|MadSci Network: General Biology|
I was stumped by your question, so I wrote the Sourdough International Organization on the Internet. The response that they gave me was that the reason the sourdough bread did not grow mold is because of the sourdough's acidity. I don't think you would need to repeat your experiment because the sourdough bread does shouldn't grow mold.
If you have any other questions about the sourdough bread, I recommend that you look on the Internet at:
They also have an e-mail address if you have any unanswered questions. Thanks for your interesting question!
The low acidity will certain prevent many species of molds from growing. However, some molds and yeasts can grow under mildly acidic conditions. Thus if exposed to enough species of mold spores or yeast cells, it's possible one will literally "take root" on the bread, particularly if it retains sufficient moisture.
Try the links in the MadSci Library for more information on General Biology.