Advances in Scalp Microbiome Research: Molecular Insights into the Metabolism-Inflammation-Barrier Axis and Dandruff Pathogenesis

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors performed a detailed literature review on microbiological, biochemical and genetic aspects of Dandruff's hair pathology, a chronic, recurrent disease caused primarily by increased colonization of the Malassezia fungus. The evolution of its treatment is discussed.

This is a review article, so it has brought together articles published on the composition of the hair microbiota and its importance in maintaining homeostasis and the evolution of the disease. The review is broad, and brings relevant information in the area.

In fact, the review discusses the need to consider aspects related to residual inflammation derived from a bacterial imbalance in the evolution of the disease. Based on the literature, it analyzes changes in the treatment, in a transition from reactive symptom relief to preemptive, personalized intervention.

The conclusions were based on the analysis of published articles and highlight the importance of considering residual inflammation in the treatment of the disease;

Although the review is long and occasionally repetitive, it is nevertheless informative and enjoyable to read.

Author Response

Response to Reviewer #1

Reviewer Comment:
The authors performed a detailed literature review on the microbiological, biochemical, and genetic aspects of dandruff hair pathology… Although the review is long and occasionally repetitive, it is nevertheless informative and enjoyable to read.

Response:
We are sincerely grateful to the reviewer for the highly encouraging feedback and for recognizing the clinical relevance and originality of our review, particularly regarding our emphasis on residual inflammation and the transition toward preemptive, personalized microecological interventions. Your recognition means a great deal to us.

In alignment with your valuable suggestions to enhance the reading experience, we have carefully optimized the manuscript’s structure and refined its content. Specifically, we have eliminated redundant and repetitive phrasing in the sections on microecology and therapeutics (Sections 4.2.2, 4.2.3, and 5.2), consolidated the advanced biochemical pathways, and thoroughly polished the language to ensure a more concise, precise, and fluid narrative.

We believe these revisions have significantly improved the clarity and readability of our work. Once again, thank you for your insightful guidance.

Closing Statement for Reviewer #1

Once again, we extend our sincere thanks to the reviewer for the encouraging and constructive feedback. We greatly appreciate your recognition of the value of our work, as well as your patience with its length and occasional repetition. We believe that the structural and language refinements we have made have effectively addressed these issues and significantly improved the overall quality of the manuscript. We respectfully hope that the revised version meets your expectations and would be grateful for any further guidance.

Reviewer 2 Report

Comments and Suggestions for Authors

Title: Advances in Scalp Microbiome Research: Molecular Insights into the Metabolism-Inflammation-Barrier Axis and Dandruff Pathogenesis. The manuscript presents a comprehensive and emerging role of the scalp microbiome in dandruff pathogenesis. The discussion on Malassezia-mediated lipid metabolism, AhR/NF-κB signaling, and microbiome-targeted therapeutic strategies is particularly interesting and clinically relevant. In addition, the figures and tables are informative and help readers understand the complex interactions involved in scalp microecology. However, the manuscript still requires major revision before it can be considered for publication. 

Comments

1. The authors should further strengthen the discussion on recent advances in skin microbiome research, particularly regarding strain-level functional heterogeneity, host–microbiome interactions, and microbiome-derived metabolites PMID: 41155313.
2. The authors should provide a clearer distinction between dandruff and seborrheic dermatitis (SD) throughout the manuscript.
3. Please add a dedicated paragraph discussing the similarities and differences in immune activation, microbial composition, and barrier dysfunction between these two conditions.
4. I would recommend that the authors elaborate more critically on the causal versus associative role of Malassezia in dandruff pathogenesis.
5. The authors should expand the section discussing host immune responses and neuroimmune interactions. Please add more information regarding cytokine signaling, keratinocyte–immune cell communication, inflammasome activation, and the role of innate immune pathways beyond NF-κB and AhR signaling. Inclusion of IL-17, IL-23, TLR signaling, and inflammasome-mediated pathways would strengthen the mechanistic depth of the review.
6. I would recommend including a more detailed discussion regarding oxidative stress and mitochondrial dysfunction in scalp inflammation.
7. The authors should improve the section on therapeutic strategies and microbiome modulation. Please add more critical discussion regarding probiotics, postbiotics, microbiome transplantation concepts PMID: 40299368.
8. Several sections require language editing and structural refinement. For example, repeated phrases related to “prebiotic/postbiotic strategies” and occasional formatting inconsistencies reduce readability. Careful professional editing is recommended.

