The Surface Microbiome of Clinically Unaffected Skinfolds in Hidradenitis Suppurativa: A Cross-Sectional Culture-Based and 16S rRNA Gene Amplicon Sequencing Study in 60 Patients

Antimicrobial Resistance Trends in Hidradenitis Suppurativa Lesions

Background/Objectives: Antibiotic (AB) therapy is the first step in managing hidradenitis suppurativa (HS). Knowledge of the local patterns of antimicrobial resistance is paramount for the appropriate selection of antimicrobials. This study aimed to assess the occurrence of antibiotic resistance in patients with HS. Methods: A cross-sectional study was conducted on 103 patients with HS seen at the Dermatology Department at the University Hospital of Heraklion, Heraklion, Crete, Greece, from January 2019 to December 2023, who were not on any antibiotics in the last three months. Results: A total of 103 patients with HS participated in this study. Purulent material from 139 skin lesions of these patients was swabbed, and 79.86% (111/139) tested positive for bacteria. Gram-positive isolates accounted for 73%, whereas Gram-negative isolates comprised 27%. Among the isolates, 85.1% were aerobes, and 14.9% were anaerobic. The most common bacterial families isolated were Staphylococcaceae (48.27%), Enterobacteriaceae (14.94%), and Streptococcaceae (6.89%). The antibiogram profiles of bacterial cultures revealed a 57.1% resistance to levofloxacin and a 53.3% resistance to penicillin in Staphylococcus lugdunensis, whereas Staphylococcus aureus showed a 76.9% resistance to penicillin and a 58.3% resistance to fusidic acid. High resistance rates of 63.5% for tigecycline, 63.3% for ampicillin, and 40.5% for colistin were observed for Gram-negative isolates. Resistances of 62.5%, 61.5%, and 53.8% to erythromycin, clindamycin, and penicillin, respectively, were observed in the anaerobes. Conclusions: Patients with HS displayed considerable resistance to bacterial proliferation. The revised therapeutic guidelines for HS should incorporate the latest insights into bacterial antibiotic resistance.

Gram-negative anaerobes elicit a robust keratinocytes immune response with potential insights into HS pathogenesis

Hidradenitis Suppurativa (HS) is a chronic autoinflammatory skin disease with activated keratinocytes, tunnel formation and a complex immune infiltrate in tissue. The HS microbiome is polymicrobial with an abundance of commensal gram-positive facultative (GPs) Staphylococcus species and gram-negative anaerobic (GNA) bacteria like Prevotella, Fusobacterium and Porphyromonas with increasing predominance of GNAs with disease severity. We sought to define the keratinocyte response to bacteria commonly isolated from HS lesions to probe pathogenic relationships between HS and the microbiome. Type strains of Prevotella nigrescens, Prevotella melaninogenica, Prevotella intermedia, Prevotella asaccharolytica, Fusobacterium nucleatum, as well as Staphylococcus aureus and the normal skin commensal Staphylococcus epidermidis were heat-killed and co-incubated with normal human keratinocytes. RNA was collected and analysed using RNAseq and RT-qPCR. The supernatant was collected from cell culture for protein quantification. Transcriptomic profiles between HS clinical samples and stimulated keratinocytes were compared. Co-staining of patient HS frozen sections was used to localize bacteria in lesions. A mouse intradermal injection model was used to investigate early immune recruitment. TLR4 and JAK inhibitors were used to investigate mechanistic avenues of bacterial response inhibition. GNAs, especially F. nucleatum, stimulated vastly higher CXCL8, IL17C, CCL20, IL6, TNF and IL36γ transcription in normal skin keratinocytes than the GPs S. epidermidis and S. aureus. Using RNAseq, we found that F. nucleatum (and Prevotella) strongly induced the IL-17 pathway in keratinocytes and overlapped with transcriptome profiles of HS patient clinical samples. Bacteria were juxtaposed to activated keratinocytes in vivo, and F. nucleatum strongly recruited murine neutrophil and macrophage migration. Both the TLR4 and pan-JAK inhibitors reduced cytokine production. Detailed transcriptomic profiling of healthy skin keratinocytes exposed to GNAs prevalent in HS revealed a potent, extensive inflammatory response vastly stronger than GPs. GNAs stimulated HS-relevant genes, including many genes in the IL-17 response pathway, and were significantly associated with HS tissue transcriptomes. The close association of activated keratinocytes with bacteria in HS lesions and innate infiltration in murine skin cemented GNA pathogenic potential. These novel mechanistic insights could drive future targeted therapies.

