Ch 19 Microbial Diseases of the Skin and Wounds: Structure, Microbiome, and Pathogenesis

Microbial Diseases of the Skin and Wounds

Structure of the Skin

The skin is the largest organ of the human body and serves as a primary barrier against microbial invasion. It consists of multiple layers, each with distinct functions and cell types.

• Epidermis: The outermost layer, composed mainly of dead keratinized cells. Basal cells at the base of the epidermis are actively dividing.

• Dermis: Contains connective tissue, blood vessels, sweat glands, and hair follicles.

• Hypodermis: The deepest layer, primarily made up of fat and connective tissue.

• Functions: Protection, sensation, temperature regulation, and synthesis of vitamin D.

Wounds and Microbial Entry

Wounds are any trauma to body tissues, such as cuts, scrapes, burns, or surgical incisions. They provide entry points for microbes, potentially leading to severe or fatal infections if body defenses are compromised.

• Types of wounds: Cuts, scrapes, burns, bites, surgical wounds.

• Risk: Microbes can infect deeper tissues, but most are eliminated by immune defenses.

Microbiome of the Skin

The skin microbiome consists of various microorganisms that are typically harmless and play a role in protecting against pathogens. The skin's high salt concentration, sebum, and slightly acidic pH limit microbial survival.

• Key features: Harmless microbes compete with pathogens for nutrients and space, produce chemicals that inhibit other microbes, and cannot be completely removed by cleansing.

• Common microbes:

  • Yeast: Malassezia

  • Gram-positive bacteria: Staphylococcus epidermidis, Micrococcus, Cutibacterium acnes

  • Diphtheroids: Cutibacterium acnes

• Pathogenic potential: Disease occurs if microbes penetrate the epidermis or if the immune system is suppressed.

Bacterial Diseases of the Skin

Folliculitis

Folliculitis is an infection of the hair follicle, commonly presenting as a pimple or, when at the eyelid base, as a sty. Severe cases can develop into furuncles (boils) or carbuncles (clusters of boils).

• Pathogen: Most commonly caused by Staphylococcus species, especially S. epidermidis and S. aureus.

• Virulence factors: S. aureus is more virulent due to enzymes, toxins, and factors that inhibit phagocytosis.

• Transmission: Direct contact or fomites; infection can spread to blood and organs.

• Diagnosis: Isolation of Gram-positive cocci in clusters from pus.

• Treatment: Mupirocin, dicloxacillin, or vancomycin for resistant strains.

• Prevention: Hand antisepsis and proper hospital procedures to minimize MRSA.

Staphylococcal Scalded Skin Syndrome (SSSS)

SSSS is caused by exfoliative toxins from certain S. aureus strains, leading to red, wrinkled skin and blistering. The outer epidermis peels off in sheets, but the dermis remains unaffected, so scarring does not occur.

• Primarily affects: Infants and young children.

• Transmission: Person-to-person spread.

• Treatment: Penicillin-derived drugs.

• Prevention: Difficult due to widespread presence of S. aureus.

Impetigo

Impetigo is characterized by red patches on the face and limbs that develop into pus-filled vesicles. It is common in children and athletes in contact sports.

• Pathogens: S. aureus (80% of cases), Streptococcus pyogenes (20%).

• Treatment: Oral and topical antimicrobials, careful cleaning.

Erysipelas

Erysipelas is an infection that spreads to the lymph nodes, causing reddening of the face, arms, or legs. It is potentially fatal if untreated.

• Pathogens: Most often Streptococcus, sometimes Staphylococcus.

• Treatment: Penicillin.

Cellulitis

Cellulitis is an infection of the deeper dermis and subcutaneous fat, presenting as a red, swollen area that is hot to the touch. It is treated with oral or IV antibiotics.

• Pathogens: Group A β-hemolytic Streptococcus, Streptococcus pneumoniae, S. aureus.

• Risk factors: Poorly controlled diabetes, compromised skin.

Necrotizing Fasciitis

Necrotizing fasciitis is a rapidly progressing infection characterized by redness, intense pain, and swelling. It is most often caused by S. pyogenes, which produces enzymes and toxins that destroy tissue.

• Symptoms: Fever, nausea, malaise, mental confusion.

• Diagnosis: Early diagnosis is difficult; intense pain is a unique symptom.

• Treatment: Removal of affected tissue, broad-spectrum antimicrobials.

• Prevention: Difficult due to common presence of S. pyogenes.

Acne

Acne is commonly caused by Cutibacterium acnes, a Gram-positive, rod-shaped diphtheroid. It is a normal member of the skin microbiome and typically begins in adolescence.

• Pathogenesis: Blockage of hair follicles by sebum and dead cells leads to inflammation and pus formation.

