Chapter 21
Microbial Diseases
of the Skin and Eyes
The first lines of defense of the body are the skin and the mucous membranes. Recently, it was found
that perspiration contains an antimicrobial peptide, dermicidin.
Structure and Function of the Skin
The epidermis is the outer, thinner portion of the skin; it is composed of several layers of epithelial
cells. The outermost epidermal layer, the stratum corneum, consists mainly of dead cells containing the
protein keratin. The inner layer of the skin is the dermis, composed of connective tissue with numerous
blood and lymph vessels, nerves, hair follicles, and sweat and oil glands. Perspiration provides the moisture necessary for microbial growth; however, salts interfere with the growth of many organisms, and
the enzyme lysozyme in perspiration digests the cell walls of many bacteria.
Sebum from oil glands is mainly a mixture of unsaturated fatty acids, proteins, and salts. The fatty
acids inhibit the growth of certain pathogens.
Mucous Membranes
In the lining of body cavities such as the mouth, nasal passages, urinary and genital tracts, and gastrointestinal tract, there is a layer of specialized epithelial cells, which are attached at their bases to a layer of
extracellular material called the basement membrane. Some cells produce mucus, which traps particles,
including microbes, hence the name mucous membrane (mucosa). Other cells are ciliated, functioning
to sweep particles and mucus from the body.
Normal Microbiota of the Skin
Many skin bacteria are gram-positive spherical bacteria such as staphylococci and micrococci. Grampositive pleomorphic rods, diphtheroids, such as Propionibacterium acnes, metabolize the sebum secretions in hair follicles. Acid produced by them keeps the skin pH between 3 and 5, a condition that tends
to be bacteriostatic. Other diphtheroids, such as Corynebacterium xerosis, are aerobic. The yeast Pityrosporum ovale grows on oily skin secretions. High-moisture regions such as the armpit and between the
legs are moist enough to support the growth of microorganisms, and populations there are high compared to dry regions like the scalp.
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Microbial Diseases of the Skin
Vesicles are small, fluid-filled lesions on the skin. Skin vesicles larger than about 1 cm in diameter are
bullae. Flat, reddened lesions are macules. Raised lesions are called papules, or pustules when they
contain pus. 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.
Bacterial Diseases of the Skin
Staphylococcal Skin Infections. Staphylococci are spherical gram-positive bacteria that form irregular grapelike clusters of cells. Staphylococcus aureus is the most pathogenic of the staphylococci. It forms
golden yellow colonies on agar, and almost all pathogenic strains produce coagulase, an enzyme that
coagulates the fibrin in blood. The coagulase test is used to distinguish S. aureus from other species of
Staphylococcus. The predominant species of coagulase-negative staphylococci is S. epidermidis. S. aureus
produces a number of toxins, such as enterotoxins, that affect the gastrointestinal tract (which will be discussed in Chapter 25), leukocidins, which destroy phagocytic leukocytes, and exfoliative toxin, which is
responsible for scalded skin syndrome. S. aureus is found primarily in nasal passages and is a common
hospital problem. It is universally present, and constant exposure to antibiotics causes rapid development of resistance. The species, therefore, causes many infections of surgical wounds and other artificial
breaches of the skin barrier.
Pimples are infections of hair follicles, which are natural openings in the skin barrier; an eyelash
follicle infection is a sty. Furuncles (boils) are a type of abscess usually arising from a hair follicle infection. In furuncles, a region of pus is surrounded by inflamed tissue; more extensive invasion of tissue is
termed a carbuncle.
Impetigo of the newborn is troublesome in hospital nurseries, and prevention of this staphylococcal infection is based largely on the use of hexachlorophene-containing skin lotions. The circulation of
bacterial toxins is called toxemia. When the outer skin layers peel away in reaction to staphylococcal
toxins, it is known as the scalded skin syndrome. This syndrome is one of the characteristics of toxic
shock syndrome, caused by growth of staphylococcal bacteria.
Streptococcal Skin Infections. Streptococci are gram-positive spherical bacteria that tend to grow in
chains. They are aerotolerant anaerobes that do not use oxygen and are catalase-negative. They produce
several toxins, as well as enzymes such as hemolysins. Group A beta-hemolytic streptococci, such as
Streptococcus pyogenes, are the most important from the standpoint of human disease. The M protein distributed on the cell surface on fibrils (which is also the basis for subdividing them into over 80 immunological types) contributes to pathogenicity by its antiphagocytic properties and as an aid in adherence.