Epidermal barrier function in dry, flaky and sensitive skin: A narrative review

The stratum corneum (SC)-the outermost layer of the epidermis-is the principal permeability and protective barrier of the skin. Different components of the SC, including corneocytes, natural moisturizing factor, a variety of enzymes and their inhibitors, antimicrobial peptides and lipids, work interactively to maintain barrier function. The main barrier properties of the SC are the limitation of water loss and the prevention of infection and contact with potentially harmful exogenous factors. Although the SC functions consistently as a protective barrier throughout the body, variations in functions and morphology occur across body sites with age and skin type. Healthy SC function also depends on the interplay between the chemosensory barrier, the skin's microbiome and the innate immune system. Dysregulation of SC barrier function can lead to the development of skin disorders, such as dry, flaky or sensitive skin, but the complete underlying pathophysiology of these are not fully understood. This review provides insight into the current literature and emerging themes related to epidermal barrier changes that occur in the context of dry, flaky and sensitive skin. Additional studies are needed to further elucidate the underlying aetiology of dry, flaky and sensitive skin and to provide tailored treatment.

Hidradenitis suppurativa and follicular occlusion syndrome: Where is the pathogenetic link?

The follicular occlusion tetrad complex encompasses several entities (hidradenitis suppurativa, acne conglobata, dissecting cellulitis of the scalp, and pilonidal cyst) that share common clinical features, risk factors, and pathophysiology. Follicular occlusion is a crucial triggering mechanism in the etiology in each of these disorders, leading to development of distinctive skin lesions such as deep-seated nodules, abscesses, comedones, and draining sinuses, often with accompanying scarring. Despite the fact that the follicular occlusion tetrad components manifest multiple similarities, they also exhibit many differences among themselves and require individual approaches and treatment.

Bacterial Metabolites and Inflammatory Skin Diseases

The microbiome and gut-skin axis are popular areas of interest in recent years concerning inflammatory skin diseases. While many bacterial species have been associated with commensalism of both the skin and gastrointestinal tract in certain disease states, less is known about specific bacterial metabolites that regulate host pathways and contribute to inflammation. Some of these metabolites include short chain fatty acids, amine, and tryptophan derivatives, and more that when dysregulated, have deleterious effects on cutaneous disease burden. This review aims to summarize the knowledge of wealth surrounding bacterial metabolites of the skin and gut and their role in immune homeostasis in inflammatory skin diseases such as atopic dermatitis, psoriasis, and hidradenitis suppurativa.

Skin and Gut Microbiome in Hidradenitis Suppurativa: A Systematic Review

Hidradenitis suppurativa (HS) is a chronic skin disease that significantly impairs the quality of life of affected individuals. The disease is characterized by persistent purulent lesions in specific anatomical areas, and its pathophysiology involves multiple factors, including inflammation, genetics, the microbiome, and environmental components. Recent research suggests a potential role for pathogenic bacteria in HS, highlighting the importance of the communication between the human host and the microbiome in maintaining homeostasis and immune system reactivity. However, the exact mechanisms underlying the gut-skin microbial interactions in HS remain unclear. This systematic review aims to examine the existing literature on the differences in skin and gut microbiome composition between HS patients and healthy controls. The review identifies methodological inconsistencies and calls for further research to elucidate the microbiome's role in HS pathogenesis and to explore new therapeutic interventions. The review highlights the need for advancements in microbiome research methodologies, such as metataxonomics and metagenomics, to improve our understanding of the microbiota's impact on health and disease.