Author Response

Response to Reviewer #2

Reviewer Comment:
Title: Advances in Scalp Microbiome Research: Molecular Insights into the Metabolism-Inflammation-Barrier Axis and Dandruff Pathogenesis. The manuscript presents a comprehensive and emerging role of the scalp microbiome in dandruff pathogenesis… However, the manuscript still requires major revision before it can be considered  for publication.

Response:
We would like to express our sincere gratitude to the reviewer for the highly positive and encouraging evaluation of our comprehensive review. We are truly honored that you found our discussion on Malassezia metabolism, AhR/NF-κB signaling, and therapeutic strategies to be particularly interesting and clinically relevant. We are also deeply grateful for your constructive, rigorous, and insightful suggestions, which have greatly strengthened the mechanistic depth and scientific rigor of our manuscript.

We have carefully and thoroughly addressed all the comments and revised the manuscript accordingly. Below, we provide our point-by-point responses with the utmost respect for the reviewer’s valuable input.

Point 1:
The authors should further strengthen the discussion on recent advances in skin microbiome research, particularly regarding strain-level functional heterogeneity, host–microbiome interactions, and microbiome-derived metabolites (PMID: 41155313).

Response:
We fully agree with this excellent suggestion. In accordance with the reviewer’s advice, we have substantially expanded our discussion on recent advances in skin microbiome research in Sections 2.2, 2.3, and 2.4. Specifically, we have incorporated detailed insights into strain-level functional heterogeneity, multi-omics/metabolite profiles (e.g., strain-specific extracellular matrix degradation, postbiotic signaling, and metabolic fingerprints), and we have cited the recommended landmark reference (PMID: 41155313). We thank the reviewer for pointing us to this important literature.

Point 2:
The authors should provide a clearer distinction between dandruff and seborrheic dermatitis (SD) throughout the manuscript.

Response:
We sincerely apologize for the previously ambiguous distinction between these two conditions. In the revised manuscript, we have meticulously refined the terminology and narrative throughout the entire text to ensure a crystal-clear distinction between dandruff (DF) and seborrheic dermatitis (SD). More specifically, we have rewritten and substantially expanded Section 3.3 to serve as a comprehensive comparative framework, ensuring that DF is strictly presented as a localized, subclinical desquamative condition, while SD is appropriately characterized as a systemic, clinically overt inflammatory dermatosis. We are very grateful to the reviewer for highlighting this important issue.

Point 3
Please add a dedicated paragraph discussing the similarities and differences in immune activation, microbial composition, and barrier dysfunction between these two conditions.

Response:
We greatly appreciate this thoughtful recommendation. Following the reviewer’s advice, we have added a dedicated and highly detailed paragraph in Section 3.3 that systematically synthesizes the precise similarities and differences between DF and SD across three critical dimensions:

Barrier dysfunction: Differentiating moderate, superficial desquamation with compensatory keratin upregulation (K16/K17) in DF from systemic macromolecular breakdown, ceramide depletion, and differentiation arrest (downregulated K1/K10) in SD.

Microbial composition: Differentiating the contained numerical shift among indigenous taxa (elevated M. restricta/M. globosa ratio) in DF from the multi-kingdom collapse, near-total depletion of C. acnes, and opportunistic invasion of external fungi (Candida/Aspergillus) in SD.

Immune activation: Differentiating homeostatic immune tolerance and basal IgG/IgM complement clearance in DF from the full-scale TLR2/NLR/NLRP3 inflammasome activation, caspase1-mediated IL-1β cleavage, and Th17 axis cascade amplification in SD.

To further enhance readability, we have also structurally integrated and visualized these comparative dimensions in the newly added Table 3. We are sincerely thankful for the reviewer’s guidance in improving the clarity of our manuscript.

Point 4.
I would recommend that the authors elaborate more critically on the causal versus associative role of Malassezia in dandruff pathogenesis.

