Acne vulgaris affects approximately 85% of adolescents and persists into adulthood in a significant proportion, making it among the most common skin conditions. Despite this prevalence, acne pathogenesis remains incompletely understood, with traditional focus on Cutibacterium acnes (formerly Propionibacterium acnes) missing important nuances about phylotype diversity, resistance to commensalism, and immune dysregulation.
Cutibacterium acnes colonizes sebaceous pilosebaceous units (hair follicles with attached sebaceous glands) in virtually all humans, yet only some develop acne. This paradox highlights that C. acnes alone doesn't cause acne—rather, specific C. acnes phylotypes combined with immune dysregulation and altered follicular microenvironment drive pathogenesis. C. acnes encompasses distinct phylotypes with dramatically different inflammatory properties. The IA1 phylotype is predominantly associated with inflammatory acne and shows enhanced ability to trigger TLR2-mediated inflammation compared to other phylotypes. Non-inflammatory phylotypes exist as apparent commensals producing less inflammation. During acne development, IA1 phylotype prevalence increases, suggesting that selective expansion of inflammatory phylotypes, rather than absolute C. acnes abundance alone, drives acne.
The lipogenic mechanism provides the nutrient substrate for C. acnes proliferation. Sebaceous glands produce sebum composed primarily of triglycerides, with smaller proportions of free fatty acids, waxes, squalene, and cholesterol. C. acnes produces lipases that hydrolyze sebum triglycerides into free fatty acids. These free fatty acids are C. acnes's preferred nutrient and also have pro-inflammatory properties—free fatty acids activate TLR2 and NLRP3 inflammasome, driving IL-1β production. Notably, sebum lipid composition varies among individuals, with some having higher proportions of pro-inflammatory lipid species, contributing to acne susceptibility.
The bacterial biofilm in comedones represents a critical pathogenic structure. In acne, C. acnes organisms aggregate in follicular microcomedo microenvironments, embedded in biofilms composed of bacterial polysaccharides and host cell material. Within biofilms, C. acnes cells become phenotypically distinct from planktonic cells: they show altered gene expression patterns, reduced antibiotic susceptibility, and enhanced persistence despite immune attack. The biofilm provides physical protection from immune responses while concentrating inflammatory mediators and bacterial toxins. Biofilm formation is facilitated in the acne follicle by altered follicular environment: reduced oxygen (favoring C. acnes), accumulated sebum, and specific small molecules that induce biofilm formation.
Normal skin microbiota colonization of hair follicles prevents pathogenic C. acnes expansion. Healthy skin contains abundant Staphylococcus epidermidis and Corynebacterium species that apparently inhibit C. acnes through competitive exclusion and antimicrobial compound production. The loss of this protective microbiota in acne individuals allows C. acnes selective expansion. Why protective microbiota is lost in acne remains incompletely understood but likely involves both intrinsic sebaceous unit factors (altered lipid composition, follicular plugging) and systemic factors (hormonal changes driving sebaceous gland hyperactivity).
Immune dysregulation in acne involves multiple abnormalities. Follicular epithelial cells and sebocytes in acne-prone individuals produce elevated IL-1β in response to C. acnes and free fatty acids, partially through NLRP3 inflammasome activation. This IL-1β amplifies neutrophil recruitment and keratinocyte proliferation. Additionally, acne-prone individuals show altered Th1/Th17 responses to C. acnes antigens, with some evidence suggesting enhanced Th17 responses that might drive IL-17-mediated inflammation.
Antibiotic resistance in C. acnes has emerged as a critical clinical problem. Widespread use of oral antibiotics (primarily doxycycline and minocycline) and topical antibiotics (erythromycin, clindamycin) has generated widespread resistance among C. acnes populations. Resistance rates to erythromycin exceed 60% in some populations. This creates a therapeutic dilemma: antibiotics remain effective initially but resistance rapidly develops, and antibiotic use selects for resistant C. acnes while eliminating susceptible commensal bacteria that provide protective functions.
Microbiome-preserving treatment approaches are increasingly advocated. Retinoids (particularly isotretinoin for severe acne) directly suppress sebaceous gland sebum production and follicular epithelial hyperkeratinization, addressing upstream pathogenic mechanisms without directly killing C. acnes. Notably, isotretinoin dramatically reduces but doesn't eliminate C. acnes, suggesting that reduced sebum and follicular obstruction are sufficient to control acne despite C. acnes persistence. Benzoyl peroxide has selective antimicrobial activity against C. acnes while sparing other bacteria—this selectivity compared to antibiotics makes it antimicrobially non-disruptive. Oral isotretinoin treatment, despite its efficacy in severe acne, causes transient dysbiosis with subsequent microbiota recovery, illustrating the disruption of protective bacteria by intensive therapies.
Emerging approaches target C. acnes biofilms through biofilm-disrupting enzymes or combination therapies targeting multiple mechanisms. Probiotics colonizing sebaceous units could theoretically outcompete pathogenic C. acnes strains, though feasibility remains unproven. Understanding acne as a dysbiosis-driven condition with specific pathogenic C. acnes phylotypes, rather than a simple bacterial infection, supports more nuanced approaches preserving protective microbiota while selectively targeting pathogenic strains.