|MadSci Network: Microbiology|
This is a very good question and an excellent demonstration of the world that we share with microbes! First, let's talk about molds. Basically, these can be classified as a type of fungi; that is, molds are colonies made up of millions of fungal cells. Fungal cells are considered eukaryotic in that they have membrane bound organelles and a nucleus. Fungal cells can reproduce by sexual reproduction (cell division, with formation of a new cell wall) or by asexual reproduction (formation of spores).
A spore is a very sturdy structure that can resist high temperatures, dry environments, and other adverse conditions. Once the spores are released from the mold, they may be carried by air currents, on hands or on animals to an energy source (for example, food) where there are ample nutrients for the spore to germinate and form a mold.
So, a likely answer as to why you had molds on your agar dish is because there were mold spores present in the air which ended up on the agar. Because the agar has lots of nutrients, the spore germinated and grew into a mold. Another possible explanation (maybe less likely) is that there were still spores present on your hand (you may have picked up spores from any surface that you touched (for example, a door handle, petri dish lid, etc.) after drying but before putting your finger on the agar. I hope this answers your question. For more information on microbes, check out this fun website!!
Sincerely, Glynis K. Rutgers University
Try the links in the MadSci Library for more information on Microbiology.