http://www.archive.org/stream/questionofliving614roys/questionofliving614roys_djvu.txt
[snip] The bacteria described by Lipman appear to be those most frequently met as contaminants in laboratories; this fact throws even more burden of proof upon the modern sponsor of extraterrestrial origin of life. [end]
Yeah, I wonder about that myself, cross contamination. I don’t think this automatically kills the idea but it just makes it harder to prove. I think the only way we will find out is if we go to Mars, the likeliest place I know of, and bring back samples for us to analyze in a sterile lab.