A high-throughput, tiered screening method of indentifying test agents that exhibit prebiotic activity on human skin commensal microorganisms and cosmetic compositions that include such agents. The method includes determining the metabolite level of a culture and comparing the metabolite level to a control value. The assay further comprises performing a plate count using the culture when the metabolite level of the culture is greater than its corresponding control value. The plate count results are compared to another control value, and the test agent is identified as a prebiotic agent when the number of colonies present in the plate count assay of the culture is greater than the second control value.