• When unbroken, is an effective physical barrier against microbes
  • First line of defense, providing protection by shedding, dryness, acidity, toxicity to pathogens
  • Perspiration
    • Salt is inihibitory to many microbes
    • Lysozyme breaks down cell walls, forming protoplasts and spheroplasts
    • Sebum inhibits the growth of some microbes
  • Is acidic, which is inhibitory

• Also First Line of Defense
  • Body cavity lining (GI, RT, GU)
  • Basement membrane secretes mucus
  • Some mucosal cells have cilia (e.g., ciliary elevator of lower respiratory tract)
  • Mucous membranes often acidic
  • Mucous membranes of eyes contain lysozyme
  • Large surface area due to infoldings

• Resistant to drying
• Halophiles
• Microbial antagonism becomes competitive exclusion
• Gram positive cocci
  • Staphylococcus: universally present
  • Micrococcus
• Gram positive diptheroids are important in acne and body odor; they metabolize oily secretions of sebaceous glands to produce fatty acids, that keep skin acidic
  • C. xerosis
  • P. acnes
• Yeast (fungi)
  • Pityrosporumn ovale
  • Candida species

1. Three most common pathogens:
   • S. aureus
   • S. pyogenes
   • P. aeruginosa
2. Fungal
   • Dermatophytes
3. Viral
4. Systemic diseases with skin components

1. Vesicles: small, fluid-filled lesions
2. Bullae: vesicles that are larger than one centimeter
3. Macules: flat, reddened lesions
4. Papules: raised macules
5. Pustules: papules filled with pus

1. Epidemiology
   • Reservoir: primarily humans
   • Spread by direct and indirect contact
     • Either from colonizing sources on mucous membranes such as nose
     • Or from draining lesions
     • Use contact precautions if major skin, wound, or burn infection; standard precautions acceptable for all other syndromes
   • Immunity: primarily nonspecific mechanisms
     • Persons with specific immunity against TSST-1 do not develop toxic shock syndrome
2. Infections of the hair follicle
   • Folliculitis: infection of hair follicle; 1 mm in size; papule or pustule
   • Furuncle: deep infection of a hair follicle; 1 cm in size; pustule or nodule
   • Carbuncle: deepest; several centimeters in diameter; usually on nape of neck
3. Impetigo
   • Superficial (just beneath the stratum corneum)
   • Redness, swelling, scaling and blistering
   • Generally occurs in the paranasal or perioral area in young people.
   • Also caused by Streptococcus
4. Scalded Skin Syndrome
   • Also known as impetigo of the newborn and bullous impetigo
     • In children, is often systemic
       • Hospital nurseries use hexachlorophene to contain outbreaks
         • Bisphenol
         • pHisoHex can lead to neurological damage
     • In adults, is usually a focal infection
   • Thin-walled vesicles that rupture and crust over
5. Toxic Shock Syndrome
   • Also known as staphylococcal scarlet fever
   • Sudden onset of high fever, hypotension, headache, severe myalgia, vomiting and diarrhea, mucous membrane inflammation, diffuse neurologic abnormalities, desquamating skin rash (skin peels)
   • Can lead to shock, cardiac arrhythmia, renal failure, respiratory failure
   • This is all due to Toxic Shock Syndrome Toxin-1 (TSST-1), another type of staphylococcal exotoxin

