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
The dermis is the inner, relatively thick portion of skin,
composed mainly of connective tissue. The hair follicles, sweat
gland ducts, and oil gland ducts in the dermis provide passageways
through which microorganisms can enter the skin and
penetrate deeper tissues
provides moisture and some nutrients for microbial
growth. However, it contains salt, which inhibits many
microorganisms; the enzyme lysozyme, which is capable of
breaking down the cell walls of certain bacteria; and antimicrobial
secreted by oil glands, is a mixture of lipids (unsaturated
fatty acids), proteins, and salts that prevents skin and hair
from drying out. Although the fatty acids inhibit the growth of
certain pathogens, sebum, like perspiration, is also nutritive for
linings of body cavities, such as those associated with the
gastrointestinal, respiratory, urinary, and genital tracts, the outer
protective barrier differs from the skin. It consists of sheets of tightly packed epithelial cells
What do you perceive from this illustration to be the weak
points that would allow microbes to reach the underlying
tissue by penetrating intact skin?
between the hair follicle and hair shaft, through which microbes can
penetrate the deeper tissues. They can also enter the skin through sweat
The moisture provided by perspiration encourages microbial
growth on the skin. What factors in perspiration discourage
Normal Microbiota of the Skin
The skin’s normal microbiota contain relatively large
numbers of gram-positive bacteria, such as staphylococci and
micrococci. Gram-positive cocci tend to be relatively resistant
to environmental stresses such as drying and the high osmotic
pressures found in concentrated salt or sugar solution.
Also part of the skin’s normal microbiota are gram-positive
pleomorphic rods called diphtheroids. Some diphtheroids, such
as Propionibacterium acnes, are typically anaerobic and inhabit
A few gram-negative
bacteria, especially Acinetobacter, colonize the skin.
Malassezia furfur, is capable of growing on oily skin secretions
and is thought to be responsible for the scaling skin condition
known as dandruff
Are skin bacteria more likely to be gram-positive or gram-negative?
Gram-positive. Gram-positive cocci tend to be relatively resistant
to environmental stresses such as drying and the high osmotic
pressures found in concentrated salt or sugar solution.
small, fluid-filled lesions
Vesicles larger than about 1 cm in
diameter are termed bullae
Flat, reddened lesions
are known as macules
papules or, when they contain pus, pustules
Raised lesions are called
papules or, when they contain pus, pustules
when they contain pus
A skin rash that arises from disease
rash on mucous membranes, such
as the interior of the mouth, such a rash is called an enanthem
Bacterial Diseases of the Skin
Two genera of bacteria, Staphylococcus and Streptococcus, are frequent
causes of skin-related diseases
genera also may produce invasive enzymes and damaging toxins.
Staphylococcal Skin Infections
spherical gram-positive bacteria that form irregular
clusters like grapes
For almost all clinical purposes, these bacteria can be
divided into those that produce coagulase, an enzyme that coagulates
(clots) fibrin in blood, and those that do not
Coagulase-negative strains, such as Staphylococcus epidermidis,
are very common on the skin, where they may represent 90% of the normal microbiota
S. aureus is the most pathogenic of the staphylococci (also
see the discussion of MRSA in Chapter 20). It is a permanent
resident of the nasal passages of 20% of the population, and an
additional 60% carry it there occasionally. Exposed on surfaces,
it can survive for months. Typically, it forms golden-yellow colonies.
This pigmentation is protective against the antimicrobial
effects of sunlight; mutants without it are also more susceptible
to killing by neutrophils
all pathogenic strains of S. aureus are coagulase-positive
What is the most likely source of the bacteria that grew on the
infections of the hair follicle often occur as pimples.
