Engraftment of staphylococcal strains on human skin can competitively displace native staphylococci

Topical probiotic formulations containing live bacteria are being explored as treatments for skin disorders such as atopic dermatitis and acne vulgaris. Certain bacterial strains with antimicrobial and/or anti-inflammatory properties show promise as bacterial therapeutics that could improve these conditions and serve as alternatives to antibiotics, which are increasingly compromised by rising antimicrobial resistance. However, little is known about the engraftment efficacy of such bacterial strains or their impact on the native skin microbiome. In this study, we applied two different coagulase-negative staphylococcal strains, one with antimicrobial activity (Staphylococcus epidermidis 2C-5) and one lacking any activity (Staphylococcus hominis H2-S92), on human back skin of 14 healthy volunteers. Engraftment was assessed using strain-specific PCR and three amplicon-based sequencing approaches at 7 and 30 days after application. Microbial profiles shifted modestly, showing a relative increase in staphylococci and a decrease in Cutibacterium spp. S. epidermidis 2C-5 drastically increased from 0.8% pre-application to 46.9% and 12.1% at days 7 and 30, respectively. S. hominis H2-S92 showed a relative rise from 1.4% to 35.8% at day 7, declining to 2.4% by day 30. Interestingly, Staphylococcus capitis relative abundance decreased by 50-60% at the application sites. These findings indicate that both strains can temporarily engraft and competitively displace native staphylococci. S. epidermidis 2C-5 appeared to colonize more effectively, possibly due to its bacteriocin production. Neither strain affected the phylotype composition of Cutibacterium acnes, suggesting lack of reach to sebaceous follicles, C. acnes' primary niche. This study supports the potential of staphylococcal probiotics for modulating the skin microbiome. While they may be effective for conditions involving staphylococcal dysbiosis, such as atopic dermatitis, they appear less suited for treating disorders like acne vulgaris, which are associated with C. acnes imbalance.