Microbial Diseases of the Skin and Eyes

Introduction

The skin is a physical and chemical barrier against microorganisms.

Moist areas of the skin (such as the armpit) support larger populations of bacteria than dry areas (such as the scalp).

Structure and Function of the Skin

The outer portion of the skin, called the epidermis, contains keratin, a waterproof coating.

The inner portion of the skin, the dermis, contains hair follicles, sweat ducts, and oil glands that provide passageways for microorganisms.

Sebum and perspiration are secretions of the skin that can inhibit the growth of microorganisms.

Sebum and perspiration provide nutrients for some microorganisms.

Body cavities are lined with epithelial cells. When these cells secrete mucus, they constitute the mucous membrane.

Normal Microbiota of the Skin

Microorganisms that live on skin are resistant to desiccation and high concentrations of salt.

Gram-positive cocci such as staphylococci and micrococci predominate on the skin. Diphtheroids (gram-positive pleomorphic rods), such as Propionibacterium acnes and Corynebacterium xerosis, are also present.

The normal skin microbiota are not completely removed by washing.

Members of the genus Propionibacterium metabolize oil from the oil glands and colonize hair follicles.

Pityrosporum ovale yeast grows on oily secretions and may be the cause of dandruff.

Microbial Diseases of the Skin

• Vesicles are small fluid-filled lesions.
• Bullae are vesicles larger than 1 cm.
• Macules are flat, reddened lesions.
• Papules are raised lesions.
• Pustules are raised lesions containing pus.

Bacterial Diseases of the Skin

Staphylococcal Skin Infections

Staphylococci are gram-positive bacteria that often grow in clusters.

The majority of skin microbiota consist of coagulase-negative S. epidermidis

Almost all pathogenic strans of S. aureus produce coagulase.

Pathogenic S. aureus can produce enterotoxins, leukocidins, and exfoliative toxin.

Many strains of S. aureus produce penicillinase; these are treated with methicillin. Methicillin resistant strains are treated with vancomycin. Vancomycin resistance has been documented and may be treated with a combination of antibiotics but we are on dangerous ground here.

Localized infections (sties, pimples, and carbuncles) result from S. aureus entering openings in the skin.

Impetigo of the newborn is a highly contagious superficial skin infections caused by S. aureus.

Toxemia occurs when toxins enter the bloodstream; staphylococcal toxemias include scalded skin syndrome and toxic shock syndrome.

Streptococcal Skin Infections

Streptococci are gram-positive cocci that often grow in chains.

Streptococci are classified according to their hemolytic enzymes and cell wall antigens.

Group A beta-hemolytic streptococci (serological types of S. pyogenes) produce a number of virulence factors: M protein, erythrogenic toxin, deoxyribonuclease, streptokinases, and hyaluronidase.

Erysipelas (reddish patches) and impetigo (isolated pustules) are skin infections cause by S. pyogenes.

Invasive group A beta-hemolytic streptococci cause severe and rapid tissue destruction.

Infections by Pseudomonads

Pseudomonads are gram-negative rods. They are aerobes found primarily in soil and water that are resistant to many disinfectants and antibiotics.

Pseudomonas aeruginosa produces an endotoxin and several exotoxins.

Diseases caused by P. aeruginosa include otitis externa, respiratory infections, burn infections, and dermatitis.

Infections have a characteristic blue-green pus caused the pigment pyocyanin.

Fluoroquinolones are useful in treating P. aeruginosa infections.

Acne

Propionibacterium acnes can metabolize sebum trapped in hair follicles.

Metabolic end-products (fatty acids) cause inflammatory acne.

Tretinoin, benzoyl peroxide, erythromycin, and Accutane are used to treat acne.

Viral Diseases of the Skin

Warts

Papillomaviruses cause skin cells to proliferate and produce a benign growth called a wart or papilloma.

Warts are spread by direct contact.

Warts may regress spontaneously or be removed chemically or physically.

Smallpox (Variola)

Variola virus causes two types of skin infections: variola major and variola minor. Variola major has a mortality rate of 20% or higher while variola minor has a mortality rate of less than 1%.

Smallpox is transmitted by the respiratory route, and the virus is moved to the skin via the bloodstream. Twelve to fourteen days after infection patients become febrile and exhibit severe aching pains and prostration.

Two to three days after the appearance of first symptoms a papular rash appears, which soon becomes vesicular and then pustular. Pustules expand, eventually scab, and heal leaving deep pitted scars.

Patients remain febrile throughout the development of the rash and experience severe pain. Death usually occurs during the second week.

