Since the discovery of the toxic perchlorate being globally distributed in the martian soil the chances for microscopic life there are thought to be greatly reduced..
Even in the RSL's, once thought to be avoided for fear of contamination by the Curiosity rover, signs of perchlorates were spotted.
Over 40 diverse microorganisms capable of growth via perchlorate reduction have been isolated. They use 2 types of enzymes that collectively take perchlorate into harmless chloride, while free oxygen is generated.
In a protected environment, like a greenhouse, those microbes could be used to clean up soil and dust that would then be suitable for agriculture and would also help in creating a breathable atmosphere.
Outside that protected environment the microbes would barely have a chance to survive the harsh martian conditions so contamination seems out of the question.
Although planetary protection rules forbid biological contamination of Mars, could not nevertheless perchlorate consuming bacteria be introduced there, at least for controlled tests ?
Because the introduced microbe would be well examined, it could always be well distinguished from whatever martian microbe would appear.
And how could a toxic consuming species, producing only harmless chloride and oxygen escaping in the atmosphere, be a threat for potential life forms native to Mars?