Evolving approaches to profiling the microbiome in skin disease

Despite its harsh and dry environment, human skin is home to diverse microbes, including bacteria, fungi, viruses, and microscopic mites. These microbes form communities that may exist at the skin surface, deeper skin layers, and within microhabitats such as the hair follicle and sweat glands, allowing complex interactions with the host immune system. Imbalances in the skin microbiome, known as dysbiosis, have been linked to various inflammatory skin disorders, including atopic dermatitis, acne, and psoriasis. The roles of abundant commensal bacteria belonging to Staphylococcus and Cutibacterium taxa and the fungi Malassezia, where particular species or strains can benefit the host or cause disease, are increasingly appreciated in skin disorders. Furthermore, recent research suggests that the interactions between microorganisms and the host's immune system on the skin can have distant and systemic effects on the body, such as on the gut and brain, known as the "skin-gut" or "skin-brain" axes. Studies on the microbiome in skin disease have typically relied on 16S rRNA gene sequencing methods, which cannot provide accurate information about species or strains of microorganisms on the skin. However, advancing technologies, including metagenomics and other functional 'omic' approaches, have great potential to provide more comprehensive and detailed information about the skin microbiome in health and disease. Additionally, inter-species and multi-kingdom interactions can cause cascading shifts towards dysbiosis and are crucial but yet-to-be-explored aspects of many skin disorders. Better understanding these complex dynamics will require meta-omic studies complemented with experiments and clinical trials to confirm function. Evolving how we profile the skin microbiome alongside technological advances is essential to exploring such relationships. This review presents the current and emerging methods and their findings for profiling skin microbes to advance our understanding of the microbiome in skin disease.

Consistency of Bacterial Triggers in the Pathogenesis of Hidradenitis Suppurativa

Hidradenitis suppurativa (HS) is an inflammatory skin disease whose pathogenesis remains poorly defined. Over the past decades, the bacterial role in HS patients has been a focus of research. According to the literature, the HS skin (and probably gut) bacterial composition is different to that of healthy controls. To date, a key question is whether compositional changes in the microbial populations are responsible for the development of HS (primum movens), or only secondarily reflect the ongoing inflammatory process. The great diversity of methodologies that have been used to study microbial role in HS have led to an accumulation of conflicting results. Thus, in view of these considerations, the aim of this article is to provide the reader with an overview about different hypotheses proposed to explain the bacterial role in HS pathogenesis.

[Impact of preservatives in topicals on the cutaneous microbiota]

Preservatives are used to stabilize topical preparations and protect the user from the influence of pathogenic microbes. After the application of a topical preparation, the matrix undergoes a metamorphosis, and by proportional evaporation of the hydrophilic phase the preservative may accumulate on the skin surface. This is believed to lead to antiseptic effects and may influence the diversity of the cutaneous microbiota. The regulation of the cutaneous microbiome and the associated influencing factors is a complex system that results in highly individualized conditions. Therefore, investigations on the influence of defined interventions are methodologically difficult. In the present proof-of-concept study, potential antiseptic effects of preservatives were investigated in a combination of in vitro and in vivo methods using microbiological culture tests. In addition, the investigations served to develop a clinical study design to answer further questions and use of an extended range of methods. The results support the hypothesis of an antiseptic effect of the tested preservatives (methyl-4-hydroxybenzoate and propyl-4-hydroxybenzoate, potassium sorbate and propylene glycol) on prominent reference bacteria, which could also be observed in clinical settings.

