How to protect Mars from contamination by germs from planet Earth

#planetmars #curiosityrover #astronauts #apollomission The search for life on the Red Planet Mars has been taken up by various agencies and the time has now come to work out methods of how to protect the environment of Mars from contamination by germs from our planet Earth. This is a major task as reported in space.com dated 11 February 2015 because life on the alien plane would be mostly of microbes and contamination could prove disastrous.
The questions that would naturally arise are - how can it be ensured that microbes from Earth do not hitchhike to the Red Planet and spread there? Obviously, it would be necessary to ensure that once a spacecraft is there on the surface of Mars, there must be mechanisms to protect the environment from changes that could hurt any existing Martian life.
At the other extreme is the question – if we do come across some form of life on Mars, how would it affect us humans? As biologist John Rummel, currently a visiting scholar at McGill University, has put it – no one would want to contaminate what we want to study. And, in case there is any life out there, how safe would it be to bring it back to Earth either robotically, or through Mars astronauts returning to Earth.
The problem would mostly arise in habitable locations on Mars - Curiosity rover mission, which landed on Mars in 2012, has come across a probable site around Gale Crater. There was ample evidence of past water in the form of an ancient stream bed. Obviously, these are danger prone areas where contamination could occur and which must be kept in check.
Incidentally, during the Apollo missions, when astronauts returned to Earth, they were placed under quarantine for 21 days.