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Microbiology: Infection of the Skin and Soft Tissue (Ebright)
OVERVIEW:

Skin can be infected by microorganisms of all types: bacteria, mycobacteria, fungi, virus and parasites

Skin disease caused by these agents in 3 ways:
Exogenous: direct infection by agents external to the skin (ie. bacteria entering through a cut)
Endogenous: spread of organisms to the skin via the blood stream, along nerve pathways, or by extension from
an adjacent site
Indirectly: from toxins produced by bacteria located outside the integument
EXOGENOUS INFECTIONS:

General:
In the majority of cases, some type of skin injury proceeds infection
Most common infecting organisms of the skin are bacterial:
o S.aureus
o S.pyogenes

Common Infections:
Infection
Location
Typical Organism
Clinical Information
Impetigo
Epidermis
S.aureus/S.pyogenes
Erysipelas
Epidermis and
dermis
S.pyogenes
Cellulitis
Epidermis, dermis
and subcutaneous
S.aureus
Streptococcus
Folliculitis
Hair follicle
(superficial)
Hair follicle (deep)
S.aureus
Carbuncle
Multiple furuncles
in confined area
S.aureus
Fasciitis
Fascia
S.pyogenes
S.aureus
Vibrio vulnificus (sea
water exposure)
Myonecrosis
(Gas
Gangrene)
Muscle
Anaerobic streptococci
Gram (-) enterics
Clostridia spp. (gas
gangrene)
Furuncle
(Boil)
S.aureus
-Common in children
-Weeping, crusted lesions (honeycolored)
-Patient is irritable and
uncomfortable, but not febrile or
seriously ill
Post-Streptococcal
Glomerulonephritis: can occur due
to impetigo caused by S.pyogenes
-More serious
-Common in older adults
-Elevation of involved tissue,
forming sharp borders
-Bright red and painful
-Fever common
-Borders not as clear
-Fever is present
-Skin is edematous, warm,
erythematous, tender and painful
-Vesicles/bullae common
-Ecchymosis in severe cases
-Superficial
-Usually multifocal pustules
-Deeper and more extensive
-Large, painful local boils
-Surrounding inflammation or
cellulitis
-Typical area is back of the neck
-Patient often acutely ill
-Requires systemic Abx and surgical
drainage
-Patient acutely ill and in marked
pain
-Skin overlying the infected area
can be unremarkable
-Can also have changes such as gas
in the tissue, hemorraghic bullae
and ecchymosis
Same as above
Treatment
Topical Abx
(Mupirocin)
Oral Abx
Oral or IV penicillin
Oral or IV penicillin
Cefazolin
-Lance, drain (often all
that is needed)
Cephalosporin or
Dicloxacillin
Emergency surgery
and debridement
Systemic Abx (IV)
Same as above
ENDOGENOUS INFECTIONS:

General:
Skin can become infected by microorganisms that spread from another infected site in 3 ways
o Direct extension
o Hematogeneous Spread
o Spread along neurons

Typical Appearance:
Hematogenous Bacterial Infections: lesions are circumscribed (confined to a limited area) and appear singly or
multiply as macule, papules or pustules (not spreading cellulitis)
Hematogenous Viral Infections: present as widespread, symmetric lesions that frequently become confluent
after initially presenting as discrete macules, papules, or vesicles (these rashes are called exanthems)
Viral Infections with Neuronal Spread: only local skin involvement (follow neuron)

Sources of Endogenous Skin Infections:
Direct Extension:
o Osteomyelitis: draining sinus
o Septic Arthritis: draining sinus
o Lymphadenitis:

TB

Atypical mycobacteria

Streptococcal infections
o Oral/Dental Infection:

Actinomycosis

Mixed cellulitis
Hematogeneous Spread:
o Bacteremia:

Menigococcus

Gonococcus: lesions spread out, usually on extremities (macules or papules)

S.aureus

Pseudomonas
o Endocarditis
o Fungemia (candidemia)
o Viremia:

Measles (rubeola): exanthem (macular-papular)
 Koplik spots in oral mucosa very diagnostic;

Rubella (German Measles): exanthema, macular-papular

Chicken pox: exanthem (vesicles)
 Characterized by multiple different patterns in clusters (not all spots of same type
due to the fact that break-outs occur in waves and not all at once)
o Rocky Mountain Spotted Fever
o Secondary Syphilis: lesions on soles of the feet and palms of the hands (as well as elsewhere)
Spread Along Neurons:
o Herpes Simplex Infection:

Herpes labialis: HSV-1 (vesicles on lip)

Herpes genitalis: HSV-2 (genital vesicles/ulcers)
o Varicella Zoster (Shingles): dermatomal spread in older and immunocompromised (vesicles)
TOXIN MEDATED SKIN DISEASE:

Scarlet Fever:
Causative Agent: S.pyogenes
Description: primary strep throat infection that releases an erythrogenic toxin
o Toxin spreads systematically and causes diffuse red rash that feels like sandpaper
o Associated Symptoms:

Circumoral pallor (white area around the mouth)

Deep red lines (Pastia’s lines) in skin fold of neck, axilla, elbows and knees

Strawberry tongue

Extensive desquamation of superficial skin layers on recovery

Staphylococcus aureus:
2 different toxins that cause skin disease:
o Exfoliatin: results in bullous impetigo or scalded skin syndrome (more extensive)

Bullous Impetigo: differs from the S.pyogenes impetigo due to the fact that there are large
vesicles/bullae

Scalded Skin Syndrome: widespread intra-epidermal cleavage, blistering and superficial
epidermal sloughing (scary, but kids often make full recovery)
o Toxic Shock Syndrome Toxin (TSST-1): spread of the toxin results in high fever, hypotension, multiorgan injury and diffuse erythematous rash that desquamates on recovery (especially around nail beds,
hands and feet)