Is sourdough more resistant to mold than other breads? My partner works at a restaurant where they serve sourdough as the bread you snack on to tide you over while perusing the menu. They are closed Sundays, which means any leftover bread on Saturday evenings goes home with the employees (since it can't be sold as day-old at the adjacent bakery the next day, as with other days). This means my partner frequently comes home with five or so loaves of sourdough.

Occasionally she'll also end up with a loaf or two of a different sort of bread. A nice wheat, or this particular bread full of nuts and herbs that they make. 

It takes a long time for two of us to get through five large loaves of bread, so when she brings them home they go straight into Ziploc bags to keep them as fresh as possible for as long as possible. The sourdough loaves last positively forever (albeit with a little bit of loss of texture), but the wheat loaves or nutty-herby-loaves-I-forget-the-name-of always end up with mold spots after five days to a week. 

I know nothing about sourdough bread other than that it tastes delicious. Is it more resistant to molding than other types of bread?
The short answer is yes, sourdough breads are generally more resistant to fungus due to the fermentation process of the sourdough starter. The reasons for this are only now becoming understood.  This study from the Journal of Applied and Environmental Microbiology says:


  Sourdough is different from traditional bread because it takes an
  extra fermentation step, which uses lactic acid bacteria to metabolize
  sugars and add that particular spunky flavor. The researchers found
  that during sourdough production, lactobacilli bacteria convert
  another acid found in bread flour--linoleic acid--into hydroxy fatty
  acids that resist fungus.


So there you have it, it's possibly better for you, tastes good (I think so), and lasts longer without going moldy.