Skin infections 1

Lecture Overview

  • Topic: Skin Infections

  • Major Sections:

    • Innate Defenses of the Skin

    • Skin and Soft Tissue Infections

    • Bacterial Pathogens

    • Staphylococcus aureus

    • Streptococcus pyogenes

    • Pseudomonas aeruginosa

    • Fungal Pathogens

    • Candida albicans

    • Dermatophytes

  • Source Material: Openstax Microbiology textbook, Chapter 21 (Skin and Eye Infections)

  • Abbreviations Used:

    • Gram Positive Cocci (GPC)

    • Gram Positive Bacillus (GPB)

    • Gram Negative Bacillus (GNB)

    • STAU = Staphylococcus aureus

    • GAS = Group A Streptococcus (Streptococcus pyogenes)

    • PSAE = Pseudomonas aeruginosa

The Skin’s Innate Defense Mechanisms

  • Melanin: Provides pigmentation and protection from UV radiation.

  • Antimicrobial Properties: Skin has natural barriers to pathogens through various components.

  • Perspiration:

    • Function: Washes away microbes.

    • Characteristics: Contains a slightly acidic pH and high salt content.

  • Lysozyme:

    • Enzyme present in perspiration.

    • Breaks down peptidoglycan (PG) in bacterial cell walls.

  • Sebum:

    • Oily substance produced by sebaceous glands.

    • Contains lipids and proteins that moisturize skin and lower pH.

Skin Microbiome

  • Skin Microbiome:

    • Comprises a diverse range of microorganisms.

    • Metabolizes sebum leading to buildup of toxic fatty acids.

    • Competes for space and resources with pathogenic organisms.

    • Includes both Gram-positive and Gram-negative bacteria, as well as fungi.

    • Contains both aerobic and anaerobic species.

  • Microbial Diversity: Varies in different skin regions:

    • Oily environments (e.g., scalp)

    • Moist environments (e.g., armpits)

    • Dry environments (e.g., forearms)

Diseases of Skin and Soft Tissue

  • Source:

    • Trauma/wounds, can be endogenous (originating from within) or exogenous (from outside).

    • May involve polymicrobial infections.

  • Transmission: Usually through direct contact.

  • Etiology:

    • Bacterial Pathogens:

    • Staphylococcus aureus

    • Streptococcus pyogenes

    • Pseudomonas aeruginosa

    • And many more.

    • Fungal Pathogens:

    • Candida albicans

    • Dermatophytes

    • Ectoparasites: E.g., Scabies mite.

Infections of the Skin and Soft Tissues

  • Importance of skin as a barrier to pathogens and its role in potentially developing infections.

  • Conditions:

    • Folliculitis: Inflammation of hair follicles characterized by raised lesions.

    • Acne lesions: Result from clogged hair follicles caused by complex lipids, fatty acids, and dead skin cells.

Pyoderma

  • Definition:

    • Pyoderma refers to any skin disease that is pyogenic (produces pus).

  • Signs and Symptoms: Presence of pus.

  • Pathogenesis:

    • Commonly involves S. aureus or S. pyogenes (both are Gram-positive cocci).

    • Part of the skin microbiome that frequently causes skin infections.

    • Known as “staph infections” when caused by S. aureus.

  • Disease features:

    • Abscess: Collection of pus that can form in various body sites.

    • Folliculitis: Infection of hair follicles.

    • Furuncles: Small abscesses deeper in hair follicles.

    • Carbuncles: Connected clusters of furuncles.

    • Impetigo: Characterized by superficial pus-filled vesicles crusting into honey-colored lesions.

Furuncle versus Carbuncle

  • Furuncle (Boil):

    • Contains pus, swelling present.

  • Carbuncle:

    • A deeper pus-filled lesion that often results from multiple furuncles.

Staphylococcal Scalded Skin Syndrome (SSSS)

  • Signs and Symptoms:

    • Skin appearance resembling burns from boiling water.

    • Peeling of the epidermal layer.

  • Transmission:

    • Can occur via infections of the umbilical stump or eyes in infants.

  • Pathogenesis:

    • Caused by S. aureus strains producing exfoliative toxin, leading to layers of skin peeling.

Staphylococcus aureus

  • Characteristics:

    • Gram positive cocci (GPC) arranged in clusters.

    • Catalase positive.

    • Coagulase positive.

  • Diseases:

    • Skin conditions: Impetigo, Erysipelas, Cellulitis, Folliculitis, SSSS.

    • Other: Surgical wound infections, hospital-acquired pneumonia, bacteremia, endocarditis, toxic shock syndrome, food poisoning.

  • Transmission:

    • Direct contact

    • Can be part of normal skin microbiota.

  • Virulence Factors:

    • Numerous exotoxins and enzymes.

  • Lab Diagnosis:

    • Gram stain and culture.

    • NAAT for nasal carriers of S. aureus.

  • Treatment:

    • Methicillin-sensitive Staphylococcus aureus (MSSA): Penicillin.

    • Methicillin-resistant Staphylococcus aureus (MRSA): Vancomycin.

