A sourdough starter is a self‑sustaining microbial ecosystem that consists primarily of wild yeasts and lactic‑acid bacteria (LAB) that have been recruited from flour, water, and the surrounding environment. These microorganisms coexist in a mutually beneficial relationship: yeasts metabolize sugars to produce carbon dioxide and ethanol, while LAB ferment sugars into lactic and acetic acids. The acids lower the pH, creating an environment that suppresses many spoilage organisms yet remains hospitable to the starter’s resident microbes. The continuous exchange of metabolites, along with the ability of both groups to reproduce, constitutes the biological life of the starter.