• Treatment: Antimicrobial drugs, exfoliation, Accutane, ultraviolet light.

Cat Scratch Disease

Cat scratch disease is caused by Bartonella henselae, typically following a scratch or bite from an infected cat.

Pseudomonas Infection

Pseudomonas infections are caused by Pseudomonas aeruginosa, a Gram-negative bacillus found in soil and moist environments. It is notable for its blue-green pigment, pyocyanin, and multidrug resistance.

• Symptoms: Fever, chills, shock; blue-green discoloration in massive infections.

• Treatment: Combination of antimicrobials; debridement of burns is required.

Spotted Fever Rickettsiosis (Rocky Mountain Spotted Fever)

Rocky Mountain spotted fever is caused by Rickettsia rickettsii, a Gram-negative intracellular parasite. It is transmitted via tick bites and presents with a non-itchy spotted rash (petechiae) and potential organ failure.

• Diagnosis: Serological testing.

• Treatment: Antimicrobials.

• Prevention: Tick repellents and avoidance of tick-infested areas.

Cutaneous Anthrax

Cutaneous anthrax is caused by Bacillus anthracis and is characterized by a black, painless ulcer called an eschar. It is treated with antimicrobial drugs and prevented by controlling the disease in animals.

Gas Gangrene

Gas gangrene is caused by Clostridium perfringens and occurs when endospores are introduced into dead tissue. It is a rapidly progressing and often fatal infection.

• Treatment: Surgical removal of dead tissue, antitoxin, antimicrobials.

• Prevention: Proper cleaning of wounds.

Viral Diseases of the Skin and Wounds

Poxviruses

Poxviruses, including smallpox, orf, cowpox, and monkeypox, cause skin lesions that progress through distinct stages. Smallpox is the first human disease to be eradicated.

• Pathogenesis: Smallpox spreads via inhalation; other poxviruses by direct contact.

• Diagnosis: Based on lesion stages.

• Treatment: Immediate vaccination.

Herpes Infections

Herpes infections are caused by human herpesviruses 1 and 2, resulting in slow-spreading, painful, and itchy skin lesions. The virus can remain latent and reactivate, causing recurrent lesions.

• Sites: Oral cavity (herpetic gingivostomatitis), finger (whitlow), skin (herpes gladiatorum).

• Diagnosis: Characteristic lesions, immunoassay for viral antigens.

• Treatment: Chemotherapeutic drugs to control symptoms.

Warts

Warts are benign growths caused by human papillomavirus (HPV). The virus infects skin cells, leading to rapid cell growth and formation of papillomas.

Chickenpox and Shingles

Chickenpox is a highly contagious disease characterized by widespread skin lesions. The virus becomes latent in sensory nerves and can reactivate later in life as shingles, which presents as localized lesions along an infected nerve.

• Epidemiology: Chickenpox mostly affects children; shingles risk increases with age.

• Treatment: Symptom relief, antiviral medications, vaccines available.

Summary Table: Major Bacterial Skin Diseases

Disease	Pathogen	Key Symptoms	Treatment
Folliculitis	Staphylococcus aureus, S. epidermidis	Pimple, sty, boil	Mupirocin, dicloxacillin, vancomycin
Impetigo	S. aureus, S. pyogenes	Pus-filled vesicles	Oral/topical antimicrobials
Erysipelas	Streptococcus	Reddening, lymph node infection	Penicillin
Cellulitis	S. pyogenes, S. aureus	Red, swollen, hot area	Antibiotics (oral/IV)
Necrotizing Fasciitis	S. pyogenes	Intense pain, tissue destruction	Surgical removal, antimicrobials
Acne	Cutibacterium acnes	Pimples, pustules	Antimicrobials, exfoliation
Cat Scratch Disease	Bartonella henselae	Lesion at scratch site	Antimicrobials
Pseudomonas Infection	Pseudomonas aeruginosa	Fever, shock, blue-green pigment	Combination antimicrobials
Rocky Mountain Spotted Fever	Rickettsia rickettsii	Spotted rash, organ failure	Antimicrobials
Cutaneous Anthrax	Bacillus anthracis	Black eschar	Antimicrobials
Gas Gangrene	Clostridium perfringens	Tissue necrosis, gas production	Surgical removal, antitoxin

Key Concepts and Review Questions

• Why is skin an effective barrier to microbes? The outer layers are dead, covered in salt, and have a lower pH, making it inhospitable to most microbes.

• What is the most common species of Staphylococcus found on the skin? Staphylococcus epidermidis.

• What feature differentiates S. aureus from other staphylococci? Production of coagulase and β-lactamase.

• How does a person acquire Rocky Mountain spotted fever? Tick bite.

• What is shingles? Reactivation of the chickenpox virus, resulting in localized skin lesions along a nerve.