Skin Infections and Structure Notes

Skin Structure and Function

  • Human skin consists of two primary layers: the epidermis and the dermis, which lie atop the hypodermis.
  • The skin serves as a crucial barrier, protecting against microbial invasion.
  • Skin dryness and the presence of normal microbiota inhibit colonization by transient microbes.
  • The composition of the normal microbiota varies across different regions of the body.
  • The conjunctiva is susceptible to microbial infections; however, deeper eye infections are less prevalent. Conjunctivitis presents in various forms.

Bacterial Skin Infections

  • Staphylococcus and Streptococcus species can cause skin infections upon entering the body through cuts or wounds.
  • S. aureus often results in purulent infections, including:
    • Folliculitis: Inflammation of hair follicles.
    • Furuncles: Boils.
    • Carbuncles: Clusters of boils.
    • Staphylococcal Scalded Skin Syndrome (SSSS): A severe skin blistering condition.
  • S. aureus frequently exhibits drug resistance, with Methicillin-resistant Staphylococcus aureus (MRSA) strains showing resistance to numerous antibiotics.
  • Asymptomatic carriers play a role in the transmission of both community-acquired and hospital-acquired staph infections.
  • Group A streptococci (S. pyogenes) can lead to:
    • Cellulitis: A deep skin infection.
    • Erysipelas: A superficial skin infection.
    • Erythema Nodosum: Inflammatory condition causing painful nodules, often on the shins.
    • Necrotizing Fasciitis: A severe, rapidly spreading infection that destroys tissue.
  • P. aeruginosa is responsible for skin and eye infections, such as:
    • Wound and burn infections
    • Hot tub rash (folliculitis)
    • Otitis Externa: Outer ear infection.
    • Bacterial Keratitis: Inflammation of the cornea.
  • Acne can become inflammatory when Propionibacterium acnes infects clogged hair follicles.
  • Cutaneous anthrax, caused by Bacillus anthracis, manifests as a black eschar and can be fatal if it spreads to the bloodstream.
  • Common bacterial conjunctivitis is often caused by Haemophilus influenzae and typically resolves spontaneously. More serious forms include:
    • Gonococcal Ophthalmia Neonatorum: A severe conjunctivitis in newborns.
    • Inclusion Conjunctivitis (Chlamydial)
    • Trachoma: A chronic, contagious bacterial infection of the eye; all of which can lead to blindness if untreated.
  • Biofilms complicate the treatment of wound and eye infections due to their resistance to elimination.

Viral Skin Infections

  • Papillomas (warts) are caused by human papillomaviruses (HPV).
  • Herpes simplex virus type 1 (HSV-1) primarily causes oral herpes but can also affect other skin areas.
  • Roseola (caused by HHV-6 and HHV-7) and Fifth disease (caused by parvovirus B19) are common viral illnesses that result in skin rashes.
  • Viral conjunctivitis is often caused by adenoviruses and can be associated with the common cold. Herpes keratitis is caused by herpesviruses spreading to the eye.

Fungal Infections (Mycoses)

  • Mycoses can be categorized as cutaneous, subcutaneous, or systemic.
  • Common cutaneous mycoses include tineas (ringworm), caused by dermatophytes like Trichophyton, Epidermophyton, and Microsporum.
  • Aspergillosis caused by Aspergillus, can enter through skin breaks (primary) or spread from the respiratory system (secondary).
  • Sporotrichosis (rose gardener's disease) is the most common subcutaneous mycosis, caused by Sporothrix schenkii.
  • Candida yeasts can cause opportunistic skin infections referred to as candidiasis.