Propionibacterium acnes: an update on its role in the pathogenesis of acne

Topical acne treatments in Europe and the issue of antimicrobial resistance

Acne vulgaris (acne) is a chronic inflammatory disease of the sebaceous gland, characterized by follicular hyperkeratinization, excessive colonization by Propionibacterium acnes (P. acnes) as well as immune reactions and inflammation. Despite an armamentarium of topical treatments available including benzoyl peroxide, retinoids and azelaic acid, topical antibiotics in monotherapies, especially erythromycin and clindamycin, are still used in Europe to treat acne. This intensive use led to antimicrobial-resistant P. acnes and staphylococci strains becoming one of the main health issues worldwide. This is an update on the current topical acne treatments available in Europe, their mechanism of action, their potential to induce antimicrobial resistance and their clinical efficacy and safety.

Multiplex touchdown PCR for rapid typing of the opportunistic pathogen Propionibacterium acnes

The opportunistic human pathogen Propionibacterium acnes is composed of a number of distinct phylogroups, designated types IA1, IA2, IB, IC, II, and III, which vary in their production of putative virulence factors, their inflammatory potential, and their biochemical, aggregative, and morphological characteristics. Although multilocus sequence typing (MLST) currently represents the gold standard for unambiguous phylogroup classification and individual strain identification, it is a labor-intensive and time-consuming technique. As a consequence, we developed a multiplex touchdown PCR assay that in a single reaction can confirm the species identity and phylogeny of an isolate based on its pattern of reaction with six primer sets that target the 16S rRNA gene (all isolates), ATPase (types IA1, IA2, and IC), sodA (types IA2 and IB), atpD (type II), and recA (type III) housekeeping genes, as well as a Fic family toxin gene (type IC). When applied to 312 P. acnes isolates previously characterized by MLST and representing types IA1 (n=145), IA2 (n=20), IB (n=65), IC (n=7), II (n=45), and III (n=30), the multiplex displayed 100% sensitivity and 100% specificity for detecting isolates within each targeted phylogroup. No cross-reactivity with isolates from other bacterial species was observed. This multiplex assay will provide researchers with a rapid, high-throughput, and technically undemanding typing method for epidemiological and phylogenetic investigations. It will facilitate studies investigating the association of lineages with various infections and clinical conditions, and it will serve as a prescreening tool to maximize the number of genetically diverse isolates selected for downstream higher-resolution sequence-based analyses.

Recent advances in acne pathogenesis: implications for therapy

Acne pathogenesis is a multifactorial process that occurs at the level of the pilosebaceous unit. While acne was previously perceived as an infectious disease, recent data have clarified it as an inflammatory process in which Propionibacterium acnes and innate immunity play critical roles in propagating abnormal hyperkeratinization and inflammation. Alterations in sebum composition, and increased sensitivity to androgens, also play roles in the inflammatory process. A stepwise approach to acne management utilizes topical agents for mild to moderate acne (topical retinoid as mainstay ± topical antibiotics) and escalation to oral agents for more resistant cases (oral antibiotics or hormonal agents in conjunction with a topical retinoid or oral isotretinoin alone for severe acne). Concerns over antibiotic resistance and the safety issues associated with isotretinoin have prompted further research into alternative medications and devices for the treatment of acne. Radiofrequency, laser, and light treatments have demonstrated modest improvement for inflammatory acne (with blue-light photodynamic therapy being the only US FDA-approved treatment). However, limitations in study design and patient follow-up render these modalities as adjuncts rather than standalone options. This review will update readers on the latest advancements in our understanding of acne pathogenesis and treatment, with emphasis on emerging treatment options that can help improve patient outcomes.

Propionibacterium acnes activates the NLRP3 inflammasome in human sebocytes

Propionibacterium acne and sebaceous glands are considered to have an important role in the development of acne. Although information regarding the activation of innate immunity by P. acnes in the sebaceous gland is limited, different P. acnes phylotypes and a higher prevalence of follicular P. acnes macrocolonies/biofilms in sebaceous follicles of skin biopsies from acne compared with control skin and occasionally single P. acnes clusters in single sebaceous glands have been detected. In this study, we investigated whether P. acnes activates the inflammasome in human sebaceous glands in vivo and in vitro. We found that IL-1β expression was upregulated in sebaceous glands of acne lesions. After stimulation of human sebocytes with P. acnes, the activation of caspase-1 and secretion of IL-1β were enhanced significantly. Moreover, knocking down the expression of NLRP3 abolished P. acnes-induced IL-1β production in sebocytes. The activation of the NLRP3 inflammasome by P. acnes was dependent on protease activity and reactive oxygen species generation. Finally, we found that NALP3-deficient mice display an impaired inflammatory response to P. acnes. These results suggest that human sebocytes are important immunocompetent cells that induce the NLRP3 inflammasome, and that P. acnes-induced IL-1β activation in sebaceous glands may have a role in combating skin infections and in acne pathogenesis.

Immunotherapy for acne vulgaris: current status and future directions

There is a high unmet clinical need for new and better treatments in acne vulgaris. Propionibacterium acnes has a strong proinflammatory activity and targets molecules involved in the innate cutaneous immunity, keratinocytes and sebaceous glands of the pilosebaceous follicle. The role of P. acnes in acne confers legitimacy on the possible benefits of immunization-based approaches, which may represent a solution for limiting the development of antibiotic-resistant P. acnes. Various immunization-based approaches have been developed over the last decades, including killed pathogen-based vaccines, vaccination against cell wall-anchored sialidase, monoclonal antibodies to the Christie, Atkins, Munch-Peterson factor of P. acnes, anti-Toll-like receptors vaccines and natural antimicrobial peptides. This review summarizes the current evidence and explores the challenges to making this a realistic treatment option for the future.