A rare loss-of-function genetic mutation suggest a role of dermcidin deficiency in hidradenitis suppurativa pathogenesis

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease with a multifactorial aetiology that involves a strict interplay between genetic factors, immune dysregulation and lifestyle. Familial forms represent around 40% of total HS cases and show an autosomal dominant mode of inheritance of the disease. In this study, we conducted a whole-exome sequence analysis on an Italian family of 4 members encompassing a vertical transmission of HS. Focusing on rare damaging variants, we identified a rare insertion of one nucleotide (c.225dupA:p.A76Sfs*21) in the DCD gene encoding for the antimicrobial peptide dermcidin (DCD) that was shared by the proband, his affected father and his 11-years old daughter. Since several transcriptome studies have shown a significantly decreased expression of DCD in HS skin, we hypothesised that the identified frameshift insertion was a loss-of-function mutation that might be associated with HS susceptibility in this family. We thus confirmed by mass spectrometry that DCD levels were diminished in the affected members and showed that the antimicrobial activity of a synthetic DCD peptide resulting from the frameshift mutation was impaired. In order to define the consequences related to a decrease in DCD activity, skin microbiome analyses of different body sites were performed by comparing DCD mutant and wild type samples, and results highlighted significant differences between the groins of mutated and wild type groups. Starting from genetic analysis conducted on an HS family, our findings showed, confirming previous transcriptome results, the potential role of the antimicrobial DCD peptide as an actor playing a crucial part in the etio-pathogenesis of HS and in the maintenance of the skin's physiological microbiome composition; so, we can hypothesise that DCD could be used as a novel target for personalised therapeutic approach.

Microbiome in Hidradenitis Suppurativa—What We Know and Where We Are Heading

Recently, interest in the microbiome of cutaneous diseases has increased tremendously. Of particular interest is the gut-brain-skin axis proposed by Stokes and Pillsbury in 1930. The microbiome has been suggested in the pathogenesis of hidradenitis suppurativa, however the link between the commensals and the host is yet to be established. Across all studies, the increased abundance of Porphyromonas, Peptoniphilus, and Prevotella spp., and a loss of skin commensal species, such as Cutibacterium in HS lesions, is a consistent finding. The role of gut and blood microbiome in hidradenitis suppurativa has not been fully elucidated. According to studies, the main link with the intestine is based on the increased risk of developing Crohn’s disease and ulcerative colitis, however, further research is highly needed in this area. Lifestyle, dietary approaches, and probiotics all seem to influence the microbiome, hence being a promising modality as adjuvant therapy. The aim of this review was to present the latest reports in the field of research on skin, blood, and gut microbiome in terms of hidradenitis suppurativa.

Innate immunity and microbial dysbiosis in hidradenitis suppurativa – vicious cycle of chronic inflammation

Hidradenitis Suppurativa (HS) is a chronic multifactorial inflammatory skin disease with incompletely understood mechanisms of disease pathology. HS is characterized by aberrant activation of the innate immune system, resulting in activation of pathways that aim to protect against pathogenic microorganisms, and also contribute to failure to resolve inflammation. Imbalance in innate immunity is evident in deregulation of host antimicrobial peptides (AMPs) and the complement system associated with the microbiome dysbiosis. The pathology is further complicated by ability of pathogens associated with HS to overcome host immune response. Potential roles of major AMPs, cathelicidin, defensins, dermcidin, S100 proteins, RNAse 7 and complement proteins are discussed. Dysregulated expression pattern of innate immunity components in conjunction with bacterial component of the disease warrants consideration of novel treatment approaches targeting both host immunity and pathogenic microbiome in HS.