Response:
We highly value this insightful recommendation. In Section 3.1 (Theoretical Framework), we have added a dedicated and critical evaluation addressing the chronological and etiological role of Malassezia. From a pharmacological and microbiological perspective, we now explicitly discuss why therapeutic resolution with antifungals does not automatically equate to etiological initiation. This has allowed us to shift the paradigm toward viewing Malassezia as a context-dependent pathogen whose active virulence is strictly gated by host susceptibility and concurrent bacterial dysbiosis. We are very grateful to the reviewer for pushing us to think more critically about this fundamental question.

Point 5:
The authors should expand the section discussing host immune responses and neuroimmune interactions. Please add more information regarding cytokine signaling, keratinocyte–immune cell communication, inflammasome activation, and the role of innate immune pathways beyond NF-κB and AhR signaling. Inclusion of IL-17, IL-23, TLR signaling, and inflammasome-mediated pathways would strengthen the mechanistic depth of the review.

Response:
We completely agree with the reviewer that these pathways are essential for the mechanistic depth of the review. In response, we have extensively enriched Section 3.3.4 by introducing detailed cascades of TLR2, NOD-like receptors (NLRs), and C-type lectin receptor activation. We have specifically integrated the intracellular assembly of the NLRP3 inflammasome, caspase1 activation, active IL-1β cleavage, and subsequent IL-8/neutrophil amplification loops. Furthermore, we have comprehensively detailed the involvement of the Th17 axis (IL-17, IL-23) as well as neuroimmune-driven autonomic/pruritus interactions. We sincerely thank the reviewer for guiding us to significantly improve this section.

Point 6.
I would recommend including a more detailed discussion regarding oxidative stress and mitochondrial dysfunction in scalp inflammation.

Response:
We are very thankful for this valuable suggestion. Accordingly, we have systematically supplemented and expanded Section 3.2.2 to critically discuss the pathways linking squalene peroxidation (SQOOH), reactive oxygen species (ROS) accumulation, intracellular oxidative stress, and subsequent mitochondrial distress/collapse. We have highlighted how this biochemical cascade triggers downstream macromolecular barrier degradation and exacerbates localized tissue inflammation. We believe this addition substantially strengthens the molecular foundation of our review.

Point 7:
The authors should improve the section on therapeutic strategies and microbiome modulation. Please add more critical discussion regarding probiotics, postbiotics, and microbiome transplantation concepts (PMID: 40299368).

Response:
This is an excellent point, and we are truly grateful to the reviewer for bringing it to our attention. In Section 4.2.3, we have added a comprehensive and critical evaluation of microbiome-targeted interventions, citing the recommended landmark study (PMID: 40299368). We have carefully delineated their technological and safety boundaries, including: formulation/viability conflicts for live probiotics on the scalp; biochemical heterogeneity and the lack of universally accepted Quality Markers (QMarkers) for postbiotics; and the risks of hostversusgraft immunological rejection or pathogen transmission in Scalp Microbiome Transplantation (SMT). We thank the reviewer for helping us refine this important therapeutic section.

Point 8
Several sections require language editing and structural refinement. For example, repeated phrases related to “prebiotic/postbiotic strategies” and occasional formatting inconsistencies reduce readability. Careful professional editing is recommended.

Response:
We sincerely apologize for these formatting and stylistic oversights. We have now carefully revised the entire manuscript, thoroughly checked for grammatical errors, and polished the language to improve readability. The redundant phrasing regarding “prebiotic/postbiotic strategies” in Sections 4.2.2, 4.2.3, and 5.2 has been completely addressed. Moreover, all spacing before citation brackets, doubleletter typographical errors (e.g., DDandruff, BBacterial), and full-width punctuation marks have been fully corrected and standardized. We are deeply thankful to the reviewer for pointing out these issues and for recommending professional editing, which has greatly improved the overall quality of our manuscript.

