Interaction of skin cells with skin microbiota

The interaction between skin cells and skin microbiota is complicated and not fully understood. A diverse community of microbes resides on the skin that participates in shaping its physiology. Skin microbiota can modulate skin immunity and barrier, thus, contributing to skin health. Also, the microbiota’s imbalance and dysbiosis can cause skin disorders. Yet the mechanisms of function of skin microbiota on skin health and disease are only beginning to be discovered. Pseudomonas aeruginosa is one of the leading opportunistic pathogens in chronic wound infections. Biofilm formation of P. aeruginosa in the wound area following its attachment to the keratinocytes has a deleterious impact on wound healing. On the other hand, Staphylococcus epidermidis, as a skin commensal, has been shown to protect the skin against pathogens. Therefore, this study aims to investigate the effect of skin commensal bacteria on the skin cells and the opportunistic bacteria inhabiting the skin.

The effect of S. epidermidis NCTC 11047 supernatant on the biofilm formation, virulence factors production, and quorum sensing network of P. aeruginosa strains NCTC 10662 and PAO1 was investigated using a range of colorimetric assays and RT-qPCR. Also, the effect of S. epidermidis supernatant on the viability, migration, and proliferation of the human skin keratinocytes (HaCaT cell line) was investigated using scratch and MTT assays. Furthermore, the protective impact of S. epidermidis and its supernatant on the skin cells against P. aeruginosa infection was explored by co-culturing keratinocytes with the bacterial cells. The co-culture of S. epidermidis and P. aeruginosa in planktonic and biofilm states was also investigated in order to investigate the interaction of commensal and opportunistic microbes.

S. epidermidis supernatant diminished biofilm formation by 68% in NCTC 10662 and 34% in PAO1 (P=0.0001 and P=0.002, respectively). It also diminished the virulence factor production of P. aeruginosa (such as elastase production P=0.02) without any impact on its growth through the modulation of the P. aeruginosa quorum sensing network. Furthermore, the S. epidermidis supernatant improved proliferation of keratinocytes (P=0.001). It also increased the re-epithelisation rate of an in vitro scratch model wound on a monolayer of keratinocytes in 24 hours (P=0.01). S. epidermidis in co-culture with P. aeruginosa decreased the adhesion of NCTC 10662 (P=0.02) and PAO1 (P=0.02) to the abiotic surface. However, S. epidermidis, during the co-culture formed on keratinocytes, was unable to decrease the adhesion of P. aeruginosa to the keratinocytes (biotic surface). With increase in the prevalence of multi-drug resistant pathogens, the discovery and application of molecules from the supernatant of commensals offering anti-virulence properties may be a promising alternative therapy for the healing of infected wounds.