  • Antibiotic Resistance:

    • MRSA is categorized as a serious threat.

    • Vancomycin-resistant Staphylococcus aureus (VRSA) is a concerning threat.

Differentiating Staphylococcus and Streptococcus in the Laboratory

  • Catalase Test:

    • Staphylococcus is catalase positive.

    • Streptococcus is catalase negative.

  • Coagulase Test:

    • Coagulase positive identifies S. aureus.

    • Coagulase negative identifies other Staphylococcus species.

  • Lancefield Grouping:

    • Utilizes agglutination tests for cell wall antigens in Streptococcus, categorized into groups A through T.

Streptococcus pyogenes (Group A Streptococcus)

  • Characteristics:

    • Beta-hemolytic Streptococcus, Lancefield grouping = Group A (GAS).

    • Produces various virulence factors.

  • Virulence Factors:

    • Streptolysins: lyse red blood cells (RBCs).

    • M Proteins: Block complement activation, help evade phagocytes and kill neutrophils; act as adhesins.

    • Hyaluronidase: Dissolves connective tissue.

    • Streptokinases: Dissolve blood clots.

Necrotizing Fasciitis

  • Epidemiology:

    • Rare, but serious invasive skin/wound infection, also known as flesh-eating disease.

    • Estimated 650–850 cases per year.

  • Signs and Symptoms:

    • Skin swelling, increased heat, and darkening.

    • Necrosis and tissue death due to loss of blood flow.

    • Pain that is disproportionate to the injury.

    • Flu-like symptoms.

  • Pathogenesis:

    • Invasive Group A Streptococcus (GAS) strains may also include S. aureus.

    • Exotoxins destroy superficial fascia, subcutaneous fat, and deep fascia.

Pseudomonas aeruginosa Infections

  • Presentation:

    • Hot tub folliculitis: red, itchy rash typically caused by P. aeruginosa in wet conditions.

    • Otitis externa (swimmer’s ear): inflammation may be due to P. aeruginosa, leading to painful swelling.

Pseudomonas aeruginosa Characteristics

  • Characteristics:

    • Gram negative bacilli (GNB).

  • Diseases:

    • Involved in wound and burn infections, otitis externa, urinary tract infections (UTI), pneumonia, bacteremia (sepsis).

  • Transmission:

    • Direct contact from soil and water.

  • Virulence Factors:

    • Biofilm formation, production of pigments like pyocyanin, and endotoxins (LPS).

  • Lab Diagnosis:

    • Gram stain and culture.

  • Treatment:

    • Antibiotics, however, multidrug-resistant strains are identified as a serious threat.

Fungal Skin Infections

Cutaneous Candidiasis

  • Epidemiology:

    • Yeast infection affecting damp, friction-prone skin; considered opportunistic.

  • Signs and Symptoms:

    • Bright red macular rash that is itchy and burning; may exhibit scaling.

  • Pathogenesis:

    • Caused by Candida albicans, which breaks down keratin.

Candida albicans Characteristics

  • Characteristics:

    • Yeast, associated with various diseases including:

    • Cutaneous candidiasis, wound infections, thrush, vaginal yeast infections, and disseminated candidiasis.

  • Transmission:

    • Direct contact or due to superinfection; part of normal microbiota in skin, gastrointestinal (GI) tract, and vaginal tract.

  • Virulence Factors:

    • Biofilm formation, keratinases.

  • Lab Diagnosis:

    • Fungal culture and microscopy smear.

  • Treatment:

    • Oral/topical antifungals (nystatin, fluconazole) and amphotericin B for disseminated disease.

Fungal Skin Infections: Tinea (Ringworm)

  • Epidemiology:

    • Fungal infection caused by molds (dermatophytes) presenting as round skin lesions.

  • Signs and Symptoms:

    • Round scaly lesions with colored rings, thicken and discolored nails/hair.

  • Pathogenesis:

    • Typically affects healthy individuals and is not usually serious.

  • Diagnosis:

    • Dermatophytes grown on Sabouraud dextrose agar for identification.

Dermatophytes Characteristics

  • Types:

    • Include Trichophyton, Microsporum, and Epidermophyton species.

  • Diseases:

    • Cause tinea (ringworm).

  • Transmission:

    • Direct contact with infected surfaces or individuals.

  • Lab Diagnosis:

    • Clinical presentation and fungal culture.

  • Treatment:

    • Topical antifungals (Azoles), oral medications like griseofulvin for persistent cases.

Learning Objectives

  • After attending the lecture and completing assigned readings, students should:

    • Describe innate factors that limit skin infections.

    • Provide examples of normal skin microbiota and their ecological roles.

    • Differentiate between staphylococci and streptococci, naming specific skin infections.

    • Describe key lab tests for differentiating Staphylococcus from Streptococcus.

    • Explain the clinical significance of coagulase-positive Staphylococcus.

    • For bacterial pathogens, articulate Gram morphology, diseases, transmission, virulence factors, diagnosis, and available vaccines (if any).

    • For fungal pathogens, describe morphology, diseases, transmission, diagnosis, and treatment.