Also produced by these cells are erythrogenic toxins (scarlet fever rash), deoxyribonucleases (enzymes
degrading DNA), streptokinases (enzymes dissolving blood clots), and hyaluronidase (an enzyme that dissolves hyaluronic acid, the cementing substance of connective tissue).
S. pyogenes is the most common cause of erysipelas, a disease in which the dermis is affected and
the skin erupts into reddish patches. Usually it is preceded by a streptococcal infection, such as a sore
throat, elsewhere in the body.
Impetigo is a superficial skin infection affecting grade-school-age children, characterized by pustules that become crusted and rupture. The bacteria enter through minor lesions. Staphylococci often are
present but are probably secondary invaders.
Streptococci may attack solid tissue (cellulitis) or muscle covering (necrotizing fasciitis). An exotoxin,
exotoxin A, acts as a superantigen, causing the immune system to contribute to damage (see Chapter 19).
Infections by Pseudomonads. Pseudomonads, particularly Pseudomonas aeruginosa, may cause
opportunistic skin infections. These bacteria are common in soil, water, and plants, and grow on minimal
organic material. Resistance to antibiotics often makes them a hospital problem. P. aeruginosa produces
several exotoxins and endotoxins. It may cause respiratory infections, growing in dense piofilms, in compromised hosts (especially cystic fibrosis) or in burn patients, in whom infection may produce a characteristic blue-green pus from pyocyanin pigments. Otitis externa, or swimmer’s ear, is a pseudomonad
Microbial Diseases of the Skin and Eyes
247
infection of the outer ear. Skin infections, collectively called Pseudomonas dermatitis, are contracted in
such places as whirlpool baths and the like; the organisms are relatively resistant to chlorines.
Gentamicin and carbenicillin, often used in combination, are effective; silver sulfadiazine is used to treat
P. aeruginosa infections of burn patients.
Acne. Acne can be classified into three categories: comedonal acne, inflammatory acne, and nodular
cystic acne. Acne develops when channels for the passage of sebum to the skin surface are blocked. As
sebum accumulates, whiteheads (comedos) form; if the blockage protrudes through the skin a blackhead
(comedone or open comedo) forms. Comedonal acne is usally treated with topical agents such as azelaic
acid (Azelex), salicylic acid preparations, or retinoids. These are derivatives of vitamin A such as
tretinoin, tazarotene (Tazorac) or adapalene (Differin). They do not affect sebum formation.
Inflammatory acne arises from action of the anaerobic bacterium Propionibacterium acnes. It metabolizes sebum and causes an inflammatory response resulting in pustules and papules. Drugs that reduce
sebum formation such as isotretinoin (Accutane) are very effective. However, it is teratogenic, meaning
it causes serious birth defects. Also used are nonprescription benzoyl peroxide compounds and prescription antibiotics. A recent development in the treatment of moderate inflammatotory acne is the Clear
Light system. The affected skin is exposed to blue light that destroys Propionibacterium acnes bacteria.
Some patients progress to nodular cystic acne that leaves prominent scars. Isotretinoin is the usual
treatment.
Viral Diseases of the Skin
Warts. More than 50 types of Papillomavirus cause the uncontrolled nonmalignant growth of skin cells
called warts. Warts can be spread by contact; the incubation period is several weeks. Treatment of warts
includes burning them with acids, or the use of liquid nitrogen (cryotherapy), or electrodesiccation.
Genital warts have been successfully treated with injected interferon and lasers.
Smallpox (Variola). Smallpox is caused by a poxvirus known as the smallpox (variola) virus. Variola
major smallpox has a mortality rate of 20% or more, and variola minor has a mortality rate of less than
1%. Recovery from one form gives immunity to both. Transmission is by the respiratory route, and the
viruses eventually move through the bloodstream to infect the skin. Vaccination has resulted in the elimination of smallpox in the world. Monkeypox closely resembles smallpox and has been mistaken for it.
Chickenpox (Varicella) and Shingles (Herpes Zoster)
Chickenpox (varicella) is a relatively mild disease, the second most common reportable disease in the
United States. Infection is by the respiratory route, and after about two weeks of incubation it localizes
in the skin. The skin is vesicular for three or four days. The herpesvirus varicella-zoster (official name,
human herpesvirus 3) that causes chickenpox may remain latent in nerve cells and later in life be reactivated to cause herpes zoster (shingles). In shingles, vesicles similar to chickenpox occur in areas such
as the waist, upper chest, and face, affecting nerve branches of the cutaneous sensory nerves.
