The human skin, being the body’s largest organ, hosts a complex and dynamic ecosystem of microorganisms, including bacteria, fungi, and viruses. Maintaining the dynamic equilibrium (homeostasis) between resident commensal bacteria, such as Staphylococcus epidermidis, and transient pathogenic bacteria, like Escherichia coli and Staphylococcus aureus, is crucial for overall skin health. Resident bacteria support skin health through nutrient supply, metabolic regulation, and immune modulation. However, when this balance is disrupted (dysbiosis), the skin barrier function is compromised, leading to the excessive proliferation of pathogens and a decrease in commensals, which can result in skin diseases like acne and atopic dermatitis.
Traditional strategies for addressing microbial infections often rely on natural broad-spectrum antibacterial active substances, such as limonene and usnic acid, used in cosmetics and pharmaceuticals. Despite their efficacy against pathogens, the non-selective nature and long-term use of these agents can unfortunately lead to further dysbiosis, exacerbating skin issues. This inherent limitation necessitates a shift toward innovative, selective microbial regulation strategies that specifically inhibit harmful bacteria while preserving or enhancing beneficial commensal flora. In recent years, microecological regulation technologies, including prebiotics, probiotics, and postbiotics (and their filtrates), have become a major research focus due to their potential to reduce damage to the microbial community. Drawing on previous reports that peptides and organic acids produced during the fermentation of milk by Lactobacillus plantarum exhibit anti-pathogen effects, this study developed Lactobacillus plantarum fermented milk (FM) as an innovative strategy specifically designed to selectively regulate microbial communities and restore skin microbiota balance.
Methods
Lactobacillus plantarum fermented milk (FM) was prepared by first removing lactose from skimmed milk, followed by protease hydrolysis, and finally, 12 hours of controlled fermentation after the addition of L. plantarum seeds, yeast extract, and glucose. The selective antimicrobial properties were assessed using monoculture systems (involving E. coli, S. aureus, and S. epidermidis) and pathogen-commensal co-culture systems (EC_SE and SA_SE). Metabolomic profiling using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) was then performed on the co-culture supernatants to identify FM-induced metabolic alterations. Differential metabolites (VIP > 1, p < 0.05) were screened via Orthogonal Projections to Latent Structures-Discriminant Analysis (OPLS-DA) and subjected to pathway enrichment analysis.
Key Findings
The study provided comprehensive evidence for the selective regulatory effects of FM, primarily driven by metabolic manipulation:
• Selective Growth Modulation in Monoculture: FM demonstrated a proliferative effect specifically on the commensal S. epidermidis, while significantly inhibiting the growth of both pathogenic E. coli and S. aureus in monoculture conditions. In contrast, the unfermented control (PM) promoted the growth of all three bacterial species.
• Restoration of Microbiota Balance in Co-culture: In both the E. coli/S. epidermidis (EC_SE) and S. aureus/S. epidermidis (SA_SE) co-culture systems, FM reduced the growth and competitiveness of the pathogens. The addition of FM reversed the trend seen in control groups—where E. coli or S. aureus became dominant—and relatively increased the colony count of the beneficial S. epidermidis.
• Activation of Pyruvate Metabolism: Metabolomic analysis indicated that FM significantly activated the pyruvate metabolic node in the co-culture systems.
• Enhanced Organic Acid Production: The enhancement of pyruvate metabolism increased metabolic fluxes, resulting in the significant upregulation of organic acids, including lactic acid, citric acid, and short-chain fatty acids (SCFAs).
• Differential Stress Response Mechanism: The increased presence of organic acids triggered an energy-consuming acid stress response in the pathogenic bacteria (E. coli and S. aureus), disrupting their proton gradients and significantly limiting their reproductive capacity.
• Commensal Tolerance: Conversely, S. epidermidis was found to be less sensitive to these organic acids. It maintained intracellular homeostasis and ensured normal growth by enhancing glutamate metabolism flux, which helps buffer protons in acidic environments.
This research successfully developed Lactobacillus plantarum fermented milk (FM) as a selective regulator of the skin microbiota, offering antimicrobial activity against pathogens (E. coli and S. aureus) while simultaneously preserving the commensal S. epidermidis. The novelty of this work lies in its breakthrough from the traditional single-strain research paradigm, providing a new theoretical dimension for developing microecological balance-based skin care products through systematic metabolic regulation. Specifically, FM enhances the survival advantage of S. epidermidis by increasing pyruvate metabolic flux, leading to organic acid accumulation, which compels harmful bacteria to activate high-energy-consuming acid stress responses. This metabolic-interaction-based approach provides a foundational reference for the development of postbiotics aimed at maintaining cutaneous microbial homeostasis. The future implication of this work includes plans to assess FM’s regulatory effects using a broader range of skin strains and performing assessments in animal models or on human skin, which are needed to more accurately reflect its effects on the immense diversity of the skin microbiota.
Schematic diagram of fermented milk (FM) with (A) S. epidermidis and E. coli co-culture (EC_SE) group, and (B) S. epidermidis and S. aureus co-culture (SA_SE) group.
Link to the study: https://www.mdpi.com/2079-9284/12/5/232