Insights into hidradenitis suppurativa

Hidradenitis suppurativa (HS) is a chronic, debilitating, inflammatory skin disorder with a prevalence of around 1% and a profound impact on patients' quality of life. Characteristic lesions such as inflammatory nodules, abscesses, and sinus tracts develop in the axillae, inguinal, and gluteal areas, typically during or after puberty. A complex interplay of genetic predisposition, hormonal factors, obesity, and smoking contributes to development and maintenance of the disease. HS is considered to arise from an intrinsic defect within the hair follicle, leading to follicular plugging, cyst formation, and subsequent rupture that in turn induce an acute inflammatory response characterized by elevated levels of IL-1β, IL-17, and TNF. Over time, acute lesions transition into chronic disease, with active draining sinus tracts accompanied by extensive fibrosis. HS is associated with other immune-mediated inflammatory diseases, metabolic and cardiovascular disorders, and psychiatric comorbidities. Treatment of HS often requires a combination of antibiotic or immunosuppressing therapies and surgical intervention. Nonetheless, the currently available treatments are not universally effective, and many drugs, which are often repurposed from other inflammatory diseases, are under investigation. Studies into the early stages of HS may yield treatments to prevent disease progression; yet, they are hampered by a lack of appropriate in vitro and animal models.

Is cutaneous microbiota a player in disease pathogenesis? Comparison of cutaneous microbiota in psoriasis and seborrheic dermatitis with scalp involvement

BACKGROUND

Knowledge about cutaneous microbiota in psoriasis vulgaris and seborrheic dermatitis is limited, and a comparison of microbiota in the two diseases was not yet previously undertaken.

AIMS/OBJECTIVES

This study aimed to compare the scalp lesional and non-lesional microbiota in psoriasis vulgaris and seborrheic dermatitis with that in a healthy control group.

METHODS

Fifty samples were taken with sterile swabs from patients' and controls' scalps, and 16S rRNA gene sequencing analyses were performed.

RESULTS

Alpha and beta diversity analyses showed that bacterial load and diversity were significantly increased in psoriasis vulgaris and seborrheic dermatitis lesions compared to the controls. As phyla, Actinobacteria decreased and Firmicutes increased, while as genera, Propionibacterium decreased; Staphylococcus, Streptococcus, Aquabacterium, Neisseria and Azospirillum increased in lesions of both diseases. Specifically, Mycobacterium, Finegoldia, Haemophilus and Ezakiella increased in psoriasis vulgaris and Enhydrobacter, Micromonospora and Leptotrichia increased in seborrheic dermatitis lesions. Mycobacterium, Ezakiella and Peptoniphilus density were higher in psoriasis vulgaris compared to seborrheic dermatitis lesions. The bacterial diversity and load values of non-lesional scalp in psoriasis vulgaris and seborrheic dermatitis lay between those of lesional areas and controls.

LIMITATIONS

The small sample size is the main limitation of this study.

CONCLUSION

Higher bacterial diversity was detected in lesions of both psoriasis and seborrheic dermatitis compared to the controls, but similar alterations were observed when the two diseases were compared. Although these differences could be a result rather than a cause of the two diseases, there is a need to analyze all members of the microbiota and microbiota-host interactions.

Hidradenitis Suppurativa: Host-Microbe and Immune Pathogenesis Underlie Important Future Directions

Hidradenitis suppurativa (HS) is an inflammatory disease of the skin with a chronic, relapsing-remitting course. The pathogenesis of the disease is poorly understood and involves multiple factors, including genetics, environment, host-microbe interactions, and immune dysregulation. In particular, the composition of the cutaneous microbiome shifts as the disease progresses, although it is unclear whether this is a primary or secondary process. Trials with immunomodulatory therapy elucidate the role of specific immune pathways and cytokine signaling in disease mechanism, such as TNF-α, IL-1β, IL-12, IL-17, IL-23, and complement. Future studies should continue examining the causes of and contributing factors to microbial changes and immune dysregulation in HS pathogenesis.

The Clinical Relevance of the Microbiome in Hidradenitis Suppurativa: A Systematic Review

Hidradenitis suppurativa is a chronic disease of the pilosebaceous unit. The name of the condition is a testament to the presumed relationship between the disease and the microbiome. The pathophysiology of hidradenitis suppurativa is, however, complex and believed to be the product of a multifactorial interplay between the interfollicular epithelium, pilosebaceous unit, microbiome, as well as genetic and environmental factors. In this review we assimilate the existing literature regarding the role played by the human microbiome in HS in various contexts of the disease, including the pathophysiologic, therapeutic, and potentially, diagnostic as well prognostic. In conclusion, the role played by the microbiome in HS is extensive and relevant and can have bench-to-bedside applications.