1. Descriptive Nomenclature
   • Gram positive cocci, in chains
   • Can be categorized based on the hemolysin they produce
     • These are called a, b, or g hemolytic strep
       • Of these, the b hemolytic strep are most likely to be associated with skin diseases
   • Are also categorized by serological group
     • Named Group A through T, based on antigens found in their cell walls
     • Of these, the Group A strep are most likely to be associated with disease
     • Group A, beta hemolytic strep is synonymous with Streptococcus pyogenes, an important skin pathogen
2. Epidemiology of Streptococcal Infections
   • Group A b hemolytic Streptococcus is etiologic agent
   • Acute glomerulonephritis is an important complication of streptococcal impetigo
   • Humans are the reservoir and may present as asymptomatic carriers
   • Transmission is by direct and indirect contact for most of the streptococcal skin infections, although erysipelas may start as a pharyngitis, spread by droplets
   • Immunity is mostly non-specific
     • Antibodies to M protein develop (but person is not immune to a serologically different M-protein streptococcal infection)
     • Antitoxin immunity to streptococcal pyrogenic exotoxin prevents scarlet fever, but not streptococcal pharyngitis
3. Erysipelas
   • This is an infection of the dermis.
   • May progress to local tissue destruction and septicemia
4. Impetigo
   • In older children, impetigo is caused by Streptococcus pyogenes
   • Disease is spread by contact
   • Bacteria penetrate through a break in the skin (e.g., mosquito bite)
5. Cellulitis
   • Infection of the underlying solid tissue
6. Necrotizing Fasciitis
   • Infection of the the muscle covering
   • Exotoxin A, a superantigen, must be present for the disease to progress to necrotizing fasciitis
     • Superantigens cause an overwhelming immune response that stimulates massive numbers of helper T lymphocytes to form
       • The result is destructive, rather than constructive

1. Epidemiology
   • Aerobic, Gram negative rods
   • Reservoir is soil and water
   • Survive on biofilms
   • Resistant to many antibiotics and disinfectants
2. Pseudomonas aeruginosa is most important pathogen
   • Cause a dermatitis that is linked to hot tubs and "swimmer's ear," otitis externa
   • Opportunistic
   • Produces exotoxins
   • Also has an endotoxin (why?)
   • Common pathogen of burn wounds and wounds of otherwise-compromised patients

1. Microsporum
2. Epidermophyton
3. Trichophyton
4. Tinea (ringworm)
5. Use KOH test to diagnose in clinical setting
6. Epidemiology of the Dermatophytes
   • Reservoir in both humans and animals
   • Transmission is through direct or indirect contact
   • Use skin scrapings for diagnosis
     • Sample from active margins of advancing lesions
     • Use 10% KOH solution (this clears cellular debris and allows for visualization of hyphae and yeast cells)

1. Warts (Papillomavirus)
2. Smallpox (Variola)
3. Herpes (HSV-1 and HSV-2)
4. Chickenpox (Varicella)
5. Measles (Rubeola)
   • Very contagious
   • Transmission is airborne
   • Macular rash
   • Koplik’s spots are pathognomic
     • Tiny red patches with central white specks on oral mucosa opposite the molars
6. German Measles (Rubella)
   • The red macules and papules of the eruption start at the forehead, and then continue to erupt in a downward direction.
   • The eruption generally clears starting from the forehead after about three days
7. Fifth Disease (Erythema Infectiosum caused by Human parvovirus 19)
   • It infects the red blood cell line, and can cause severe anemia and even death in the fetus or in those with a red blood cell dyscrasia
   • Called "Fifth Disease" because it was 1 of 5 common rash diseases of childhood
   • Note characteristic "slapped cheek" appearance
8. Roseola
   • The child normally looks and feels well
   • But has a temperature of about 104 degrees Fahrenheit for about three days
   • The temperature then drops, and as it does numerous red macules and papules appear predominantly on the trunk; they occassionally appear on the extremities and the face

1. Continuation of skin
2. Covered with mucous membrane
3. Conjunctivitis (pink-eye)
   • Hemophilus aegyptius
   • Pseudomonas
4. Neonatal gonorrheal ophthalmia
   • N. gonorrheae
   • Treatment within 1 hr of birth with antibiotics
   • Also effective against birth-acquired chlamydia infections
5. Chlamydia trachomatis
   • Sexually transmitted disease
   • Obligate intracellular parasite
   • Causes two types of infections of the eye
     • Inclusion conjunctivitis acquired during birth
     • Trachoma--more serious infection
       • Also transmitted by hand contact or by fomites such as towels
       • Number 1 infectious cause of blindness in world
6. Blepharitis
   • Caused by Staphylococcus aureus
7. Inflamation of cornea
   • Cause is HSV-1
   • Also known as herpetic keratitis (inflammation of cornea)

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