The only host for smallpox is humans.

Smallpox has been eradicated as a result of a vaccination effort by the WHO (yeah, except for those bioterror stockpiles everybody's holding on to).

Chickenpox (Varicella) and Shingles (Herpes Zoster)

Varicella-zoster virus is transmitted by the respiratory route and is localized in skin cells, causing a vesicular rash. Vesicles fill with pus, rupture, and form a scab within 3-4 days. The disease is usually mild.

Complications of chickenpox include encephalitis and Reye's syndrome.

After chickenpox, the virus can remain latent in nerve cells and subsequently activate as shingles.

Shingles (herpes zoster) is characterized by a vesicular rash along the affected cutaneous sensory nerves.

The virus can te treated with acyclovir. An attenuated live vaccine is available.

Herpes Simplex

Herpes simplex infection of mucosal cells results in cold sores and occasionally encephalitis.

The virus remains latent in nerve cells (HSV-1 in the trigeminal nerve ganglion), and cold sores can recur when the virus is activated.

HSV-1 is transmitted primarily by oral and respiratory routes.

Herpes encephalitis occurs when herpes simplex viruses infect the brain.

Acyclovir has proven successful in treating herpes encephalitis.

Measles (Rubeola)

Measles is caused by measles virus and transmitted by the respiratory route.

Vaccination provides effective long-term immunity.

After the virus has incubated in the upper respiratory tract, macular lesions appear on the skin, and Koplik's spots appear on the oral mucosa.

Complications of measles include middle ear infections, pneumonia, encephalitis, and secondary bacterial infections.

Rubella (German measles)

The rubella virus is transmitted by the respiratory route.

A red rash and light fever might occur in an infected indivitual; the disease can be asymptomatic.

Congenital rubella syndrome can affect a fetus when a woman contracts rugella during the first trimester of her pregnancy.

Damage from congenital rubella syndrome includes stillbirth, deafness, eye cataracts, heart defects, and mental retardation.

Vaccination with live rubella virus provides immunity of unknown duration.

Other Viral Rashes

Human parvovirus B19 causes fifth disease and HHV-6 causes roseola.

Fungal Diseases of the Skin and Nails

Cutaneous Mycoses

Fungi that colonize the outer layer of the epidermis cause dermatomycoses.

Microsporum, Trychophyton, and Epidermophyton cause dermatomycoses called ringworm, or tinea.

These fungi grow on keratin-containing epidermis, such as hair, skin, and nails.

Ringworm and athlete's foot are usually treated with topical antifungal chemicals.

Diagnosis is based on the microscopic examination of skin scrapings or fungal culture.

Subcutaneous Mycoses

Sporotrichosis results from a soil fungus that penetrates the skin through a wound.

The fungi grow and produce subcutaneous nodules along the lymphatic vessels.

Candidiasis

Candida albicans causes infections of mucous membranes and is a common cause of thrush (in oral mucosa) and vaginitis.

C. albicans is an opportunistic pathogen that may proliferate when the normal bacterial microbiota are suppressed.

Topical antifungal chemicals may be used to treat candidiasis.

Parasitic Infestation of the Skin

Scabies is cause by a mite burrowing and laying eggs in the skin.

Pediculosis is an infestation by P. humanus.

Microbial Diseases of the Eye

The mucous membrane lining the eyelid and covering the eyeball is the conjunctiva.

Inflammation of the Eye Membranes: Conjunctivitis

Conjunctivitis is caused by several bacteria and can be transmitted by improperly disinfected contact lenses.

Bacterial Diseases of the Eye

Bacterial microbiota of the eye usually originate from the skin and upper respiratory tract.

Neonatal gonorrheal ophthalmia is caused by the transmission of Neisseria gonorrhoeae from an infected mother to an infant during its passage through the birth canal.

All newborn infants are treated with an antibiotic to prevent the growth of Neisseria and Chlamydia infection.

Inclusion conjunctivitis is an infection of the conjunctiva cause by Chlamydia trachomatis. It is transmitted to infants during birth and is transmitted in unchlorinated swimming water.

In trachoma, which is caused by C. trachomatis, scar tissue forms on the conrea.

Trachoma is transmitted by hands, fomites, and perhaps flies.

Other Infectious Diseases of the Eye

Inflammation of the cornea is called keratitis.

Herpetic keratitis causes corneal ulcers. The etiology is HSV-1 that invades the central nervous system and can recur.

Trifluridine is an effective treatment for herpes keratitis.

Acanthamoeba protozoa, transmitted via water, can cause a serious form of keratitis.