Decreased Arterial Vascular Tone in Small Arteries in Severe Hidradenitis Suppurativa - A Study Using Finger Photopulseplethysmography.

A previous study has found an association between chronic inflammatory disorders e.g. psoriasis, rheumatoid arthritis, and inflammatory bowel disease and increased vascular stiffness(1). Psoriasis and hidradenitis suppurativa (HS) are believed to have shared comorbidities and pathophysiology despite their morphologically different manifestations in the skin. In order to evaluate a putative association between the chronic inflammatory skin disease HS and arterial stiffness, an observational cross-sectional retrospective study was carried out as part of the Danish General Suburban Population Study (GESUS) (1), in which 430 patients with HS from the general population (representing mild HS; Table 1), 32 patients with HS from a hospital-based out-patient clinic (representing severe HS, Table 1), and 20,780 controls underwent measurements of arterial vascular tone and stiffness using photoplethysmography (Pulse Trace PCA2®; Micro Medical Ltd, Kent, UK). The method of Pulse Trace has been validated by correlation with intra-arterial sensing techniques, and is a simple cost-effective screening method[2]. All analyses were performed using SAS 9.3. This study was accepted by the ethics committee of Region Zealand (project number SJ-191, SJ-113, SJ-114) in Denmark (2,3). RESULTS Reflection index (RI) is an expression of arterial vascular tone and stiffness of small arteries. The raw data showed a significantly lower RI for both HS groups groups, compared to controls. The results remained significant when adjusting for confounders (age, sex, smoking and metabolic syndrome) in the out-patient clinic HS group (-11.26 (-17.75- -4.76), P=0.0002*), but not in the population HS group (Table 2). Stiffness index (SI) expresses arterial stiffness in large arteries. Both HS groups showed no significant difference in either SI or vascular age in multivariate analysis, when compared with controls (Table 2). DISCUSSION This study suggests that decreased vascular tone and stiffness of small arteries may be associated with severe HS, and at the same time found no difference in arterial stiffness in large arteries. The significance for the out-patient clinic HS group, but not the population HS group may reflect a dose-response relationship. Vascular tone in vascular smooth muscle cells of small arteries depends on competing vasodilators and vasoconstrictors. We speculate that the inflammation of HS may induce a dysfunctional balance e.g. through increased TNF-alpha with subsequent increase of the vasodilator nitric oxide resulting in the lower arterial vascular tone observed. Additionally, mast cells are increased in HS [4], possibly increasing levels of the vasodilator histamine. HS patients often suffer from stress which could increase sympathetic activity, thereby adrenalin/cortisol and subsequent vasodilation in e.g. muscles. The more peripheral an artery is, the more collagen it contains and the stiffer it is. The finding of lower vascular tone may also be suggestive of a different elastin:collagen ratio in small arteries in HS. The healing process of HS lesions is known to involve scarring formation of sinus tracts [5], which may suggest a hypothesis of altered connective tissue. This study found no difference in SI expressing arterial stiffness of large arteries between HS and controls. Our previous study found an association between HS and myocardial infarction, but no association with stroke, nor peripheral arterial stiffness of lower extremities in medium/large arteries [6], suggesting regional differences in vascular beds in HS. The major limitation of the study is the missing values of pulse trace measurement (Table 1) creating possible selection bias. Although unable to draw any clinical conclusions, we believe these results may contribute to the future research of the complexity of HS and cardiovascular risk profiling. This study suggests that decreased vascular tone and stiffness of small arteries may be associated with severe HS, and at the same time found no difference in arterial stiffness in large arteries. The significance for the out-patient clinic HS group, but not the population HS group may reflect a dose-response relationship. Vascular tone in vascular smooth muscle cells of small arteries depends on competing vasodilators and vasoconstrictors. We speculate that the inflammation of HS may induce a dysfunctional balance e.g. through increased TNF-alpha with subsequent increase of the vasodilator nitric oxide resulting in the lower arterial vascular tone observed. Additionally, mast cells are increased in HS [4], possibly increasing levels of the vasodilator histamine. HS patients often suffer from stress which could increase sympathetic activity, thereby adrenalin/cortisol and subsequent vasodilation in e.g. muscles. The more peripheral an artery is, the more collagen it contains and the stiffer it is. The finding of lower vascular tone may also be suggestive of a different elastin:collagen ratio in small arteries in HS. The healing process of HS lesions is known to involve scarring formation of sinus tracts [5], which may suggest a hypothesis of altered connective tissue. This study found no difference in SI expressing arterial stiffness of large arteries between HS and controls. Our previous study found an association between HS and myocardial infarction, but no association with stroke, nor peripheral arterial stiffness of lower extremities in medium/large arteries [6], suggesting regional differences in vascular beds in HS. The major limitation of the study is the missing values of pulse trace measurement (Table 1) creating possible selection bias. Although unable to draw any clinical conclusions, we believe these results may contribute to the future research of the complexity of HS and cardiovascular risk profiling.

