21.1 Structure and Function of the Skin

• The epidermis is the thin outer portion, composed of several layers of epithelial cells.

  • The outermost layer of the epidermis, the stratum corneum, consists of many rows of

dead cells that contain a waterproofing protein called keratin.

• The dermis is the inner, relatively thick portion of skin, composed mainly of connective tissue.

• Sebum, secreted by oil glands, is a mixture of lipids (unsaturated fatty acids), proteins, and salts that prevents skin and hair from drying out.

• Mucous membranes are often acidic, which tends to limit their microbial populations

  • Mucous membranes are often folded to maximize surface area

21.2 Normal Microbiota of the Skin

• On superficial skin surfaces, certain aerobic bacteria produce fatty acids from sebum

• Microorganisms that find the skin a satisfactory environment are resistant to drying and to relatively high salt concentrations.

  • Also part of the skin’s normal microbiota are gram-positive pleomorphic rods called diphtheroids

• A yeast, Malassezia furfur, is capable of growing on oily skin secretions and is associated with the scaling skin condition known as dandruff

21.3 Microbial Diseases of the Skin

• Vesicles larger than about 1 cm in diameter are termed bullae

• Flat, reddened lesions are known as macules

  • Raised lesions are called papules or, when they contain pus, pustules.

• A skin rash that arises from disease conditions is called an exanthem; on mucous membranes, such as the interior of the mouth, such a rash is called an enanthem.

• Staphylococci are spherical gram-positive bacteria that form irregular clusters like grapes

  • It is a permanent resident of the nasal passages of 20% of the population, and an additional 60% carry it there occasionally.

• Almost all pathogenic strains of S. aureus are coagulase-positive.

  • This is significant because there is a high correlation between the bacterium’s ability to form coagulase and its production of damaging toxins, several of which facilitate the spread of the organism in tissue, damage tissue, or are lethal to host defenses

• All humans possess antibodies against S. aureus, but they do not effectively prevent repeated infections.

  • Antibiotic-resistant strains of S. aureus have emerged in hospitals and in the community.

• The infected follicle of an eyelash is called a sty.

• A more serious hair follicle infection is the furuncle (boil), which is a type of abscess, a localized region of pus surrounded by inflamed tissue.

• The extensive damage is called a carbuncle, a hard, round deep inflammation of tissue under the skin

• Staphylococci are the most important causative organism of impetigo.

• The other type of impetigo, bullous impetigo, is caused by a staphylococcal toxin and is a localized form of staphylococcal scalded skin syndrome.

  • Scalded skin syndrome is also characteristic of the late stages of toxic shock syndrome (TSS).

• As streptococci grow, they secrete toxins and enzymes, virulence factors that vary with the different streptococcal species.

  • Necrotizing fasciitis is often associated with streptococcal toxic shock syndrome (streptococcal TSS), which resembles staphylococcal TSS

  • This causes activation of neutrophils, precipitating the release of damaging enzymes and consequent shock and organ damage.

• Pseudomonads frequently cause outbreaks of Pseudomonas dermatitis

• Competition swimmers are often troubled with otitis externa, or “swimmer’s ear,” a painful infection of the external ear canal leading to the eardrum that is frequently caused by pseudomonads

• P. aeruginosa produces several exotoxins that account for much of its pathogenicity. It also produces an endotoxin.

• Infection may produce bluegreen pus, whose color is caused by the bacterial pigment pyocyanin.

• Buruli ulcer, named for a now-renamed region of Uganda in Africa, is an emerging disease found primarily in western and central Africa

• Acne is probably the most common skin disease in humans, affecting an estimated 17 million people in the United States.

• Comedonal (mild) acne is usually treated with topical agents such as azelaic acid, salicyclic acid preparations, or retinoids (which are derivatives of vitamin A, such as tretinoin, tazarotene, or adapalene).

• Inflammatory (moderate) acne arises from bacterial action, especially Cutibacterium acnes, an anaerobic diphtheroid commonly found on the skin. P. acnes has a nutritional requirement for glycerol in sebum; in metabolizing the sebum, it forms free fatty acids that cause an inflammatory response.

  • Some patients with acne progress to nodular cystic (severe) acne.

  • Warts, or papillomas, are generally benign skin growths caused by viruses.

• During the Middle Ages, an estimated 80% of the population of Europe contracted smallpox* at some time during their lives

  • There are two basic forms of this disease: variola major, with a mortality rate of 20–60% and over 80% in children, and variola minor, with a mortality rate of less than 1%

• Today, only two sites are known to maintain the smallpox virus, one in the United States and one in Russia.

• Reye’s syndrome is an occasional severe complication of chickenpox, influenza, and some other viral diseases.

• Shingles is simply a different expression of the virus that causes chickenpox: different because the patient, having had chickenpox, now has partial immunity to the virus.

  • Exposing unvaccinated children to shingles has led to their contracting chickenpox

• Herpes simplex viruses (HSV) can be separated into two identifiable groups, HSV-1 and HSV-2. The name herpes simplex virus, used here, is the common or vernacular name

• Frequently, this infection is subclinical, but many cases develop lesions known as cold sores or fever blisters.

  • Cold sores, caused by herpesvirus infections, are often confused with canker sores.

• HSV-1 infection can be transmitted by skin contact among wrestlers; this is colorfully termed herpes gladiatorum.

  • Nurses, physicians, and dentists are occupationally susceptible to herpetic whitlow, infections of the finger caused by contact with HSV-1 lesions—as are children with herpetic oral ulcers

• Very rarely, either type of the herpes simplex virus may spread to the brain, causing herpes encephalitis.

• Measles (rubeola) is an extremely contagious viral disease spread by the respiratory route.

• Rubella, or German measles (so called because it was first described by German physicians in the eighteenth century), is a much milder viral disease than rubeola (measles) and often goes undetected.

  • The seriousness of rubella was not appreciated until 1941, when certain severe birth defects were associated with maternal infection during the first trimester (3 months) of pregnancy, a condition called congenital rubella syndrome.

21.4 Diseases of the Eye

• Conjunctivitis is an inflammation of the conjunctiva, often called by the common name red eye, or pinkeye.

• Ophthalmia neonatorum is a serious form of conjunctivitis caused by Neisseria gonorrhoeae, the cause of gonorrhea. Large amounts of pus are formed; if treatment is delayed, ulceration of the cornea will usually result.

• Chlamydial conjunctivitis, or inclusion conjunctivitis, is quite common today

• A serious eye infection, and probably the greatest single cause of blindness by an infectious disease, is trachoma—an ancient name derived from the Greek word for rough.

• Herpetic keratitis is caused by the same herpes simplex type 1 virus (HSV-1) that causes cold sores and is latent in the trigeminal nerves

  • Herpetic keratitis is caused by the same herpes simplex type 1 virus (HSV-1) that causes cold sores and is latent in the trigeminal nerves