Hidradenitis Suppurativa in the Pediatric Population: An International, Multicenter, Retrospective, Cross-sectional Study of 481 Pediatric Patients

Importance

Hidradenitis suppurativa (HS) in pediatric patients has been understudied. Increased awareness and recognition of HS prevalence in children demand efforts to better understand this condition.

Objective

To describe the demographics, clinical features, treatment, associated comorbidities, and outcomes in a large cohort of pediatric patients with HS.

Design, Setting, and Participants

International, multicenter, retrospective medical record review of pediatric patients (aged 1-18 years) with a clinical diagnosis of HS carried out in 10 dermatology clinics across the US, Canada, Israel, Australia, and Italy from January 1996 to January 2017.

Main Outcomes and Measures

Patient demographics, clinical features, severity, associated comorbidities, and treatments in pediatric patients with HS.

Results

This cross-sectional study included 481 patients diagnosed with HS. Overall, 386 (80%) were girls. The mean (SD) age of disease onset was 12.5 (2.9) years, and the mean (SD) age at diagnosis was 14.4 (3.5) years. Family history of HS was present in 111 of 271 (41%) patients. First signs/symptoms reported at disease onset were cyst/abscess in 229 of 481 (48%), pain/tenderness in 118 of 481 (25%), and papules/pustules in 117 of 481 (24%). At initial dermatologic assessment, 233 of 481 (48%) patients already had evidence of skin scarring. Disease severity (Hurley staging) was documented in 288 of 481 (60%) patients (47% stage 1, 45% stage 2 and 8% stage 3). Comorbid conditions were reported in 406 of 481 (85%) patients, the most common being obesity (263/406 [65%]) and acne vulgaris (118/406 [29%]). Complications occurred in 378 of 481 (79%) patients, the most common of which were scars or contractures (301/378 [80%]).

Conclusions and Relevance

The findings of this study indicate that there is a gap in recognizing and diagnosing pediatric HS. Pediatric patients with HS are likely to present with other comorbidities. Prospective observational and interventional studies are needed to better understand clinical course and optimal treatments for pediatric HS.

Hidradenitis suppurativa: A folliculotropic disease of innate immune barrier dysfunction?

The innate immune system of human skin consists of a multi-layered barrier consisting of cells and soluble effector molecules charged with maintaining homeostasis and responding to insults and infections. It has become increasingly clear that these barrier layers become compromised in skin diseases, especially in disorders of an (auto)inflammatory nature. In the case of hidradenitis suppurativa, great strides have been made in recent years in characterizing the underlying breakdown in homeostatic innate immunity, including an increasing understanding of the central role of the hair follicle in this process. This breakdown appears to occur at multiple levels: the pilosebaceous unit, associated epithelium, the cutaneous microbiome, alteration of immune cell function and local molecular events such as complement activation. This review seeks to summarize, contextualize and analyse critically our current understanding of how these innate immune barriers become dysregulated in the early stage(s) of hidradenitis suppurativa, and to speculate on where potential hidradenitis suppurativa research could be most fruitful.

Histopathological progression of hidradenitis suppurativa/acne inversa : A morphological study with a closer look on the early changes of the folliculosebaceous apocrine apparatus

BACKGROUND

It is generally acknowledged that the first morphological change of hidradenitis suppurativa/acne inversa (HS/AI) consists of infundibular plugging of the folliculosebaceous apocrine apparatus, which is followed by acute and chronic inflammation, cysts with sinus formation, and fibrosis. Alternatively, it has been hypothesized that HS/AI is primarily a neutrophilic autoinflammatory disease and that the follicular plugging typical of this disease is secondary to inflammation.