Reye’s syndrome is an occasional, severe complication of chickenpox, influenza, and sometimes
other viral diseases. Brain damage or death may result from brain swelling, which prevents blood circulation. Use of aspirin may increase chances for the Reye’s syndrome complication.
Herpes Simplex
The herpes simplex virus is able to remain latent for long periods of time following an initial infection,
which usually occurs in infancy and affects the oral mucous membrane. Infections recur in the form of
cold sores or fever blisters when triggered by stress at a later time. Cold sores are often mistaken for
canker sores that are similar in appearance but appear in different areas such as the tongue, cheeks, and
inner surface of the lips. Their cause is unknown but may be related to stress. Transmission by skin
contact among wrestlers is common (herpes gladiatorum). These infections are due to herpes simplex
type 1 virus (HSV-1). Another infection caused by a similar virus, herpes simplex type 2 virus (HSV-2),
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is transmitted primarily by sexual contact and will be discussed in Chapter 26. Either virus may occasionally cause herpes encephalitis. The official names of these viruses are human herpesvirus 1 and 2.
Measles (Rubeola)
The measles virus is spread by the respiratory route. Measles is extremely dangerous; complications
such as encephalitis, which may result in brain damage, occur once in every 1000 cases, and once in
about 3000 cases there is a fatality. Koplik’s spots on the oral mucosa are diagnostically useful.
Rubella
Rubella (German measles) is caused by a virus and is much milder than rubeola, often being subclinical.
There is a light fever and a macular rash of small red spots. Transmission is by the respiratory route.
Infections during the first trimester of pregnancy can lead to the congenital rubella syndrome, causing
serious defects such as deafness, eye cataracts, hearing defects, and mental retardation. Diagnostic tests
are mainly used to determine the immune status of pregnant women.
Other Viral Rashes
Fifth Disease (Erythema Infectiosum). The name fifth disease derives from a 1905 list of skin rash
diseases; it was fifth on the list. Caused by human parvovirus B19, symptoms are mild and influenzalike, but with a “slapped-cheek” facial rash.
Roseola.
Roseola is a mild, common childhood disease. A high fever is followed in a few days by a
body rash lasting for a day or two. The pathogen is human herpesvirus 6 (HHV-6).
Fungal Diseases of the Skin
A fungal infection of the body is referred to as a mycosis. The ability of fungi to resist high osmotic
pressure and low moisture makes the skin susceptible to fungal infections.
Cutaneous Mycoses. Fungi that colonize hair, nails, and the outer layer of the epidermis are dermatophytes. Ringworm, or tinea (called dermatomycoses), is caused by members of the genera Microsporum,
Trichophyton, and Epidermophyton. Tinea capitis, or ringworm of the scalp, is spread by contact among
children and by contact with various fomites, dogs, and cats. Ringworm of the groin (jock itch) is tinea
cruris, and ringworm of the feet (athlete’s foot) is tinea pedis. Hair and nails, high in keratin, are areas for
which fungal diseases, such as tinea unguium (onychomycosis), have an affinity. Topical agents for tinea
infections are miconazole and clotrimazole. When nails are infected, itraconazole or terbinatine are recommended. An oral antibiotic, griseofulvin, localizes in keratinized tissue and is an effective treatment.
Subcutaneous Mycoses. Subcutaneous mycoses are caused by fungi inoculated into a wound. The
most commonly encountered is sporotrichosis, caused by Sporothrix schenckii. The nodule formed by the
growing fungus may spread along the lymphatic vessels.
Candidiasis. Candida albicans is a yeastlike fungus that causes infections of the mucous membranes of
the genitourinary tract (vaginitis) and oral mucosa (thrush). These infections, collectively called candidiasis, often appear when bacterial populations are suppressed by antibiotics that do not affect fungi. For
topical use, clotrimazole or miconazole are recommended.
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249
Parasitic Infestation of the Skin
Scabies
The tiny mite Sarcoptes scabiei burrows under the skin to lay its eggs. This sets up an inflammation
that itches intensely. Treatment is by topical application of permethrin insecticide or gamma benzene
hexachloride.
Pediculosis (Lice).
Infestation by lice is called pediculosis. It is caused by a subspecies of Pediculus
humanus, Pediculus humanus capitis. Treatment is by insecticides, but resistance is common. An alternative
treatment is to comb out the nits (egg cases attached to hairs).
