The aim of the present study was to depict the microbiome composition of our HS patients as bacterial dysbiosis has repeatedly been linked to its pathogeny. Lesional and perilesional skin was compared to that of healthy controls. The skin microbiota can exhibit substantial variations, depending on geographical location, diet, climate, or other lifestyle-related factors. Therefore, it seemed advisable to conduct a study on patients from our area of influence. To the best of our knowledge, no study has been published on this matter concerning Spanish patients, representing the characteristics of the Mediterranean Europe. Our study detected higher variation in species abundances in controls vs HS patients, but no differences in evenness or diversity, in line with previous published studies that conclude similar richness and higher evenness in HS skin4 or no differences in richness5.
The published studies on the microbiome of HS have yielded diverse results, sharing a common demonstration of an imbalance, with a higher detection of gram-negative anaerobes and opportunistic pathogens such as Prevotella or Porphyromonas, along with a decrease in Propionibacterium and gram-positive cocci. Also, some bacterial species have been related to different types of lesions,14 being Staphylococcus Lugdunensis more abundant in nodules and abscesses, and anaerobes with chronic draining lesions. However, results have not always been replicated in subsequent studies. In our results, it is possible to clearly differentiate the three types of analyzed samples: lesional, perilesional, and healthy skin, into three distinct clusters. This fact confirms the differences in the microbiota as the skin becomes affected by the disease and detects distinctions between perilesional skin and that of healthy controls, unlike some previous studies. We found an abundance of Propionibacterium and Staphylococcus in controls, while Mycoplasma and Bacillus were more predominant in patients, which partially seems to align with previous publications. Such abundance of Mycoplasma in particular, has not been previously reported. The analysis of clusters based on dermatotypes or bacterial groups yielded slightly different results. We detected a predominance of Anaerococcus and Peptoniphilus (gram-positive anaerobes) in lesional skin, and Mycoplasma (mostly facultative anaerobes) and Bacillus in lesional and perilesional skin. Corynebacterium appeared to be characteristic of healthy or perilesional skin, and Propionibacterium (Cutibacterium) was more prevalent in healthy controls. In fact, the loss of Cutibacterium is quite consistent in the results of published studies and is also observed in our findings. Regarding possible differences in HS severity, we confirmed that the most consistent finding was the decrease in Propionibacterium, mimicking the same gradient found in perilesional vs healthy skin. We failed to demonstrate microbiome differences with regards to lesion type (probably because most of the lesions we sampled were tunnels and we had less representation of nodules and abscesses) and HS endotype, which yields the conclusion that, regardless of the clinical presentation or the potentially different pathogenic pathways in every endotype, the already formed lesions exhibit a similar dysbiotic pattern.
The results of the present study offer data similar to what has already been published15, but they present new findings that could be explained by the inclusion of patients from the Mediterranean area, in whom studies have been scarcely performed compared to patients from Northern Europe. We have found that Anaerococcus and Peptoniphilus predominate in HS lesions, as well as Mycoplasma and Bacillus. Mycoplasmas are well-known proinflammatory bacteria16, and their binding to Toll-like receptor triggers several pathways responsible for inflammation, such as NF-KB, the inflammasome and proinflammatory cytokines with a well-known role in HS pathogenesis (IL-6, IL-1beta, TNF-alpha). Commonly used antibiotics for HS (doxycycline, clindamycin or minocycline) are effective against mycoplasma and part of their antiinflammatory action in HS could be explained by their capacity for controlling the abundance of this bacteria.
It should be noted that by obtaining samples through biopsy, many of which were taken from deeper areas, we have detected the presence of bacteria that may not be evident using more superficial collection techniques like swabs. Most bacteria detected are anaerobic, in line with previous publications. Microorganisms such as Porphyromonas, Bacteroides and Prevotella (anaerobic gram-negative rods), repeatedly found in previous studies as typical constituents of HS lesional tissue, have not been found as relevant in the present study
Corynebacterium appears to be a more relevant constituent in the perilesional skin and controls, and its presence decreases as the disease becomes more severe. In a similar way, Propionibacterium seems to decrease with severity, being abundant in healthy skin of controls and scarce in lesional HS skin. This disbalance in gram-positive rods is consistently found in many previous studies and seems to be crucial in HS dysbiosis.
The limitations of this study are its cross-sectional design, the limited number of samples, and the scarce representation of nodules and abscesses. Only v3-v4 regions of ribosomal 16s RNA were analyzed, and we did not attempt the detection of fungal or viral genetic material. Its strength lies in the fact that there was abundant material for the microbiome analysis, and from deeper areas of the lesions, because it was obtained by skin biopsies and not skin swabs.