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Microbiology

Summary

MicrobiologySummary
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21.1 Anatomy and Normal Microbiota of the Skin and Eyes

  • Human skin consists of two main layers, the epidermis and dermis, which are situated on top of the hypodermis, a layer of connective tissue.
  • The skin is an effective physical barrier against microbial invasion.
  • The skin’s relatively dry environment and normal microbiota discourage colonization by transient microbes.
  • The skin’s normal microbiota varies from one region of the body to another.
  • The conjunctiva of the eye is a frequent site for microbial infection, but deeper eye infections are less common; multiple types of conjunctivitis exist.

21.2 Bacterial Infections of the Skin and Eyes

  • Staphylococcus and Streptococcus cause many different types of skin infections, many of which occur when bacteria breach the skin barrier through a cut or wound.
  • S. aureus are frequently associated with purulent skin infections that manifest as folliculitis, furuncles, or carbuncles. S. aureus is also a leading cause of staphylococcal scalded skin syndrome (SSSS).
  • S. aureus is generally drug resistant and current MRSA strains are resistant to a wide range of antibiotics.
  • Community-acquired and hospital-acquired staphyloccocal infections are an ongoing problem because many people are asymptomatic carriers.
  • Group A streptococci (GAS), S. pyogenes, is often responsible for cases of cellulitis, erysipelas, and erythema nosodum. GAS are also one of many possible causes of necrotizing fasciitis.
  • P. aeruginosa is often responsible for infections of the skin and eyes, including wound and burn infections, hot tub rash, otitis externa, and bacterial keratitis.
  • Acne is a common skin condition that can become more inflammatory when Cutibacterium acnes infects hair follicles and pores clogged with dead skin cells and sebum.
  • Cutaneous anthrax occurs when Bacillus anthracis breaches the skin barrier. The infection results in a localized black eschar on skin. Anthrax can be fatal if B. anthracis spreads to the bloodstream.
  • Common bacterial conjunctivitis is often caused by Haemophilus influenzae and usually resolves on its own in a few days. More serious forms of conjunctivitis include gonococcal ophthalmia neonatorum, inclusion conjunctivitis (chlamydial), and trachoma, all of which can lead to blindness if untreated.
  • Keratitis is frequently caused by Staphylococcus epidermidis and/or Pseudomonas aeruginosa, especially among contact lens users, and can lead to blindness.
  • Biofilms complicate the treatment of wound and eye infections because pathogens living in biofilms can be difficult to treat and eliminate.

21.3 Viral Infections of the Skin and Eyes

  • Papillomas (warts) are caused by human papillomaviruses.
  • Herpes simplex virus (especially HSV-1) mainly causes oral herpes, but lesions can appear on other areas of the skin and mucous membranes.
  • Roseola and fifth disease are common viral illnesses that cause skin rashes; roseola is caused by HHV-6 and HHV-7 while fifth disease is caused by parvovirus 19.
  • Viral conjunctivitis is often caused by adenoviruses and may be associated with the common cold. Herpes keratitis is caused by herpesviruses that spread to the eye.

21.4 Mycoses of the Skin

  • Mycoses can be cutaneous, subcutaneous, or systemic.
  • Common cutaneous mycoses include tineas caused by dermatophytes of the genera Trichophyton, Epidermophyton, and Microsporum. Tinea corporis is called ringworm. Tineas on other parts of the body have names associated with the affected body part.
  • Aspergillosis is a fungal disease caused by molds of the genus Aspergillus. Primary cutaneous aspergillosis enters through a break in the skin, such as the site of an injury or a surgical wound; it is a common hospital-acquired infection. In secondary cutaneous aspergillosis, the fungus enters via the respiratory system and disseminates systemically, manifesting in lesions on the skin.
  • The most common subcutaneous mycosis is sporotrichosis (rose gardener’s disease), caused by Sporothrix schenkii.
  • Yeasts of the genus Candida can cause opportunistic infections of the skin called candidiasis, producing intertrigo, localized rashes, or yellowing of the nails.

21.5 Protozoan and Helminthic Infections of the Skin and Eyes

  • The protozoan Acanthamoeba and the helminth Loa loa are two parasites that can breach the skin barrier, causing infections of the skin and eyes.
  • Acanthamoeba keratitis is a parasitic infection of the eye that often results from improper disinfection of contact lenses or swimming while wearing contact lenses.
  • Loiasis, or eye worm, is a disease endemic to Africa that is caused by parasitic worms that infect the subcutaneous tissue of the skin and eyes. It is transmitted by deerfly vectors.
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