Microbial Diseases of the Eye
Inflammation of the Eye Membranes
Conjunctivitis is an inflammation of the conjunctiva, the mucous membrane that lines the eyelid and
covers the outer surface of the eyeball. It is often called red eye or pink eye.
Bacterial Diseases of the Eye
Neonatal gonorrheal ophthalmia. Neonatal gonorrheal ophthalmia is a dangerous eye inflammation transmitted to the newborn during passage through the birth canal. Newborn infants have their
eyes washed with 1% silver nitrate or antibiotics to prevent this infection.
Inclusion conjunctivitis.
Inclusion conjunctivitis is caused by the same organisms but involves only
the conjunctiva, not the cornea. This disease may be spread (to newborns) by an infection in the birth
canal or by unchlorinated water. Tetracycline is used topically in treatment.
Trachoma.
Trachoma is the greatest single cause of blindness in the world. It is an infection of the
epithelial cells of the eye caused by Chlamydia trachomatis. Scar tissue forms on the cornea. It is transmitted by contact with fomites. Tetracycline is useful in treatment.
Other Infectious Diseases of the Eye
Herpetic keratitis. Herpetic keratitis is caused by herpes simplex type 1 virus, with an epidemiology
similar to cold sores. Corneal ulcers occur and may lead to blindness. Trifluridine is the drug of choice
against this viral infection.
A freshwater amoeba, Acanthamoeba, causes a keratitis (Acanthamoeba keratitis) in the eyes of contact lens wearers. The damage is severe and may require a corneal transplant to correct.
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Self-Tests
In the matching section, there is only one answer to each question; however, the lettered
options (a, b, c, etc.) may be used more than once or not at all.
I. Matching
1. The inner layer of the skin, composed of connective tissue.
a. Mucous membrane
2. The lining of the inner eyelid and the surface of the
eyeball.
b. Stratum corneum
3. Some of these specialized epithelial cells are ciliated.
4. The outermost epidermal layer; consists largely of dead
cells containing the protein keratin.
5. Extracellular material to which epithelial cells of mucous
membrane are attached.
c. Dermis
d. Conjunctiva
e. Basement membrane
f. Exanthem
g. Enanthem
6. A skin rash that arises from disease conditions.
II. Matching (Skin Lesions)
1. Vesicles.
a. Flat, reddened
2. Papules.
b. Small, fluid-filled
3. Bullae.
c. Fluid-filled lesions larger than
about 1 cm
4. Macules.
d. Raised lesions
III. Matching
1. Streptococcus pyogenes.
a. Scalded skin syndrome
2. Staphylococcus aureus.
b. Acne
3. Tinea.
c. Erysipelas
4. Propionibacterium acnes.
d. Warts
5. Papillomavirus.
e. Ringworm
Microbial Diseases of the Skin and Eyes
IV. Matching
1. Variola.
a. Impetigo
2. Varicella.
b. Smallpox
3. Herpes zoster.
c. Measles
4. Rubeola.
d. Chickenpox
5. Rubella.
e. German measles
6. Shingles.
V. Matching
1. Ringworm.
a. Cutaneous mycoses
2. Tinea pedis.
b. Superficial mycoses
3. Dermatophytes.
c. Subcutaneous mycoses
4. Thrush.
d. Candidiasis
5. Sporotrichosis.
e. Nodular cystic acne
6. Treated with isotretinoin (Accutane).
f. Comedonal acne
VI. Matching
1. Swimmer’s ear, usually caused by pseudomonads.
a. Herpetic keratitis
2. Boils.
b. Trachoma
3. Idoxuridine is an effective chemotherapeutic treatment.
c. Otitis externa
4. Chlamydia-caused disease.
d. Furuncles
VII. Matching
1. Treatment of cystic acne.
a. Teratogenic
2. The location of M protein of streptococci.
b. Fibrils on cell surface
3. Causes birth defects.
c. Isotretinoin
4. Prevention of neonatal gonorrheal ophthalmia.
d. Mite
5. Scabies.
e. Silver nitrate
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VIII. Matching
1. Ringworm of the scalp.
a. Reye’s syndrome
2. First disease deliberately eliminated on Earth.
b. Measles (rubeola)
3. Koplik’s spots are diagnostic.
c. Smallpox
4. Athlete’s foot.
d. Tinea pedis
5. Occasional complication of chickenpox and influenza.
e. Tinea capitis
6. Fungal infection of the nails.
f. Tinea unguium
Fill in the Blanks
1. The eyes are washed by tears, and the enzyme
in tears destroys many
bacteria.