Development of HiSQOL: A Hidradenitis Suppurativa-Specific Quality of Life Instrument.

BACKGROUND: Hidradenitis suppurativa (HS) is a chronic and painful skin disease. In addition, HS lesions may be associated with pus and odour, potentially leading to significant stigma and, consequently, greatly affected quality of life (QOL). QOL is a multidimensional construct, which can be measured in various ways. However, generic or dermatologic QOL measures may not capture changes in QOL particularly affected in HS. Accordingly, patients and experts included in the HIdradenitis SuppuraTiva cORe outcomes set International Collaboration (HISTORIC) agreed that future clinical HS trials should measure HS-specific QOL. OBJECTIVES: To develop an HS-specific QOL instrument (HiSQOL, Hidradenitis Suppurativa Quality of life). METHOD: The initial phases of the questionnaire development, described in this study, included item generation by patient interviews, development of a pilot questionnaire, questionnaire refinement, and pilot testing. RESULTS: For item generation, 21 patients were interviewed individually or in focus groups. Analysis of the interviews identified 105 candidate items and, next, a pilot questionnaire was developed. Finally, item reduction and two rounds of pilot testing resulted in a 23-item questionnaire representing physical, psychological, and social QOL dimensions. CONCLUSIONS: We have comprehensively explored on HS's possible effect on the QOL of the affected individuals and identified a 23-item HS-specific QOL questionnaire. The questionnaire proved to be feasible, acceptable, and comprehensible in the second round of pilot testing. With HiSQOL, researchers can measure HS-specific QOL in future clinical trials, potentially enabling them to discover more effective treatment options. It is envisaged, that after thorough validation in a trial setting, a streamlined version of HISQOL may also become available for clinical use in daily practice.

Lactase Persistence, Milk Intake, and Adult Acne: A Mendelian Randomization Study of 20,416 Danish Adults.

Whether there is a causal relationship between milk intake and acne is unknown. We tested the hypothesis that genetically determined milk intake is associated with acne in adults using a Mendelian randomization design. LCT-13910 C/T (rs4988235) is associated with lactase persistence (TT/TC) in Northern Europeans. We investigated the association between milk intake, LCT-13910 C/T (rs4988235), and acne in 20,416 adults (age-range: 20⁻96) from The Danish General Suburban Population Study (GESUS). The adjusted observational odds ratio for acne in any milk intake vs. no milk intake was 0.93(95% confidence interval: 0.48⁻1.78) in females and 0.49(0.22⁻1.08) in males aged 20⁻39 years, and 1.15(95% confidence interval: 0.66⁻1.99) in females and 1.02(0.61⁻1.72) in males above 40 years. The unadjusted odds ratio for acne in TT+TC vs. CC was 0.84(0.43⁻1.62) in the age group 20⁻39 years, and 0.99(0.52⁻1.88) above 40 years. We did not find any observational or genetic association between milk intake and acne in our population of adults.

Dairy Intake and Acne Vulgaris: A Systematic Review and Meta-Analysis of 78,529 Children, Adolescents, and Young Adults.

