Your intuition is reasonable: **fresh flour and water left warm can support unwanted microbes. A mature sourdough starter develops conditions that suppress many of them—but it isn’t sterile.**

The main protection is **acidification**. Lactic acid bacteria grow on the flour’s carbohydrates and produce organic acids. As the starter becomes acidic, microbes that tolerate those conditions gain an advantage; many others struggle to grow. A study following starters over 14 days found an early, mixed bacterial community giving way to communities dominated by acid-tolerant organisms, with mature starters reaching pH 4 or below. [Research on starter development](https://pubmed.ncbi.nlm.nih.gov/37810791/)

Regular feeding maintains that community. Each feeding carries a substantial population of established bacteria and yeasts into fresh flour and water, allowing fermentation and acid production to resume. Maintaining a starter is therefore a way of managing a microbial ecosystem. Clean equipment, a loose cover, appropriate temperature, and regular feeding all matter. [Colorado State University’s safety guidance](https://extension.colostate.edu/resource/sourdough-starter-best-practices/)

There are two important limits:

- **Suppressing growth doesn’t guarantee that every pathogen has been killed.** Flour can contain *E. coli* or *Salmonella*, so raw starter and dough should not be tasted. Proper baking supplies the final pathogen-killing step. [FDA flour safety guidance](https://www.fda.gov/consumers/consumer-updates/flour-raw-food-and-other-safety-facts)
- **A starter can still become contaminated.** Visible mold means discarding the whole starter and cleaning the container; fermentation is not a reason to ignore spoilage. [CSU guidance](https://extension.colostate.edu/resource/sourdough-starter-best-practices/)

So the safety mechanism you suspected does exist: **acid-producing fermentation limits pathogen growth, careful maintenance supports that fermentation, and baking makes the finished bread safe to eat.** A newly mixed or neglected bowl of flour and water doesn’t automatically have those protections.