Closing Statement#2
Once again, we extend our sincere thanks to the reviewer for the thorough, thoughtful, and highly constructive comments. We believe the revised manuscript has been substantially improved in terms of mechanistic depth, clarity, organization, and readability. We respectfully hope that the revisions meet the reviewer’s expectations and would be grateful for any further guidance.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript is comprehensive and interesting. 
It discusses about  Advances in Scalp Microbiome Research  Molecular Insights into the Metabolism-Inflammation-Barrier Axis and Dandruff Pathogenesis
The topic is relevant and original with the journal. 
The research gap is clearly discussed as shown in the introduction that this study aims to systematically summarize recent advances in the understanding of dandruff from a microbiome-centered perspective 
The conclusion is good. 
Generally, this paper can be accepted after minor revisions. My comment is as follow:
1. The abstract is too long, please make it concise. Also please include the source and year of literature that being used in the study.
2. Please revise the writing of scientific name should be italics such as : S. capitis ,  S. aureus, C. acnes , S. epidermidis ,  etc.
3. Please check the  misspelling such as : 
-TThe follicular in-fundibulum serves as a key interface for interactions between microorganisms and host keratinocytes[23].
- [40–42]. n healthy scalps, Malassezia may account for a substantial proportion of the microbial com-munity, and this proportion often increases further under disease conditions
- BBacterial diversity on the scalp is relatively low compared with other skin sites, with microbial populations typically present at moderate densities of approximately 10⁴–10⁵ cells/cm²[17]. The
- DDandruff (DF) is increasingly recognized as a disorder involving complex interac-tions within the scalp microbial ecosystem rather than the overgrowth of a single micro-organism.
4.  How do microbial metabolites derived from the scalp microbiome regulate inflammatory signaling and epidermal barrier integrity in dandruff patients?
5. How does the interplay between scalp microbial composition, oxidative stress, and barrier impairment influence dandruff severity?
6. Can multi-omics analysis of the scalp microbiome identify key metabolic and inflammatory biomarkers linked to dandruff progression?
7. What metabolic pathways are differentially expressed in healthy versus dandruff-associated scalp microbiomes?
8. To what extent does microbiome-induced barrier dysfunction mediate chronic inflammation in dandruff-afflicted scalps?

Comments for author File: Comments.pdf

Author Response

Response to Reviewer #3

Reviewer Comment (General):

The reviewer provided five specific questions regarding microbial metabolites, triaxial interplay, multi-omics/biomarkers, differential metabolic pathways, and barrier dysfunction vs. chronic inflammation.

Response:

We sincerely thank the reviewer for these focused and insightful questions. Below we address each question in turn.

Additional points 1–3

Response:

We sincerely thank the reviewer for the thorough reading and for these valuable corrections. We have addressed the formatting and writing issues as follows:

Abstract: Thank you for your helpful suggestion. We have shortened the abstract as requested. Regarding the inclusion of literature sources and publication years, we have provided a comprehensive description of our literature search strategy, including key references and their publication years, at the end of the Introduction section, as this is the standard practice for a review article in Molecules. We hope this meets the reviewer’s expectation.Italicization of scientific names: Throughout the revised manuscript, we have carefully  italicized all microbial genus and species names (e.g.,, S. capitis, S. aureus, C. acnes, S. epidermidis, Malassezia restricta, etc.).

Typographical errors: All identified misspellings and duplicated letters have been corrected, including: “TThe” → “The” “n healthy scalps” → “In healthy scalps” “BBacterial” → “Bacterial” “DDandruff” → “Dandruff” Other similar typos have been systematically eliminated. We believe these revisions now meet the journal’s standards for language and formatting. The detailed corrections are highlighted in the revised manuscript.

Reviewer Question 1: Microbial Metabolites & Signaling Mechanisms

Response:

We thank the reviewer for this mechanics-focused question. We have thoroughly addressed this signaling mechanism in Section 2.4 of the revised manuscript. In brief, the regulatory role of microbial metabolites operates via a dual paradigm:

In healthy homeostasis: Commensal-derived metabolites (such as propionic acid and short-chain fatty acids [SCFAs] secreted by Cutibacterium acnes, and lipoteichoic acids) act as "biological shields." They neutralize reactive oxygen species (ROS), suppress host tissue-destructive matrix metalloproteinases (MMPs), and actively reinforce lamellar lipid synthesis to lock in barrier macromolecular integrity.

In dandruff-associated dysbiosis: The depletion of these protective postbiotic signals, combined with Malassezia-mediated hydrolysis of host sebum, leads to an influx of irritating fungal free fatty acids (FFAs). This influx induces severe intracellular oxidative stress and mitochondrial distress in keratinocytes, triggering inflammatory downstream cascades and causing advanced macromolecular barrier collapse.