A meta-analysis can help inform the debate about the epidemiological evidence on dairy intake and development of acne. A systematic literature search of PubMed from inception to 11 December 2017 was performed to estimate the association of dairy intake and acne in children, adolescents, and young adults in observational studies. We estimated the pooled random effects odds ratio (OR) (95% CI), heterogeneity (I²-statistics, Q-statistics), and publication bias. We included 14 studies (n = 78,529; 23,046 acne-cases/55,483 controls) aged 7⁻30 years. ORs for acne were 1.25 (95% CI: 1.15⁻1.36; p = 6.13 × 10-8) for any dairy, 1.22 (1.08⁻1.38; p = 1.62 × 10-3) for full-fat dairy, 1.28 (1.13⁻1.44; p = 8.23 × 10-5) for any milk, 1.22 (1.06⁻1.41; p = 6.66 × 10-3) for whole milk, 1.32 (1.16⁻1.52; p = 4.33 × 10-5) for low-fat/skim milk, 1.22 (1.00⁻1.50; p = 5.21 × 10-2) for cheese, and 1.36 (1.05⁻1.77; p = 2.21 × 10-2) for yogurt compared to no intake. ORs per frequency of any milk intake were 1.24 (0.95⁻1.62) by 2⁻6 glasses per week, 1.41 (1.05⁻1.90) by 1 glass per day, and 1.43 (1.09⁻1.88) by ≥2 glasses per day compared to intake less than weekly. Adjusted results were attenuated and compared unadjusted. There was publication bias (p = 4.71 × 10-3), and heterogeneity in the meta-analyses were explained by dairy and study characteristics. In conclusion, any dairy, such as milk, yogurt, and cheese, was associated with an increased OR for acne in individuals aged 7⁻30 years. However, results should be interpreted with caution due to heterogeneity and bias across studies.

Hidradenitis Suppurativa is Associated with Higher Heart Rate but Not Atrial Fibrillation: A Comparative Cross-sectional Study of 462 Individuals with Hidradenitis Suppurativa in Denmark.

Hidradenitis suppurativa (HS) is a chronic inflammatory dermatological disease with inflammatory mechanisms overlapping those of psoriasis, and both diseases have been associated with cardiovascular risk factors i.e. smoking and metabolic syndrome. Two studies have recently linked psoriasis with Atrial Fibrillation (AF). AF is the most frequently occurring cardiac arrhythmia in the general population and is typically accompanied by increased heart rate. Both AF and heart rate are linked with inflammation. The aim of the study was to investigate a potential association between HS and increased heart rate as well as AF. We performed a comparative cross-sectional study using digital measurements of heart rate and resting 12-lead electrocardiography (ECG) in combination with self-reported information when diagnosing AF. Our study comprised 32 individuals with HS from the hospital (the hospital HS group), 430 from the general population HS group (the population HS group), and 20,780 controls. Age and sex adjusted analysis demonstrated a significantly higher heart rate in the HS groups vs. controls (15% (range: 8-23%) higher for the hospital HS group and 4% (2-5%) higher for the population HS group). We found no association between HS and AF (P=0.1670).

The Follicular Skin Microbiome in Patients With Hidradenitis Suppurativa and Healthy Controls.

Importance: Although the pathogenesis of hidradenitis suppurativa (HS) remains enigmatic, several factors point to potential involvement of the cutaneous microbiome. Insight into the cutaneous microbiome in HS using next-generation sequencing may provide novel data on the microbiological diversity of the skin. Objective: To investigate the follicular skin microbiome in patients with HS and in healthy controls. Design, Setting, and Participants: This case-control study obtained punch biopsy specimens from patients with HS (lesional and nonlesional) and healthy controls between October 1, 2014, and August 1, 2016. Data were analyzed from March to November 2016. Patients with HS were recruited from the Department of Dermatology, Zealand University Hospital, Roskilde, Denmark. Biopsy specimens were analyzed at the Department of Microbiology and Infection Control, Statens Serum Institut, Copenhagen, Denmark. None of the participants received any antibiotics (systemic or topical therapy) within 1 month before the study. In patients with HS, biopsy specimens were obtained from lesional skin (axilla or groin) and nonlesional skin. Only nodules containing at least 1 visible hair follicle were biopsied. Biopsy specimens from healthy controls were obtained from the axilla only. Main Outcomes and Measures: The different microbiomes were investigated using next-generation sequencing targeting 16S and 18S ribosomal RNA. Results: The skin microbiome was characterized in 30 patients with HS (mean [SD] age, 46.9 [14.0] years; 19 [63% female]) and 24 healthy controls (mean [SD] age, 32.2 [12.0] years; 13 [54% female]). The next-generation sequencing data provided a previously unreported (to our knowledge) characterization of the skin microbiome in HS. The study demonstrated that the microbiome in HS differs significantly from that in healthy controls in lesional and nonlesional skin. Overall, the following 5 microbiome types were identified: Corynebacterium species (type I), Acinetobacter and Moraxella species (type II), Staphylococcus epidermidis (type III), Porphyromonas and Peptoniphilus species (type IV), and Propionibacterium acnes (type V). In lesional skin, microbiome types consisted predominantly of type I or type IV. Microbiome type IV was not detected in healthy controls. Several taxa, including Propionibacterium, showed a significantly higher relative abundance in healthy controls vs HS skin, indicating that Propionibacterium may be part of the pathogenesis in HS. Conclusions and Relevance: The study findings suggest a link between a dysbiotic cutaneous microbiome and HS.

