@ddominick, investigating the individual strains of bacteria and yeast in a sourdough culture is likely to require more microbiology than is usually accessible to the average 6th grader. That said, there's a lot of great data available online from the Sourdough Project where the team collected starters from around the world and did just that - analyzed the microbial content of each. You can click on their interactive map and see the percentages of different yeasts and bacteria in each one. Here's a video with some updates on the project and some Q&A that might be interesting for your daughter.
You've probably already checked this out, but if you search for "Sourdough Science Fair Projects" there are some good materials out there for grade-level appropriate measurements that your daughter could use to track the growth of your starter and maybe compare that with new starters that she makes with one or more types of flour. This link will take you to a series of blog posts where a scientist (who's also in the video) from the lab working on the Sourdough Project uses some of those materials and takes a group of middle schoolers through the process of making their own starters, choosing the best ones and collaborating with a local bakery, conducts taste-tests on the resulting breads.
I've judged quite a few state and county science fairs and my best advice would be to encourage your daughter to have fun with her project!