The infected follicle of an
eyelash is called a sty
more serious hair follicle infection is
the furuncle (boil), which is a type of abscess, a localized region
of pus surrounded by inflamed tissue. Antibiotics do not penetrate
well into abscesses, and the infection is therefore difficult
a hard, round deep inflammation of tissue under
the skin. At this stage of infection, the patient usually exhibits
the symptoms of generalized illness with fever
Staphylococci are the most important causative organism of
This is a highly contagious skin infection mostly affecting
children 2 to 5 years of age, among whom it is spread by direct
Streptococcus pyogenes, a pathogen that can also cause impetigo
two forms of impetigo
(2)bullous impetigo-is caused by
a staphylococcal toxin and is a localized form of staphylococcal
scalded skin syndrome
pemphigus neonatorum or
impetigo of the newborn
bullous impetigo are a frequent problem in hospital nurseries
toxic shock syndrome
Scalded skin syndrome is also characteristic of the late stages
of toxic shock syndrome (TSS).
In this potentially life-threatening
condition, fever, vomiting, and a sunburnlike rash are followed
by shock and sometimes organ failure, especially of the kidneys.
toxic shock syndrome toxin 1 (TSST-1)
is formed at the
growth site and circulates in the bloodstream. The symptoms are
thought to be a result of the superantigenic properties of the toxin
Streptococcal Skin Infections
are gram-positive spherical bacteria.
streptococcal cells usually grow in chains
As streptococci grow, they secrete toxins and enzymes, virulence
factors that vary with the different streptococcal species.
Among these toxins are hemolysins, which lyse red blood cells.
Depending on the hemolysin they produce, streptococci are
categorized as alpha-hemolytic, beta-hemolytic, and gammahemolytic
(actually nonhemolytic) streptococci
Hemolysins can lyse not only red blood cells, but
almost any type of cell
Streptococcal skin infections are generally localized,
but if the bacteria reach deeper tissue, they can be highly
Is the M protein more likely to be antigenic than a polysaccharide
No. This protein
is external to the cell wall on a fuzzy layer of fibrils. The
M protein prevents the activation of complement and allows
the microbe to evade phagocytosis and killing by neutrophils
enzymes that dissolve blood clots
an enzyme that dissolves the hyaluronic acid in the connective
tissue, where it serves to cement the cells together
enzymes that degrade DNA
that lyse red blood
cells and are toxic to neutrophils
S. pyogenes infects the dermal layer of the skin, it
causes a serious disease
In this disease, the skin
erupts into reddish patches with raised margins
It can progress to local tissue destruction and even enter the
bloodstream, causing sepsis.
The infection usually
appears first on the face and often has been preceded by a
streptococcal sore throat. High fever is common.
S. pyogenes has remained sensitive to β-lactam-type antibiotics,
group A streptococcal
caused by the “flesh-eating bacteria,
necrotizing fasciitis may destroy tissue
as rapidly as a surgeon can remove it, and mortality rates from
often associated with streptococcal
toxic shock syndrome (streptococcal TSS)
What is the name of the primary toxin that leads to tissue invasion
by the pathogen?
by certain streptococcal M-protein types, which acts
as a superantigen, causing the immune system to contribute to
Infections by Pseudomonads
Pseudomonads are aerobic gram-negative rods that are widespread
in soil and water
This is a self-limiting rash of about 2 weeks’ duration,
often associated with swimming pools and pool-type saunas
otitis externa, or “swimmer’s ear,” a
painful infection of the external ear canal leading to the eardrum
that is frequently caused by pseudomonads
produces several exotoxins that account for
much of its pathogenicity. It also has an endotoxin.
often grows in dense biofilms that contribute to its frequent identification as a cause of nosocomial
infections of indwelling medical tubes or devices
very common and serious opportunistic
pathogen in burn patients, particularly those with secondand
third-degree burns. Infection may produce blue-green pus,
whose color is caused by the bacterial pigment pyocyanin
disease is caused by Mycobacterium ulcerans, which is similar to
the mycobacteria that cause tuberculosis and leprosy.
pathogen is introduced into the skin, it causes a disease that progresses
slowly with few serious early signs or symptoms. ,
the result is a deep ulcer that often becomes massive
and seriously damaging.
