Key points

Introduction

Although an increasing body of literature points to dysfunctional immune responses playing a key role in hidradenitis suppurativa (HS), involving both the innate and adaptive immune systems, the role of the microbiology remains in dispute. However, the clinical presentation is strongly reminiscent of bacterial infection and patients are often treated with antibiotics (eg, penicillins or incision) and drainage by nondermatologists, even though the treatments are regularly ineffective.

Whether bacterial colonization is a primary or secondary event in the evolution of HS lesions is a subject of much debate. In acne vulgaris a well-established correlation between disease development and Propionibacterium acnes are described. No such clear association between a specific bacterium and the pathophysiology of HS has been shown. The literature on HS and its potential association with bacteria may seem bewildering, which may be attributed to the lack of consistency in methodology and different bacteriologic findings. Nevertheless, several studies have isolated an array of bacterial specimens sporadically associated with lesional HS tissue. Consistent findings of gram-positive cocci and rods, including Staphylococcus aureus , coagulase-negative staphylococci (CoNS) and Corynebacterium spp in deep tissue samples have been shown in HS. Although they are a part of the normal human skin flora they have been speculated to constitute a central target for the immune system in patients with HS. Moreover, the efficacy of antibiotics (ie, rifampicin, clindamycin, or tetracycline) in HS treatment further supports a microbial role in disease pathogenesis. However, these antibiotics also work as immunomodulators, especially of T cells, and the underlying mechanisms may therefore be more complex.

Introduction

Although an increasing body of literature points to dysfunctional immune responses playing a key role in hidradenitis suppurativa (HS), involving both the innate and adaptive immune systems, the role of the microbiology remains in dispute. However, the clinical presentation is strongly reminiscent of bacterial infection and patients are often treated with antibiotics (eg, penicillins or incision) and drainage by nondermatologists, even though the treatments are regularly ineffective.

Whether bacterial colonization is a primary or secondary event in the evolution of HS lesions is a subject of much debate. In acne vulgaris a well-established correlation between disease development and Propionibacterium acnes are described. No such clear association between a specific bacterium and the pathophysiology of HS has been shown. The literature on HS and its potential association with bacteria may seem bewildering, which may be attributed to the lack of consistency in methodology and different bacteriologic findings. Nevertheless, several studies have isolated an array of bacterial specimens sporadically associated with lesional HS tissue. Consistent findings of gram-positive cocci and rods, including Staphylococcus aureus , coagulase-negative staphylococci (CoNS) and Corynebacterium spp in deep tissue samples have been shown in HS. Although they are a part of the normal human skin flora they have been speculated to constitute a central target for the immune system in patients with HS. Moreover, the efficacy of antibiotics (ie, rifampicin, clindamycin, or tetracycline) in HS treatment further supports a microbial role in disease pathogenesis. However, these antibiotics also work as immunomodulators, especially of T cells, and the underlying mechanisms may therefore be more complex.

Normal bacterial flora of the skin

A vast variety of microorganisms such as bacteria, fungi, and arthropods colonize the human skin. About 1 trillion commensal bacteria exist on the human skin surface, with Actinobacteria, Proteobacteria, Firmicutes, and Bacteroidetes as the overall dominant phyla. The skin supports the growth of commensal bacteria, which serves as an essential defense mechanism against pathogenic bacteria both directly and indirectly. For bacteria to become pathogenic they must possess specific properties, such as the ability to adhere, grow, and invade the host. If organisms succeed in evading the cutaneous host defenses, the next line of protection involves the immune system, or skin-associated lymphoid tissue. However, the human skin is, in general, an unfavorable environment for bacterial growth because of the predominant dry areas on the skin surface.

An important feature of the skin is the topographic diversity of the bacterial populations, which depends on temperature, age, the amount of sebum, sweat production, hormonal status, and local humidity. Further, differentially distributed hair follicles, eccrine glands, apocrine glands, and sebaceous glands contribute to the variable cutaneous microenvironments, and are likely selective for the subsets of bacteria that can thrive in those specialized conditions. The sites affected in HS (intertriginous areas) offer preferable conditions for bacterial growth because of their high levels of local humidity, sebaceous glands, sweat, and hair follicles.

The spectrum of bacteria found in patients with hidradenitis suppurativa

Fig. 1 shows that CoNS (34.1%) and anaerobic bacteria (23.3%) were the most frequently encountered types of bacteria among the 324 patients included from 9 studies (see Fig. 1 , Table 1 ). With regard to the overall categorization of the bacteria, facultative anaerobic bacteria constitute the highest percentage (69.6%) followed by strictly anaerobic bacteria (18.1%) and strictly aerobic bacteria (8.5%), whereas a less significant number of species were ungroupable (3.4%).

Fractional distribution of various bacteria found in 9 studies. Swabs, biopsies, blood, and needle aspiration of purulent content are included. Bacteria that were not clearly identified as any of the bacteria listed earlier were not included in this presentation; for example, anaerobe streptococci, microaerophilic streptococci, anaerobe gram-positive cocci, and lactobacillus are classified as Other.

As seen in Fig. 1 , the studies show a high prevalence of CoNS/coagulase positive staphylococci CoPS with a considerable variety of other species isolated from the inflamed lesions of patients with HS. Although there is no consistency with regard to the occurrence of the type of bacteria, all studies show predominantly positive culturing samples from HS lesions. The polymicrobial flora, and in particular the dominating occurrence of S aureus /CoNS in the HS lesions, may raise speculations on the pathogenetic significance of this recurring bacteriologic finding. However, although the microflora/microbiota of the normal human skin has been described, no studies investigating the bacteriology in HS have included a healthy control group, which makes unequivocal conclusions on the possible pathogenic role of S aureus and CoNS, and the general polymicrobial flora, difficult. Moreover, subtyping of CoNS/CoPS is generally lacking. Nevertheless, a causal involvement of bacteria seems reasonable to many physicians, because the disease often responds to antibiotics and also has a clinical picture that resembles well-known colonization of wounds (suppuration).