly unlikely. Incredibly unlikely. So unlikely that we shouldn’t even worry about it. Wait. No, not that unlikely. We have a sample size of 1 biosphere here. So we assume that Earth’s biosphere is somewhat typical. But maybe our biosphere is totally whacked out, completely different from what you find on Mars or Europa or Tau Ceti VI. Maybe Earth is the only planet that supports life because every other planet has Chemical X or some such, and just by blind luck Earth doesn’t have any. Bring back Chemical X, or NanoBerserkers, or biosphere-eating bacteria and we’re through.
So even though I don’t expect any problems from cross-planet contamination, it wouldn’t hurt to take a few precautions. No need to get crazy, but hey, let’s be safe. Use a condom every time, you never know who that other planet has been with. It doesn’t matter if they say they’re clean, how can you trust a planet you just met?
The worst that could happen? The bacteria start building very small three-legged fighting machines and make a valiant effort at laying London to waste?
London? Heck no. That was Grover’s Mill, New Jersey. At least that’s where the original War of the Worlds was set. I used to be in possession of an LP record of the original radio broadcast.
It is not completely impossible that an extraterrestrial microbe could cause terrible devastation to the Earth’s biosphere. Very, very unlikely, but not impossible. Lemur866 is right: most probably, nothing at all would happen. But there is the possibility that the microbe has the ability to eat any organic material, with no predators or known weaknesses on Earth. That would spell big trouble for Homo sapiens, and it makes sense to take precautions.
And, whether or not he’s a flake (I haven’t read Travels), he does write some very entertaining, well researched, and usually fairly scientifically accurate stories – for instance, Airframe, Jurassic Park, and Sphere. Of course, some of the science has to pay the price of accuracy to make a good story. C’est la vie. (Then again, he does write some real crap, too – i.e., The Terminal Man and Prey.)
Maybe I’m being whooshed here, but it was London that was attacked in H. G. Wells War of the Worlds. Orson Welles changed it to NJ for the radio broadcast.
The most likely case by far is that absolutely nothing would happen. The Martian microbes, if any (and that alone is rather a long shot) would be in an environment radically different from what they’ve evolved for, and would be in competition with billions of other species which are in the environment they’ve evolved for. The worst-case mass extinction scenarios are exceedingly unlikely… But then, that’s pretty high stakes. So it makes sense to be carefull, just in case.
We find out it replicates itself like mad, consumes ozone, craps sarin, and votes Republican.
Well, no, I was referring to the radio broadcast, not the novel.