OBJECTIVE

To review the sequence of the changes that mark the disease development, we have performed a histopathologic study on the surgical material from a series of axillary and inguinal/perineal cases.

METHODS

The histologic material from surgery on Hurley's second and third stage HS/AI was retrieved and collected with the patients' clinical images. The virtually uninvolved skin peripheral to the lesions was studied together with the main inflammatory foci on vertical sections stained with hematoxylin-eosin and immunohistochemistry for the follicle sheaths.

RESULTS

The fully developed lesions showed acute and chronic, suppurative and granulomatous inflammation overlapping fibrosis, cysts, and sinuses. Instead, the skin adjacent to florid inflammation showed plugging and dysmorphic alterations of the hair follicles associated with immunopathological changes of the inner root sheath keratin expression.

CONCLUSION

Our observations coincide with the classical pathological studies on the progressive changes of HS/AI; however, in our specimens, the virtually normal skin peripheral to the fully developed lesions show seemingly initial follicular changes that suggest development error. This finding would support the hypothesis of combined mutation-induced epithelial differentiative defects and immunological derangement in HS/AI pathogenesis.

Yin and Yang: A disrupted skin microbiome and an aberrant host immune response in hidradenitis suppurativa

The skin microbiome plays an important role in maintaining skin homeostasis by controlling inflammation, providing immune education and maintaining host defense. However, in many inflammatory skin disorders the skin microbiome is disrupted. This dysbiotic community may contribute to disease initiation or exacerbation through the induction of aberrant immune responses in the absence of infection. Hidradenitis suppurativa (HS) is a complex, multifaceted disease involving the skin, innate and adaptive immunity, microbiota and environmental stimuli. Herein, we discuss the current state of HS skin microbiome research and how microbiome components may activate pattern recognition receptor (PRR) pathways, metabolite sensing pathways and antigenic receptors to drive antimicrobial peptide, cytokine, miRNA and adaptive immune cell responses in HS. We highlight the major open questions that remain to be addressed and how antibiotic therapies for HS likely influence both microbial burden and inflammation. Ultimately, we hypothesize that the two-way communication between the skin microbiome and host immune response in HS skin generates a chronic positive feed-forward loop that perpetuates chronic inflammation, tissue destruction and disease exacerbation.

Standardizing Hidradenitis Suppurativa Skin Microbiome Research: The Methods Matter

Cutaneous dysbiosis is implicated in hidradenitis suppurativa (HS) pathogenesis. Previous studies reveal skin microbiota shifts in HS lesional skin. In their report, Riverain-Gillet et al. (2020) extended these findings and reported skin microbiota shifts in unaffected HS skinfolds as well. Their study suggests that skin microbial shifts may precede clinical lesions and draws attention to study methods important for skin microbiome research.

The Microbiome in Hidradenitis Suppurativa: A Review

INTRODUCTION

Hidradenitis suppurativa (HS) is a chronic autoinflammatory skin disease. It is characterised by the development of abscesses and nodules in intertriginous anatomical sites. Whilst it is now recognised as an autoinflammatory condition rather than an infective disease, bacteria are implicated in disease pathogenesis.

METHODS

We performed a search of the literature from inception to 12 August 2020 using the search terms "hidradenitis suppurativa", "Verneuil's disease", "acne inversa", "microbiome", "bacteriology" and "microbiology". Studies were included if they assessed the cutaneous, gut or oral bacteria, bacteriology or microbiome in hidradenitis suppurativa.

