Coproporphyrin III dihydrochloride is a bioactive molecule excreted by nostril- and skin-associated Propionibacterium species that induces aggregation of Staphylococcus aureus (conditions: during early stationary phase growth, low pH: 4-6). Coproporphyrin III dihydrochloride also induces plasma-independent biofilm formation on abiotic surfaces by Staphylococcus aureus. Coproporphyrin III dihydrochloride may be an important mediator of S. aureus accumulation and/or biofilm formation in the nostrils or other sites where Propionibacterium and S. aureus inhabit.