Untreated, this can be so extensive as to
require amputation or plastic surgery. This tissue damage is attributed
to the production of a toxin, mycolactone.
the infection is associated with contact with swamps and
The pathogen probably enters through a
break in the skin from a minor cut or an insect bite
Which bacterial species features the virulence factor M protein?
What is the common name for otitis externa?
Warts, or papillomas
are generally benign skin growths caused
by viruses. It was long known that warts can be transmitted from
one person to another by contact, even sexually
most common medical
treatments for warts are to apply extremely cold liquid nitrogen
(cryotherapy), dry them with an electrical current (electrodesiccation),
or burn them with acids
caused by an orthopoxvirus
There are two basic forms of this disease:
variola major, with a mortality rate of 20% or higher, and
variola minor, with a mortality rate of less than 1%.
Transmitted by the respiratory route, the viruses infect
many internal organs before they eventually move into the
bloodstream, infecting the skin and producing more recognizable
The growth of the virus in the epidermal layers
of the skin causes lesions that become pustular after 10 days or
cold sores or fever blisters
painful, short-lived vesicles that occur near the outer red
margin of the lips
similar to cold sores in appearance, canker sores usually appear
in different areas. They occur as painful sores on movable mucous
membranes, such as those on the tongue, cheeks, and inner surface
of the lips
previously vaccinated persons,
remains latent in the trigeminal nerve ganglia
communicating between the face and the central nervous
Recurrences can be triggered by events
such as excessive exposure to ultraviolet radiation from the
sun, emotional upsets, or the hormonal changes associated with
HSV-1 infection can be transmitted by skin contact among
infections of the finger caused by contact with
HSV-1 lesions—as are children with herpetic oral ulcers
transmitted primarily by sexual
contact. It is the usual cause of genital herpes
HSV-2 is differentiated from HSV-1 by its antigenic makeup and by its effect on cells in tissue culture.
It is latent in the sacral nerve
ganglia found near the base of the spine
a different location from
that of HSV-1.
either type of the herpes simplex virus may
spread to the brain
extremely contagious viral disease (the
measles virus) that is spread by the respiratory route
measles vaccine, now usually administered as the MMR
vaccine (measles, mumps, rubella), has almost eliminated measles
in the United States
Why is it potentially possible to eradicate measles?
subacute sclerosing panencephalitis
rare complication of measles, severe neurological symptoms result in
death within a few years
Rubella, or German measles
much milder viral disease than rubeola (measles) and often goes
A macular rash of small red spots and a light fever
are the usual symptoms (Figure 21.15).
Complications are rare,
especially in children, but encephalitis occurs in about 1 case in
6000, mostly in adults.
The rubella virus is transmitted by the
respiratory route, and an incubation of 2 to 3 weeks is the norm
congenital rubella syndrome
certain severe birth defects were associated with maternal
infection during the first trimester (3 months) of pregnancy
Fifth Disease (Erythema Infectiosum)
produces no symptoms at all in about 20% of individuals infected
by the virus (human parvovirus B19, first identified in
Symptoms are similar to a mild case of influenza, but
there is a distinctive “slapped-cheek” facial rash that slowly
In adults who missed an immunizing infection in childhood,
the disease may cause anemia, an episode of arthritis, or,
Roseola is a mild, very common childhood disease.
The child has a high fever for a few days, which is followed by a
rash over much of the body lasting for a day or two. Recovery
leads to immunity.
The pathogens are human herpesviruses
6 (HHV-6) and 7 (HHV-7)—the latter is responsible for 5–10%
of roseola cases.
Both viruses are present in the saliva of most
How did the odd naming of “fifth disease” arise?