RESULTS

Twenty-one studies examining the cutaneous microbiome and two studies examining the gastrointestinal microbiome in HS were identified. No studies examining the oral microbiome in HS were identified. A total of 972 patients and 46 healthy controls were included across studies examining the cutaneous microbiome. A total of 100 patients and 36 controls were included across both gut microbiome studies. Coagulase-negative Staphylococcus, anaerobes such as Porphyromonas and Prevotella, and Staphylococcus aureus species were commonly encountered organisms across the included cutaneous microbiome studies. The studies examining the gut microbiome were limited, with one small study demonstrating an alteration in the gut microbiome composition compared to controls. The other study found no alteration to the gut microbiome in patients with HS compared to those with inflammatory bowel disease (IBD) and HS, and IBD and/or psoriasis.

CONCLUSION

Research should be undertaken into the oral microbiome in HS. Further research should be undertaken examining the cutaneous and gut microbiome in HS, and its relationship with documented co-morbidities. Additionally, metagenomics-focused studies may help identify the relationship between microorganisms and host, and this may shed light on new pathways of disease pathogenesis. This may help identify potential future therapeutic targets.

Dysregulation of tryptophan catabolism at the host-skin microbiota interface in hidradenitis suppurativa

Hidradenitis suppurativa (HS) is a chronic skin disorder of unknown etiology that manifests as recurrent, painful lesions. Cutaneous dysbiosis and unresolved inflammation are hallmarks of active HS, but their origin and interplay remain unclear. Our metabolomic profiling of HS skin revealed an abnormal induction of the kynurenine pathway of tryptophan catabolism in dermal fibroblasts, correlating with the release of kynurenine pathway–inducing cytokines by inflammatory cell infiltrates. Notably, overactivation of the kynurenine pathway in lesional skin was associated with local and systemic depletion in tryptophan. Yet the skin microbiota normally degrades host tryptophan into indoles regulating tissue inflammation via engagement of the aryl hydrocarbon receptor (AHR). In HS skin lesions, we detected contextual defects in AHR activation coinciding with impaired production of bacteria-derived AHR agonists and decreased incidence of AHR ligand-producing bacteria in the resident flora. Dysregulation of tryptophan catabolism at the skin-microbiota interface thus provides a mechanism linking the immunological and microbiological features of HS lesions. In addition to revealing metabolic alterations in patients with HS, our study suggests that correcting AHR signaling would help restore immune homeostasis in HS skin.

Loss of homeostasis of tryptophan metabolism at the host-microbiota interface may contribute to Hidradenitis Suppurativa.

Aetiology and pathogenesis of hidradenitis suppurativa

Hidradenitis suppurativa (HS) is a chronic inflammatory disorder. Patients develop inflamed nodules and abscesses and, at later stages of disease, epithelialized tunnels and scars in skinfolds of axillary, inguinal, gluteal and perianal areas. Quality of life is affected due to severe pain, purulent secretion, restricted mobility and systemic involvement. Genetics and lifestyle factors including smoking and obesity contribute to the development of HS. These factors lead to microbiome alteration, subclinical inflammation around the terminal hair follicles, and infundibular hyperkeratosis, resulting in plugging and rupture of the follicles. Cell-damage-associated molecules and propagating bacteria trigger inflammation and lead to massive immune cell infiltration that clinically manifests as inflamed nodules and abscesses. The immune system plays a key role also in the progression and chronification of skin alterations. Innate proinflammatory cytokines (e.g. interleukin-1β and tumour necrosis factor-α), mediators of activated T helper (Th)1 and Th17 cells (e.g. interleukin-17 and interferon-γ), and effector mechanisms of neutrophilic granulocytes, macrophages and plasma cells are involved. Simultaneously, skin lesions contain anti-inflammatory mediators (e.g. interleukin-10) and show limited activity of Th22 and regulatory T cells. The inflammatory vicious circle finally results in pain, purulence, tissue destruction and scarring. Chronic inflammation in patients with HS is also frequently detected in organs other than the skin, as indicated by their comorbidities. All these aspects represent a challenge for the development of therapeutic approaches, which are urgently needed for this debilitating disease. This scholarly review focuses on the causes and pathogenetic mechanisms of HS and the potential therapeutic value of this knowledge.