UC San Diego

The skin provides a protective niche and is home to a variety of microorganisms. Here, we hypothesize that the cutaneous resident microbe Staphylococcus epidermidis (S. epidermidis) plays a beneficial role by contributing to the skin's innate immune system. We investigated the ability of S. epidermidis to provide an antimicrobial barrier. Our data demonstrate that S. epidermidis inhibits the pathogen Group A Streptococcus (GAS). Purification and sequencing identified 2 putative antimicrobial peptides (AMPs): Phenol Soluble Modulin (PSM)-gamma and Phenol Soluble Modulin-delta. We found that both were alpha- helical, membrane-active, and formed complexes. Furthermore, the PSMs exerted selective antimicrobial activity on pathogens. Finally, the PSMs reduced the survival of GAS on explants. These studies illustrate that the PSMs function as AMPs. As S. epidermidis lives on the epidermis, the interaction of PSM-gamma and the host was evaluated. Immunostaining revealed deposition of PSM-gamma in human skin. PSM-gamma induced and bound neutrophils extracellular traps (NETs). In addition, PSM-gamma cooperated with host AMPs. Also, PSM-gamma rendered GAS bacteriostatic in blood and increased NET killing. Similarly, GAS survival in mouse wounds was reduced by pretreatment of wounds with PSM-gamma. To this end, PSM- gamma cooperates with the host AMPs to ward off bacterial infections. To elucidate the components of PSM-gamma required for its antimicrobial properties, PSM-gamma analogs were investigated. Analogs included mutations in the c-terminus, n-terminus, helix disruption, polarity disruption, and charge neutralization. Although all analogs lost the ability to form complexes, only the c- terminus, polarity and charge neutralization analogs showed reduced membrane binding, vesicle leakage, and bacterial killing. Overall, these data demonstrate that the c-terminus, polarity and charge are critical for PSM- gamma's antimicrobial activity. Finally, we evaluated the ability of the PSMs to induce AMPs in keratinocytes. Both PSMs induced hBD2 and hBD3 with synergy occurring with the toll-like receptor 2 agonists. Thus, PSMs increase the production of endogenous antimicrobial peptides. As a whole, this work demonstrates the beneficial relationship that S. epidermidis has with the host. A deeper understanding of bacteria:host mutualism can lead to a greater understanding of the innate immune system as well as providing novel therapeutic strategies for the treatment of disease