Reviewer Question 2: The Triaxial Interplay (Microbiome-ROS-Barrier)

Response:

We completely agree with the reviewer that these three dimensions form an intertwined network rather than isolated events. As updated in Sections 2.2, 2.3, and 2.4, dandruff severity is driven by a non-linear positive feedback loop:

Microbiome Alteration to Barrier Damage: Driven by altered host physiological or lipid states, the overgrowth of specific pro-inflammatory bacterial strains and Malassezia leads to the hyper-secretion of tissue-destructive enzymes (proteases, lipases, and elastases). These enzymes actively degrade the extracellular matrix (ECM) and structural proteins, compromising physical barrier integrity.

Barrier Failure to Oxidative Stress: The compromised barrier increases transepidermal water loss (TEWL) and permits xenobiotics/irritating FFAs to penetrate deeper epidermal layers, generating massive intracellular ROS accumulations and mitochondrial distress.

Oxidative Stress back to Dysbiosis: The resulting microenvironmental oxidative stress and localized cell death destroy the niche for beneficial symbionts, further promoting the colonization of opportunistic, keratolytic taxa (e.g., Corynebacterium), which self-perpetuates the disease and increases clinical flaking and inflammation severity.

Reviewer Question 3: Multi-omics & Biomarkers

Response:

This is an excellent point. Multi-omics analyses indeed bridge the gap between superficial taxonomy and functional mechanics. As integrated into our rationale in Section 2.3, high-resolution multi-omics datasets reveal that evaluation must occur at the strain level due to phenotypic heterogeneity. Through cross-omics correlation, specific biomarkers have been identified:

Metabolic Biomarkers: A down-regulation of functional pathways for biotin and vitamin B6 biosynthesis serves as an early metabolic signature of a failing ecosystem, while high ratios of unsaturated free fatty acids and active microbial protease/elastase genes mark active disease progression.

Inflammatory Biomarkers: Host transcriptomic readouts indicate that these strain-level disruptions strongly correlate with the up-regulation of the MAPK/AP-1 and NF-κB pathways, translating to macroscopic elevations in clinical dandruff scores and TEWL.

Reviewer Question 4: Differential Metabolic Pathways

Response:

Thank you for highlighting this crucial aspect. As detailed in the functional metagenomic analysis section of Section 2.4, functional differences between healthy and dandruff-associated microbiomes are stark:

In Healthy Communities: Microbial functional pathways are heavily enriched in biosynthetic and maintenance housekeeping operations, including amino acid metabolism, fatty acid elongation, and essential vitamin synthesis pathways (specifically biotin and vitamin B6 biosynthesis), which support host keratinocyte homeostasis.

In Dandruff-Associated Communities: Metabolic pathways shift aggressively toward host colonization and survival. There is a distinct enrichment in host interaction pathways such as N-glycan biosynthesis (primarily driven by M. restricta), glycerolipid metabolism, and virulent extracellular enzyme export pathways, facilitating  nutrient  harvesting  at  the cost of the host tissue.

Reviewer Question 5: Barrier Dysfunction vs. Chronic Inflammation

Response:

We highly appreciate the reviewer’s insight into this causal relationship. As discussed  in  Sections 2.3 and 2.4, microbial-driven barrier dysfunction acts as the primary gatekeeper and obligate amplifier that translates transient microbial shifts into chronic low-grade (subclinical) inflammation.

Quantitatively and mechanistically, microbial-derived lipases and proteases degrade the tight junctions and lamellar lipid integrity of the stratum corneum. This physical disruption compromises the scalp’s immunological privilege: it permits an uncontrolled influx of environmental xenobiotics, pollutants, and pathogen-associated molecular patterns (PAMPs) into deeper epidermal layers. Simultaneously, the physical shearing of keratinocytes triggers the release of endogenous alarmins (such as IL-33). This continuous influx locks the host into a self-sustaining loop of sustained immune cell recruitment, meaning that barrier failure  directly  mediates and  sustains  the  chronicity of the inflammation.

Closing Statement for Reviewer #3

We again thank the reviewer for these thoughtful and detailed questions, which have helped us substantially improve the clarity and mechanistic depth of our manuscript. We hope that the revisions and the above explanations are satisfactory.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Authors have revised the manuscript meticulously and I would recommend it for publication.