No, they can't, not normal widespread bacteria at least, but please consider two things: i. If there is moisture around your cultures and around your Parafilm wrap, bacteria may move and grow along the liquid film below the Parafilm. ii. Parafilm itself is not necessarily sterile (although it normally is according to the production procedure).

I was also having the same problem during my MSc. It was happening in rainy season mostly. what we have felt was that Drosophila or some other small flies get attracted to the parafilm. some how they get entered into plate. in couple of days their larva moves on media inside plate and we were getting a contamination in a line form.

Definitly no. Sealing petri dishes containing nutrient media with Parafilm, remained sterile for weeks. Test tubes with different types of sterile media did also. I checked also sterility of Parafilm on sterile nutrient media (PDA, and others) and got no growth of fungi or bacteria.

Several factors may interfere. If cultures are maintained on the same nutrient medium for a longer period of time, the Parafilm may start detaching. Too much condensation inside Petri dishes in time may favor the development of infections. Some explants contain endophytes that do not develop until after a period of time, or only if they meet certain conditions of temperature / humidity.

At one of the labs where I work, we frequently have problems with fungivorous mites that somehow crawl underneath the Parafilm and spread contaminants. Streptomyces cultures also attract mites. The problem can be solved by placing the plates inside plastic bags, and then placing the bags in plastic containers treated with a pyrethroid insecticide. Some of my colleagues say that you can seal the plastic containers with petroleum jelly and place mothballs in the containers, but I still have problems with mites if I do this.