Microbial Diseases of the Skin and Eyes

Epidermis

Thin outer portion of skin; composed of layers of epithelial cells

Keratin

Waterproofing protein coating outer layer of epidermis

Dermis

Inner, thick portion of skin; composed mainly of connective tissue

Human Skin

• Perspiration and sebum contain nutrients and keep skin from drying out
• Salt inhibits microbes
• Lysozyme hydrolyzes peptidoglycan

Mucous Membranes

• Line body cavities that lead to the outside: Digestive tract, Respiratory tract, Urogenital tract.
• Part of the first line of defense against pathogens.
• Secrete mucus that helps trap pathogens

Normal Microbiota of the Skin

• Gram-positive, salt-tolerant bacteria

• Staphylococci e.g Staph. epidermidis

• Micrococci

• Diphtheroids e.g Cutibacterium (Propionibacterium) acnes
  • Anaerobic
  • lives in hair follicles
  • Produces propionic acid, and keep pH low

Staphylococcus epidermidis

• Gram-positive cocci

• Coagulase negative.

• 90% of normal skin microbiota

• Health-care associated pathogen

• Can form biofilms on catheters

• Forms slimy layer that allows cells to attach

Staphylococcus aureus

• Gram-positive cocci

• Coagulase-positive

• Carried in the nasal passages of 20% of the population

• Produces exfoliatin that causes separation of skin layers

• Avoids host defenses in the skin - secretes proteins and toxins that kill phagocytic cells

• MRSA strains are antibiotic-resistant

Folliculitis

Infections of the hair follicles

Sty

Folliculitis of the eyelash

Impetigo

Crusting sores

Scalded Skin Syndrome (Staphylococcal Skin Infection)

• Type of impetigo where the toxin circulates and causes separation of skin layers
  ▪ Exfoliative toxin A remains localized
  ▪ Exfoliative toxin B spreads to distant sites

Streptococcus pyogenes

• Gram-positive cocci in chains

• Associated with a wide range of infections in addition to skin infections

• Beta hemolytic: ability of bacteria to completely break down red blood cells (hemolysis) in the agar, leading to a clear, transparent zone around the bacterial colonies.

• Produces several virulence factors
  ▪ Hyaluronidase: digest hyaluronic acid that keeps cells of connective tissue together
  ▪ Hemolysin that lyses red blood cells
  ▪ M protein: allows attachment to host cell and helps cell evade phagocytosis

Streptococcus pyogenes - Necrotizing fasciitis

• “Flesh-eating” disease

• Pyrogenic toxins produced by S. pyogenes acts as a superantigen

Infections by Pseudomonads - Pseudomonas aeruginosa

Pseudomonas aeruginosa

• Gram-negative, aerobic rod

• Pyocyanin produces a blue-green pus

• Opportunistic pathogen

• Causes:

  • Pseudomonas dermatitis

  • Otitis externa, or “swimmer’s ear”

  • Post-burn infections

Viral Diseases of the Skin

• Many are transmitted via respiratory routes and are systemic
• Many cause problems in children and developing fetuses

Smallpox (Variola)

• Caused by an orthropoxvirus
• Transmitted via the respiratory route, moves into the bloodstream, and infects many internal organs and the skin
• Completely eradicated from the human population by vaccination

MPOX - Monkeypox

• Related to smallpox but less severe

• Endemic to small animals in Africa

• Spill-overs from animals to humans

• Can be transmitted from human to human by prolonged close contact

• 2022 outbreak in the United States: >30,000 cases

• Prevention by the smallpox vaccination

Chickenpox (varicella)

• Herpesvirus varicella-zoster (human herpesvirus 3)

• Transmitted via the respiratory route

• Causes pus-filled vesicles after a 10–14 day incubation

• Virus becomes latent in the dorsal root ganglia

• Prevented by a live attenuated vaccine

Shingles (herpes zoster)

• Reactivation of the latent varicella-zoster virus that moves from dorsal root ganglia along peripheral nerves to the skin
  ▪ Due to stress or lowered immunity
  ▪ Highest incidence is in older adults

• Follows the distribution of affected cutaneous sensory nerves
  ▪ Painful pustular lesions over a dermatome
  ▪ Limited to one side of the body

Shingles (herpes zoster) Cont’d

• Postherpetic neuralgia—painful stinging and burning sensation for months to years following shingles

• Treatment of shingles: acyclovir, valacyclovir, famciclovir may lessen symptoms

• An individual with shingles can shed the virus and infect nonimmune individuals with chicken pox

• Prevention: Shingrix® vaccine

Herpes Virus

• Human herpesvirus 1 (HSV-1) and 2 (HSV-2)
  – HSV-31 is spread primarily by oral or respiratory routes
  – HSV-2 is spread primarily sexually, causing genital herpes

• Viruses can remain latent in nerve ganglia

• 90% of the US population is infected with HSV–1
  – Usually develop as cold sores or fever blisters
  – Not the cause of canker sores

Measles (Rubeola)

• Measles virus transmitted by the respiratory route

• Virus can be shed for several days before symptoms appear

• Cold-like symptoms, macular rash

• Koplik’s spots
  – Red spots on the oral mucosa opposite the molars

• Prevented by the MMR (measles, mumps, rubella) vaccine
  – Children under 1 year old cannot receive the vaccine

Rubella

• German measles

• Rubella virus

• Macular rash and light fever, milder than measles

• Transmitted via the respiratory route; 2-to 3-week incubation

• Congenital rubella syndrome
  – Fetal damage, deafness, heart defects, mental retardation in 35% of cases
  – 15% mortality within first year of life

• Prevented by the MMR vaccine

Bacterial Diseases of the eye: Conjungtivitis

• An inflammation of the conjunctiva (the outermost layer of the eye and inner layer of eyelids)

• Also called pinkeye or red eye

• Commonly caused by Haemophilus influenzae

• Various other microbes can also be the cause

Bacterial Diseases of the Eye - Ophthalmia Neonatorum

• Caused by Neisseria gonorrhoeae

• Large amount of pus forms; ulceration of corneas results
  – Untreated cases may lead to blindness

• Transmitted to a newborn’s eyes during passage through the birth canal

• Prevented by treating a newborn’s eyes w/ antibiotics

Bacterial Diseases of the Eyes - Chlamydia trachomatis

• Conjunctivitis, or chlamydial conjunctivitis

• Transmitted to a newborn’s eyes during passage through the birth canal

• Treated with tetracycline

• Causes trachoma

• Leading cause of blindness worldwide

• Infection causes permanent scarring; scars abrade the cornea, leading to blindness

Coagulase-Postive

refers to bacteria that produce an enzyme called coagulase, which can clot plasma (the liquid portion of blood).