Coproporphyrin III dihydrochloride, a bioactive compound secreted by nostril- and skin-associated Propionibacterium species, promotes the aggregation of Staphylococcus aureus during early stationary phase growth at a low pH of 4-6. In addition, it facilitates plasma-independent biofilm formation by S. aureus on non-living surfaces. This compound could play a crucial role in mediating the accumulation and biofilm development of S. aureus in nasal passages or other locations where both Propionibacterium and S. aureus coexist.