2. If a boil undertakes a more extensive invasion of the surrounding tissue, it is termed a
.
3. Streptococcus pyogenes is an example of group A
hemolytic streptococci.
4. The blue-green pus caused by opportunistic infections of burn patients is due to Pseudomonas aeruginosa forming water-soluble
pigment.
5. Benzoyl peroxide is useful in treating
.
6. A fungal infection of the body is referred to as a
.
7. The presence of viruses in the bloodstream is termed
.
8. The single most prevalent infectious cause of blindness in the world is the chlamydial disease
.
9. Herpetic keratitis is caused by herpes simplex type
10. An eyelash follicle infection is called a
11. Propionibacterium acne metabolizes
virus.
.
, forming free fatty acids that cause the
inflammation leading to acne scarring.
12. Congenital rubella syndrome is caused by rubella infection during the first
of pregnancy.
13. The
other species of Staphylococcus.
test is used to distinguish pathogenic Staphylococcus aureus from
Microbial Diseases of the Skin and Eyes
14. Scarlet fever rash is caused by an
-type toxin.
15. A protozoan disease of the eye is caused by the aquatic amoeba
16. Pediculosis is caused by Pediculus humanus
253
.
.
Label the Art
j.
k.
a.
l.
b.
c.
d.
e.
m.
f.
g.
h.
n.
i.
Critical Thinking
1. A surgical patient acquires a nosocomial infection. The primary organism isolated from the surgical
wound is coagulase positive and resistant to penicillin. What is the probable etiologic agent? Why is
this organism a common cause of nosocomial infections?
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Chapter 21
2. Discuss the role of Propionibacterium in acne. Does Propionibacterium provide any benefits to its host?
3. Why are the symptoms of shingles different from those of chickenpox even though they are caused
by the same virus?
4. A 43-year-old patient consulted his physician concerning mild inflammation of his eyes.
The patient was instructed to avoid wearing his contact lenses for one week. The symptoms worsened over the next few days. After eliminating the possibility of a bacterial agent, the physician
ordered a corneal scraping. What etiologic agent does the physician suspect? What evidence of this
agent is the physician looking for in the scraping?
5. Why were efforts to eliminate smallpox successful?
Answers
Matching
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
1. c
1. b
1. c
1. b
1. a
1. c
1. c
1. e
2. d
2. d
2. a
2. d
2. a
2. d
2. b
2. c
3. a
3. c
3. e
3. d
3. a
3. a
3. a
3. b
4. b
4. a
4. b
4. c
4. d
4. b
4. e
4. d
5. e 6. f
5. d
5. e 6. d
5. b 6. e
5. d
5. a 6. f
Microbial Diseases of the Skin and Eyes
255
Fill in the Blanks
1. lysozyme 2. carbuncle 3. beta- 4. pyocyanin 5. acne 6. mycosis 7. viremia 8. trachoma
9. 1 10. sty 11. sebum 12. trimester 13. coagulase 14. erythrogenic 15. Acanthamoeba
16. capitis
Label the Art
a. Stratum corneum b. Hair erector muscle c. Oil gland d. Blood vessel e. Hair follicle
f. Duct of sweat gland g. Nerve h. Sweat gland i. Adipose tissue j. Hair shaft k. Sweat pore
l. Epidermis m. Dermis n. Subcutaneous layer
Critical Thinking
1. The etiologic agent is probably Staphylococcus aureus. It is a common cause of nosocomial infections
because it is a part of the normal microbiota of most people. That means hospital staff, visitors, and
even the patient are possible sources of infection.
2. Propionibacterium acnes is part of the normal microbiota of the skin. This organism is able to metabolize sebum and produces acid by-products that discourage the growth of potential pathogens.
When sebum channels become blocked and rupture, bacteria—especially Propionibacterium acnes—
become involved. The bacterium metabolizes sebum into free fatty acids that cause an inflammatory
response. This leads to tissue damage and possible scarring.
3. The symptoms of shingles are different from those of chickenpox because shingles is a different
expression of the virus. After having had chickenpox, the patient has partial immunity to the virus.
4. The physician suspects Acanthamoeba keratitis and is looking for trophozoites or cysts in the corneal
scraping.
5. The efforts to eliminate smallpox were successful primarily because of the lack of animal reservoirs.
Once an effective vaccine was available, a concerted effort was coordinated by the World Health
Organization that resulted in elimination of the disease.