Investigating the Effect of Enzymatically-Derived Blackcurrant Extract on Skin Staphylococci Using an In Vitro Human <i>Stratum Corneum</i> Model

<b>Background/Objectives</b>: Numerous intrinsic and extrinsic stressors can disrupt the balance of the skin microbiome, leading to the development of various skin diseases. It has been proven that coagulase-negative staphylococci (CoNS) are important commensals for maintaining skin microbiome homeostasis and fighting cutaneous pathogens such as <i>Staphylococcus aureus</i> (<i>S. aureus</i>). Here, we examined the influence of polyphenol-rich enzymatic blackcurrant extract (EBCE) on pathogenic coagulase-positive <i>S. aureus</i> strains and beneficial CoNS, like <i>Staphylococcus epidermidis</i> (<i>S. epidermidis</i>), to explore its potential for rebalancing the skin microbiota. <b>Methods</b>: The polyphenol profile of EBCE was determined by ultra-high-pressure liquid chromatography–tandem mass spectrometry. Microwell plate assays were employed to study the effect of EBCE on five <i>S. aureus</i> strains isolated from the skin of atopic dermatitis patients. An in vitro human <i>stratum corneum</i> model was used to test its effect on mixed bacterial cultures. <b>Results</b>: EBCE inhibited the growth of all tested <i>S. aureus</i> strains by 80–100% at the highest tested concentration after 7 h. No microbial growth was observed at the highest tested EBCE concentration using the <i>stratum corneum</i> model inoculated with one selected pathogen (<i>S. aureus</i> SA-DUS-017) and one commensal laboratory strain (<i>S. epidermidis</i> DSM 20044). The lowest tested concentration did not interfere with <i>S. aureus</i> growth but strongly stimulated the growth of <i>S. epidermidis</i> (~300-fold colony forming unit increase). In addition, low EBCE concentrations strongly stimulated CoNS growth in microbiome samples taken from the armpits of healthy volunteers that were spiked with <i>S. aureus</i> SA-DUS-017. <b>Conclusions</b>: These preclinical data support further testing of EBCE-enriched topical preparations as potential cutaneous prebiotics in human studies.