Normal Skin Microbiota is Altered in Pre-clinical Hidradenitis Suppurativa.

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease defined by recurrent nodules, tunnels (sinus tracts) and scarring involving the intertriginous regions. The clinical course of HS is compatible with a biofilm-driven disease, and biofilm has been described in lesional HS skin. We therefore hypothesized that clinically unaffected HS skin would also have an increased presence of biofilm compared with that of healthy controls. We conducted a case-control study, investigating the morphology of the axillary skin microbiota. Peptide nucleic acid - fluorescence in situ hybridization probes were used in combination with confocal laser scanning microscopy. Significant differences were found in both distribution and quantity of the cutaneous microbiota in clinically non-affected axillary skin of patients with HS compared with healthy controls. Surprisingly, we detected fewer bacteria and less biofilm in patients with HS. The reduced microbiota in patients with HS may play an important role in the early course of the disease.

Validation of a Screening Questionnaire for Chronic Leg Ulcers.

The use of a validated screening questionnaire to identify individuals with chronic leg ulcers allows large-scale population-based studies to be conducted that measure and monitor the prevalence of the disease. The aim of this study was to design and validate such a screening questionnaire to identify patients with chronic leg ulcers. A simple 3-item questionnaire was developed at the Department of Dermatology, University Hospital of Zealand, Denmark. In total, 90 patients attending the department's outpatient clinic for dermatological diseases and chronic wounds were included in this study. All included participants completed the questionnaire and were subsequently examined by dermatologists. We found that the constructed 3-item questionnaire in this study had a sensitivity and specificity of 95% and 93% and a positive predictive value and negative predictive value of 78% and 95%, respectively. Moreover, we found that the use of the 3-item questionnaire, as compared with a single question, in which the participants were asked whether they currently have a leg ulcer, resulted in significantly higher positive predictive value (+11.6%, P = .035) and specificity (+5.6%, P = .046) of the diagnostic test. Future studies are merited to investigate the diagnostic accuracy of the questionnaire in other populations and settings.

Leukocyte Profile in Peripheral Blood and Neutrophil-Lymphocyte Ratio in Hidradenitis Suppurativa: A Comparative Cross-Sectional Study of 462 Cases.

BACKGROUND: Hidradenitis suppurativa (HS) is a chronic, inflammatory skin disease. Increasing evidence suggests that HS involves dysfunctional immune responses in both the adaptive and the innate immune system. The recently proposed association of HS with metabolic syndrome may further add to the inflammatory state in HS. OBJECTIVE: To investigate the status of inflammation and leukocyte profile in the peripheral blood of HS patients. MATERIALS AND METHODS: Using a comparative cross-sectional study design, we investigated blood samples of high-sensitivity C-reactive protein (hs-CRP) and leukocyte profile in hospital-treated HS patients (HS-HOSP), self-reported population-based HS patients (HS-POP) and population controls. RESULTS: Our study comprised 32 individuals in the HS-HOSP group, 430 in the HS-POP group, and 20,780 controls. The median hs-CRP for the HS-HOSP group was 5.1 mg/l (quartile range 2.6-8.2), 2.2 mg/l (1.0-4.3) for the HS-POP group and 1.3 mg/l (0.7-2.9) for the controls. An age-sex-adjusted analysis revealed a significantly higher hs-CRP for both HS groups compared to controls (p < 0.0001). When performing age-sex-adjusted analysis, both HS groups had significantly higher odds of leukocytosis when compared to controls with an odds ratio for the HS-HOSP group of 4.38 (95% CI = 2.18-8.80; p < 0.0001) and 1.95 (95% CI = 1.58-2.42; p < 0.0001) for the HS-POP group. The age-sex-adjusted leukocyte differential count yielded significantly higher neutrophil (p < 0.0001) and monocyte (p = 0.0014, p = 0.0004) levels in the HS groups compared with controls. CONCLUSION: The hs-CRP levels associated with HS appear to be intermediate (2.2-5.1 mg/l), implying systemic inflammation rather than infection. The peripheral blood leukocytosis in HS was dominated by neutrophils and monocytes.

Renal amyloid A amyloidosis as a complication of hidradenitis suppurativa.

