Why is acne not considered an infection, despite the presence of pathogenic bacteria, such as Propionibacterium (P.) acnes?

From the Guidelines

Acne is not considered an infection because it's primarily an inflammatory condition, not an invasion of pathogenic bacteria. While bacteria (specifically Cutibacterium acnes, formerly Propionibacterium acnes) are involved in acne development, they are normal residents of the skin and not external invaders. Acne develops when hair follicles become clogged with oil and dead skin cells, creating an environment where these normal skin bacteria can overgrow. The immune system then responds to this overgrowth and to the substances these bacteria produce, causing inflammation that manifests as pimples, nodules, or cysts. This differs from true infections where pathogenic microorganisms invade tissues where they don't normally reside.

Key Points in Acne Management

• Treatment for acne often includes topical retinoids like adapalene (0.1% gel), benzoyl peroxide (2.5-5%), or antibiotics like clindamycin (1% solution) to reduce bacteria and inflammation, as supported by moderate certainty evidence from 14 studies 1.
• For moderate to severe acne, oral medications such as antibiotics (doxycycline 100mg daily for 3 months), hormonal treatments (combined oral contraceptives), or isotretinoin (0.5-1mg/kg/day for 4-6 months) may be prescribed.
• Understanding that acne is inflammatory rather than infectious helps explain why treatment approaches focus on multiple factors including reducing oil production, preventing follicular plugging, and controlling inflammation, not just killing bacteria.

Considerations in Treatment

• The use of antibiotics in acne treatment is based on their antibacterial and anti-inflammatory effects, as noted in the guidelines of care for the management of acne vulgaris 1.
• The certainty of evidence for acne treatment is categorized into high, moderate, low, and very low certainty evidence, with recommendations made based on the highest level of evidence available 1.
• Routine endocrinologic testing is not indicated for most patients with acne, but may be considered for patients presenting with clinical signs or symptoms of hyperandrogenism 1.

From the Research

Definition of Infection and Acne

• An infection is typically defined as an invasion of pathogenic microorganisms, such as bacteria, viruses, or fungi, into a host organism, leading to a range of symptoms and potentially severe consequences.
• Acne, on the other hand, is a chronic inflammatory condition of the pilosebaceous follicles, characterized by excessive sebum production, hyperkeratinization, and the presence of Propionibacterium acnes (P. acnes) 2.
• While P. acnes is a type of bacteria, acne is not considered a traditional infection in the sense that it is not caused by an external pathogen invading the body.

Role of P. acnes in Acne

• P. acnes is a commensal bacterium that is normally present on the skin and plays a role in the development of acne by breaking down sebum and producing pro-inflammatory factors 2, 3.
• The presence of P. acnes is necessary for the development of acne, but it is not the sole cause of the condition.
• Other factors, such as hormonal fluctuations, genetics, and environmental factors, also contribute to the development of acne.

Treatment of Acne

• Acne treatment typically involves a combination of topical and oral medications, such as retinoids, antibiotics, and hormonal therapies 4, 5.
• Antibiotics, such as doxycycline and minocycline, are commonly used to treat acne, but their use can contribute to the development of antibiotic resistance 5, 3.
• Newer treatments, such as topical peroxisome proliferator-activated receptor gamma (PPARγ) modulation and probiotics, are being studied as potential alternatives to traditional antibiotic therapies 6.

Microbiome and Acne

• The skin microbiome plays a crucial role in the development and treatment of acne, with an imbalance of the microbiome (dysbiosis) potentially contributing to acne pathogenesis 6.
• Current acne treatments, such as antibiotics and benzoyl peroxide, can disrupt the balance of the skin microbiome, leading to potential long-term consequences 6.
• Further research is needed to understand the complex relationship between the skin microbiome and acne, and to develop effective treatments that minimize disruption to the microbiome.