Facultatively Anaerobic Staphylococci Enable Anaerobic <i>Cutibacterium</i> Species to Grow and Form Biofilms Under Aerobic Conditions

Facultatively anaerobic <i>Staphylococcus</i> spp. and anaerobic <i>Cutibacterium</i> spp. are among the most prominent bacteria on human skin. Although skin microbes generally grow as multispecies biofilms, few studies have investigated the interaction between staphylococci and <i>Cutibacterium</i> spp. in dual-species biofilms. Here, we measured the mono- and dual-species biofilm formation of four staphylococcal species (<i>S. epidermidis</i>, <i>S. hominis</i>, <i>S. capitis</i>, and <i>S. aureus</i>) and two <i>Cutibacterium</i> spp. (<i>C. acnes</i> and <i>C. avidum</i>) cultured in vitro under both aerobic and anaerobic conditions. The biofilms were quantitated by rinsing them to remove planktonic cells, detaching the biofilm bacteria via sonication, and enumerating the cells by dilution plating. When cultured alone, staphylococci formed biofilms under both aerobic and anaerobic conditions, whereas <i>Cutibacterium</i> spp. formed biofilms only under anaerobic conditions. In co-culture, staphylococcal biofilm formation was unaffected by the presence of <i>Cutibacterium</i> spp., regardless of oxygen availability. However, <i>Cutibacterium</i> spp. biofilm formation was significantly enhanced in the presence of staphylococci, enabling robust growth under both anaerobic and aerobic conditions. Fluorescence confocal microscopy of the aerobic dual-species biofilms suggested that staphylococci create anaerobic niches at the base of the biofilm where <i>C. acnes</i> can grow. These findings demonstrate that staphylococci facilitate the colonization of <i>Cutibacterium</i> spp. in oxygen-rich environments, potentially explaining their presence in high numbers on the oxygen-exposed stratum corneum.