Rheumatic disease is the dominant cause of amyloid A (AA) amyloidosis, but other chronic inflammatory diseases may have similar consequences. Hidradenitis suppurativa (HS) is a relatively common, but little known skin disease characterized by chronic inflammation. Here we present a case of chronic HS leading to biopsy-verified severe renal AA amyloidosis and dialysis dependency.

Differences in Disease-specific Quality of Life in Patients with Actinic Keratosis in Australia and Denmark.

Actinic keratosis (AK) negatively influences patient quality of life as measured by the disease-specific Actinic Keratosis Quality of Life (AKQoL) questionnaire. The quality of life in Australian patients was significantly less affected than in Danish patients. We hypothesize that general factors such as public awareness and cultural connotations of AK, may influence the impact of AK on quality of life (QoL).

Self-reported skin morbidity in Denmark: a population-based cross-sectional study.

Skin diseases are thought to be common in the general population. In 2004, a cross-sectional study in Norway, using a validated questionnaire for 18,770 individuals, revealed a high prevalence of skin diseases in the general population. To describe the prevalence of self-reported skin morbidities and possible socio-demographic and economic associations in the Danish general population, and furthermore compare this data to that reported in Norway. A population-based cross-sectional study of the adult Danish suburban population was performed. Participants (n = 20,164) completed the Norwegian validated questionnaire. In total, 17.2% self-reported skin complaints. The most prominent self-reported skin complaint was itch with an overall prevalence of 6.5%. The skin morbidity most influenced by age was pimples. There was a uniform pattern showing fewer skin complaints with increasing education. Women reported skin morbidities more frequently than men. Participants in employment reported fewer skin morbidities compared to unemployed participants. Skin morbidities in Denmark are common, and the distribution of prevalence estimates in the Danish population parallel those of the Norwegian population, although the prevalence is generally higher in Norway. This difference may be attributed to climatic differences, and age, education, sex, and employment status may also be influential.

Prevalence, Risk Factors, and Comorbidities of Hidradenitis Suppurativa.

It is challenging to estimate a true prevalence of hidradenitis suppurativa (HS) because it is underdiagnosed and misdiagnosed. Prevalences have been reported from 0.00033% to 4.1%. The incidence seems to be rising. In addition to dermatologic symptoms, HS is associated with metabolic syndrome, and increased cardiovascular risk. The majority of HS patients are smokers. Additional somatic comorbidities complicating HS include autoimmune conditions, follicular syndromes, rheumatologic conditions, and malignancies. HS patients are troubled by psychological comorbidities. When treating HS patients it is imperative not only to treat the skin symptoms, but also address the screening and treatment of possible comorbidities.

A Population- and Hospital-based Cross-sectional Study of Renal Function in Hidradenitis Suppurativa.

The chronic inflammatory skin diseases hidradenitis suppurativa (HS) and psoriasis have been linked to cardiovascular risk factors and the latter has also been linked to possible renal dysfunction. Since basement membrane thinning in the skin of HS patients has been described, we speculated whether similar basement membrane defects might occur in renal tissue. Our objective was to investigate a possible association between HS and renal dysfunction. We performed a hospital and population-based cross-sectional study using estimated Glomerular-Filtration-Rate (eGFR) to assess renal function. Thirty-two hospital individuals with HS, 430 population individuals with HS, and 20,780 population individuals without HS were (controls) identified. The age-, sex-, smoking-, BMI-, hypertension- and diabetes-adjusted analysis revealed a statistically significant higher eGFR for the hospital group with HS and a mean difference in eGFR of 6.81 (1.27-12.35) ml/min/1.73 m2 between the hospital group with HS and the population group without HS. The observed higher eGFR in the hospital group with HS indicates a possible association of HS and renal dysfunction.

Gallstones are associated with hidradenitis suppurativa: a population-based and hospital-based cross-sectional study from Denmark.