Fungi that colonize the hair, nails, and the outer layer (stratum
corneum) of the epidermis (see Figure
they grow on the keratin present in those locations.
these fungal infections are more
informally known as tineas or ringworm
Tinea capitis, or ringworm
common among elementary school
children and can result in bald patches. This characteristic led the
Romans to adopt the name tinea, Latin for clothes moth, because
the infection resembles the holes left by the wormlike larvae of
the moth in wool clothing
Ringworm of the groin, or jock itch
ringworm of the feet, or athlete’s foot
Three genera of fungi are involved in cutaneous mycosis
(1)Trichophyton (trik-ō-fīʹton) can infect hair, skin, or nails;
(2)Microsporum (mī-krō-spôʹrum) usually involves only the hair
(3)Epidermophyton (ep-i-de . r-mō-fīʹton) affects only the skin and nails
tinea unguium or
When nails are infected
most common disease of this type
is sporotrichosis, caused by the dimorphic fungus Sporothrix
Most cases occur among gardeners or other people
working with soil. The infection frequently forms a small ulcer
on the hands.
The fungus often enters the lymphatic system in
the area and there forms similar lesions.
overgrowths by C. albicans
Newborn infants, whose normal microbiota have
not become established, often suffer from a whitish overgrowth of
the oral cavity, called thrush
candidiasis becomes systemic, as can
happen in immunosuppressed individuals
How can antibacterial drugs lead to candidiasis?
the fungus is not affected by antibacterial drugs, it sometimes
overgrows mucosal tissue when antibiotics suppress the normal
bacterial microbiota. Changes in the normal mucosal pH may
have a similar effect
How do sporotrichosis and athlete’s foot differ? In what ways are
Differ: sporotrichosis is subcutaneous while athelet's food is cutaneous.
Similiar: both are caused by fungi
How might the use of penicillin result in a case of candidiasis?
penicillin would suppress normal microbiata allowing C. albicans to overgrow
intense local itching and is caused by the tiny mite Sarcoptes scabiei
burrowing under the skin to lay its eggs
are often visible as slightly elevated, serpentine lines about 1 mm in
Infestations by lice,
Pediculus humanus capitis,
The head louse, Pediculus humanus capitis,
is not the same as the body louse, Pediculus humanus corporis
Pediculus humanus corporis
How is pediculosis transmitted?
head to head contact
What diseases, if any, are spread by head lice, such as Pediculus
the mucous membrane that lines
the eyelids and covers the outer white surface of the eyeball
is an inflammation of the conjunctiva, often
called by the common name red eye, or pinkeye
influenzae is the most common bacterial cause;
is usually caused by adenoviruses.
Chlamydial conjunctivitis, or inclusion conjunctivitis
It is caused by Chlamydia trachomatis, a bacterium
that grows only as an obligate intracellular parasite.
In infants, who
acquire it in the birth canal, the condition tends to resolve spontaneously
in a few weeks or months, but in rare cases it can lead to
scarring of the cornea
is a serious form of conjunctivitis
caused by Neisseria gonorrhoeae (the cause of gonorrhea).
amounts of pus are formed; if treatment is delayed, ulceration
of the cornea will usually result.
The disease is acquired as the
infant passes through the birth canal, and infection carries a
A serious eye infection, and probably the greatest single cause
of blindness by an infectious disease,
It is caused by certain
serotypes of Chlamydia trachomatis but not the same ones
that cause genital infections
The disease is a conjunctivitis transmitted largely by hand
contact or by sharing such personal objects as towels
trichiasis, an in-turning of the eyelashes
Abrasion of the cornea, especially by the
eyelashes, eventually causes scarring of the cornea and blindness
What is the common name of inclusion conjunctivitis?
Why have antibiotics almost entirely replaced the less expensive use
of silver nitrate for preventing ophthalmia neonatorum?
Silver nitrate has been almost entirely replaced
by antibiotics because of frequent coinfections by gonococci and
sexually transmitted chlamydias, and silver nitrate is not effective
characterized by inflammation
of the cornea,
is caused by the same herpes simplex type 1
virus that causes cold sores and is latent in the trigeminal nerves
Of the two eye diseases herpetic keratitis and Acanthamoeba
keratitis, which is the more likely to be caused by an organism
actively reproducing in saline solutions for contact lenses?