Nice. Could I use a petri dish to figure out what sort of organism spoiled my homebrew?

yeah, colony on this plate is likely aspergillus spp.

Why would it be mold if the OP was culturing bakers yeast?

look at the 1:10 plate. there's colonies that isn't yeast there. luckily, it got diluted out, or, maybe that plating was a bit more careless than the others.

i didnt mean to imply that all of the colonies aren't yeast.

The culture was contaminated with fungal spores.

Are we great again?