BACKGROUND: Hidradenitis suppurativa (HS) is a chronic inflammatory dermatological disease that was recently linked to the metabolic syndrome (MetS). MetS has been associated with gallstones, and nonalcoholic fatty liver has been suggested to be the hepatic expression of MetS. OBJECTIVE: The objective of the study was to investigate whether there was an association of HS with gallstones as well as with hepatic dysfunction. PATIENTS AND METHODS: This was a cross-sectional study comparing a hospital-based HS group, a population-based HS group, and controls for self-reported gallstone and blood sample verified hepatic dysfunction. Blood samples were analyzed for alanine transaminase, bilirubin, alkaline phosphatase, albumin, thrombocytes, and the international normalized ratio. RESULTS: A total of 32 hospital HS patients, 430 population-based HS patients, and 20 780 non-HS controls were identified. The age-sex-smoking-adjusted analysis of gallstones revealed a significant OR of 1.72 (95% CI 1.23-2.42, P=0.0191) and a borderline significant OR of 3.28 (95% CI 1.24-8.74, P=0.0516) for the population HS group and hospital HS group versus controls, respectively. Furthermore, no clinically significant evidence was found with regard to hepatic dysfunction. CONCLUSION: This study demonstrates an association of HS with gallstones, but not with hepatic dysfunction. The association with gallstones may be partly explained by the comorbidity of hypertriglyceridemia and obesity as a part of MetS.

Co-morbidities in inflammatory dermatological diseases. Psoriasis, hidradenitis suppurativa, and cardiovascular risk factors.

In conclusion, this thesis demonstrated an association between inflammatory dermatological diseases, i.e. psoriasis and hidradenitis suppurativa, and the metabolic syndrome putting these two patient groups at cardiovascular risk. Therefore, it is recommended as a minimum to screen hidradenitis and psoriasis patients attending in/outpatient clinics for the metabolic syndrome aimed at prevention of cardiovascular disease. The increased risk of metabolic syndrome adds to the range of well-known disease-related burdens e.g. the physical skin symptoms, the psychological impact thereof, and other co-morbidities, thus highlighting that both hidradenitis and psoriasis patients require general medical attention beyond the skin.

Coagulation Status in Hidradenitis Suppurativa: A Danish Population- and Hospital-Based Cross-Sectional Study.

BACKGROUND: Chronic inflammatory diseases other than hidradenitis suppurativa (HS) have been associated with prothrombotic/hypercoagulable status. OBJECTIVE: To investigate a possible association between the chronic inflammatory skin disease HS and prothrombotic/hypercoagulable state. METHODS: We performed a hospital- and population-based cross-sectional study investigating the coagulation status (thrombocytes, mean platelet volume [MPV], international normalized ratio [INR] and activated partial thromboplastin time [APTT]). RESULTS: 32 hospital HS subjects, 430 population HS subjects and 20,780 population non-HS control subjects were identified. The adjusted analyses showed no differences in the levels of thrombocytes, MPV, INR or APTT between the HS groups (hospital HS group, population HS group) when compared to controls (p = 0.089, p = 0.3078; p = 0.5499, p = 0.0659; p = 0.0932; p = 0.3432). CONCLUSION: We did not find an association between HS and prothrombotic/hypercoagulable status. Thus, thrombocytes may not be activated in HS. Furthermore, INR may not be affected in HS, suggesting that intrinsic and vitamin K-dependent coagulation factors appear unaffected.

The bacteriology of hidradenitis suppurativa: a systematic review.

Hidradenitis suppurativa (HS) is a chronic inflammatory disabling skin disease consisting of recurrent nodules, sinuses, fistulas and scarring involving the intertriginous regions. HS is often a therapeutic challenge and most treatments are off-label. A better understanding of aetiology and pathogenesis of HS may facilitate the development of effective treatment. Although the clinical presentation is strongly reminiscent of bacterial infection, the role of bacteria remains controversial. Studies have isolated an array of different bacteria specimens. Consistent findings of Gram-positive cocci and Gram-positive rods including Staphylococus aureus, coagulase-negative staphylococci (CoNS) and Corynebacterium species in deep tissue samples have been demonstrated in HS and may constitute a central target for the immune system. Efficacy of antibiotics, that is rifampicin, clindamycin or tetracycline, supports a microbial role in disease pathogenesis. However, these antibiotics also work as immunomodulators of especially T cells, and the underlying mechanisms may therefore be more complex. We performed a systematic review of previous studies investigating the bacterial flora in hidradenitis suppurativa. We searched PubMed, EMBASE, Royal Danish Library and Cochrane library (search date 11 December 2014). A total of 66 papers were identified and nine papers published between 1988 and 2014 matched our inclusion criteria, yielding bacteriological data of a total of 324 patients with HS (mean age 36.8 years and female/male ratio 215/109). This overview of the bacteriology may aid researchers and physicians exploring the potential role of bacteria in HS. Furthermore, to stimulate a broader debate, we also present different viewpoints on the possible role of bacteria in HS.