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

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Microbial Diseases of the Skin and Wounds

Structure and Function of the Skin

The skin is the largest organ of the human body and serves as a critical barrier against microbial invasion. It is composed of two main layers: the superficial epidermis and the deeper dermis. The skin prevents excessive water loss, regulates temperature, is involved in sensory phenomena, assists in vitamin D formation, and acts as a barrier to pathogens.

Epidermis: Outermost layer, composed of dead, keratinized cells, providing a tough barrier.
Dermis: Contains connective tissue, blood vessels, nerves, hair follicles, and glands.
Functions: Sensory detection, temperature regulation, and protection from mechanical and microbial threats.

Structure of the skin showing epidermis, dermis, and associated structures

Wounds and Their Impact on Skin Integrity

Wounds are trauma to any tissue of the body that breach the mechanical barrier provided by intact skin. Examples include cuts, scrapes, burns, bites, and surgical incisions. Wounds allow microbes to infect deeper tissues, but the body responds by forming blood clots and initiating tissue repair. Most wound infections are eliminated by immune defenses, but some can overwhelm the body and cause severe disease.

Why the Skin Surface Is Inhospitable to Most Microbes

The skin is covered with salt (from sweat) and sebum (an oily lipid), both of which have antimicrobial properties.
The outermost layer consists of dead, keratinized cells, which are regularly shed, removing attached microbes.
Antimicrobial chemicals in sweat and sebum inhibit microbial growth.

Microbiome of the Skin

Composition and Role of Skin Microbiota

The skin microbiome consists of bacteria and yeasts that tolerate the harsh conditions of the epidermis, hair follicles, and sweat ducts. These microbes are generally harmless and compete with pathogens for nutrients and space, producing chemicals that inhibit the growth of other microbes. Vigorous scrubbing reduces but does not eliminate these organisms, as they recolonize from deeper layers.

Yeasts: Malassezia (rarely pathogenic, but can cause disease in immunosuppressed individuals).
Bacteria: Staphylococcus epidermidis (major member of the microbiome), Staphylococcus aureus (more virulent, transient), Micrococcus, and diphtheroids such as Propionibacterium acnes (can cause acne but also provides protection).

Benefits of the Normal Skin Microbiome

Competes with pathogens for nutrients and space.
Produces chemicals that inhibit the growth of harmful microbes.

Bacterial Diseases of the Skin and Wounds

General Overview

Despite the skin's defenses, pathogenic microbes can cause disease if they penetrate the epidermis or if the immune system is compromised.

Folliculitis

Signs and Symptoms

Infection of a hair follicle, resulting in redness, swelling, and pus (pimple).
If at the eyelid base, called a sty.
Furuncle (boil): Large, painful, raised nodule from spread of infection.
Carbuncle: Several furuncles joined together.

Pathogen and Virulence Factors

Most commonly caused by Staphylococcus species, which are facultatively anaerobic, Gram-positive cocci arranged in clusters. Two main species:

Staphylococcus epidermidis: Major member of the microbiome, rarely pathogenic.
Staphylococcus aureus: More virulent, produces a variety of disease conditions.

Scanning electron micrograph of Staphylococcus bacteria

Virulence Factors of Staphylococci

Enzymes (coagulase, staphylokinase, lipase, β-lactamase)
Structures that inhibit phagocytosis (capsule, Protein A)
Toxins (cytolysins, leukocidins, exfoliative toxins, toxic shock syndrome toxin)

Virulence Factor	S. aureus	S. epidermidis
Coagulase	+	-
Staphylokinase	+	-
Lipase	+	+
β-Lactamase	Present in 90% of strains	-
Polysaccharide slime layer	+	+
Protein A	+	-
Cytolysins	+	-
Leukocidin	+	-
Exfoliative toxin	Present in some strains	-
Toxic shock syndrome toxin	Present in some strains	-

Table comparing virulence factors of S. aureus and S. epidermidis

Pathogenesis

Transmitted via direct contact or fomites.
Bacteria invade hair follicles and sebaceous glands, causing inflammation and pus formation.
Can spread to form furuncles, carbuncles, or enter the bloodstream.

Diseases Caused by Staphylococcus aureus

Disease	Where Discussed
Skin disease: folliculitis, sty, furuncle, carbuncle	p. 554
Staphylococcal scalded skin syndrome	p. 556
Impetigo	p. 557
Staphylococcal toxic shock syndrome	Chapter 24
Bacteremia	Chapter 21
Endocarditis	Chapter 21
Pneumonia	Chapter 22
Food poisoning	Chapter 23

Table of diseases caused by Staphylococcus aureus

Epidemiology

S. epidermidis: Common on skin, rarely causes disease except in immunocompromised or with prosthetic devices.
S. aureus: Not a permanent resident but often found in nostrils and moist skin folds; about 20% of people are asymptomatic carriers.

Diagnosis, Treatment, and Prevention

Diagnosis: Detection of Gram-positive cocci in clusters from pus.
Treatment: Topical mupirocin, dicloxacillin, vancomycin for resistant strains.
Prevention: Aseptic techniques in hospitals.