The Petri dish assessment is interesting. I used to plate a lot of cultures in my molecular biology days, and that's pretty much how it goes - you get a bunch of little colonies (from your antibiotic-resistant strain of bacteria you created), and you pull from 1-2 of those to start culturing. Over time the antibiotics decompose or you get some contamination and foreign contaminates and your once-useful cultures go to shit. So you bleach the whole thing and throw it out. Idk if aliens are so fastidious as us, but if they are, I'd expect a planet bleaching anytime now.