Macules: Flat lesions with a change in skin color.
Papules: Raised, solid lesions less than 5 mm in diameter.
Plaques: Flat, elevated lesions larger than 5 mm in diameter (plateau-like).
Nodules: Rounded, raised lesions greater than 5 mm in diameter.
Urticaria (Wheals/Hives): Annular (ring-like) papules or plaques, pinkish in color.
Vesicles: Circumscribed, fluid-filled lesions less than 5 mm in diameter.
Bullae: Circumscribed, fluid-filled lesions greater than 5 mm in diameter.
Pustules: Circumscribed lesions filled with exudate.
Purpura: Skin lesions caused by bleeding into the skin.
Petechiae: Purpura less than 3 mm in diameter.
Ecchymosis: Purpura greater than 3 mm in diameter.
Ulcer: Crater-like lesion involving epidermis and dermis.
Eschar: Necrotic ulcer covered by a blackened scab.
Bacterial Skin Infections
Staphylococci: Staphylococcus aureus
Important Properties:
Common human pathogen.
Gram-positive cocci, arranged individually, in pairs, short chains, or grape-like clusters.
Found on skin and in nasopharynx.
Culture: Gray to golden yellow colonies (best at 20°C–25°C).
Coagulase-positive (differentiates from other Staphylococcus species).
Catalase-positive (differentiates from Streptococci).
Produces enzymes and toxins, responsible for invasiveness and pathogenicity.
Mode of Transmission:
Direct contact with purulent lesions
Hands of healthcare workers
Fomites (bed linens, contaminated clothing)
Clinical Findings:
Folliculitis: Pyogenic infection involving hair follicle; localized painful inflammation; heals rapidly after pus drainage.
Furuncle (Boil): Extension of folliculitis; larger, painful nodules with necrotic tissue.
Carbuncle: Coalescence of furuncles extending into subcutaneous tissue with multiple sinus tracts.
Sty (Hordeolum): Folliculitis at the base of eyelids.
Impetigo: Common in young children; involves face and limbs; begins as a flattened red spot (macule) that becomes a pus-filled vesicle, ruptures, and forms a honey-colored crust; caused by S. aureus and S. pyogenes.
Staphylococcal Scalded Skin Syndrome (Ritter’s disease): Primarily in newborns and young children; sudden perioral erythema spreading across the body within two days; positive Nikolsky sign (skin displacement with slight pressure); bullae and cutaneous blister formation followed by desquamation; exfoliativetoxin-mediated; no scarring.
Beta-lactam antibiotics (e.g., penicillin; however, resistance is common).
Oxacillin is effective against S. aureus.
Incision and drainage of localized infections.
Staphylococcus epidermidis
Part of normal skin flora.
Associated with stitch abscesses, UTIs, and endocarditis.
Causes infections in individuals with prosthetic devices.
Streptococci: Streptococcus pyogenes
Gram-positive cocci arranged in pairs or chains.
Group A Streptococci based on Lancefield classification.
Beta-hemolytic (complete hemolysis of blood).
Major virulence factor: M protein (anti-phagocytic).
Produces enzymes and toxins responsible for pathogenesis.
Some strains are encapsulated, protecting them from phagocytosis and potentially causing severe systemic infections.
Mode of Transmission:
Direct contact with infected persons.
Fomites.
Clinical Findings:
Pyoderma (Impetigo): Localized purulent skin infection involving face and extremities; vesicles progress to pustules, rupture, and form honey-colored crusts; regional lymph node enlargement may occur without systemic infection.
Erysipelas (St. Anthony’s Fire): Follows respiratory tract or skin infection; localized raised areas with pain, erythema, and warmth; distinct from normal skin; lymphadenopathy and systemic manifestations are present.
Cellulitis: Involves skin and subcutaneous tissue; less demarcated borders than erysipelas; local inflammation with systemic signs.
Necrotizing Fasciitis (Flesh-Eating Disease/Streptococcal Gangrene): Deep subcutaneous tissue infection; starts as cellulitis, becomes bullous and gangrenous, spreads to fascia, muscle, and fat; may become systemic and cause multi-organ failure and death.
Complications:
Acute glomerulonephritis (more commonly with skin infections).
Rheumatic fever (usually associated with throat infection).
Laboratory Diagnosis:
Microscopy: Gram stain shows gram-positive cocci in pairs and chains, associated with leukocytes.
Culture: Positive beta-hemolysis on blood agar.
Bacitracin test: (+) zone of inhibition of growth around the Bacitracin disc.
Treatment and Prevention:
Penicillin is the drug of choice.
Macrolides (erythromycin, azithromycin) or cephalosporins for penicillin-allergic patients.
Drainage of pus and thorough debridement of infected tissues.
Pseudomonas aeruginosa
Gram-negative bacilli arranged in pairs and encapsulated.
Opportunistic pathogen; common cause of nosocomial infections; resistant to many antibiotics.
Virulence factors: adhesins (flagella, pili, LPS, alginate), toxins (exotoxin A, pigments), and enzymes.
Mode of Transmission:
Colonization of previously injured skin.
Clinical Findings:
Colonization of burn wounds with blue-green pus and sweet, grape-like odor.
Folliculitis.
Secondary infections in acne and nail infections (from contaminated water).
Osteochondritis (inflammation of bone and cartilage) following penetrating injury.
Laboratory Diagnosis:
Gram stain: gram-negative bacilli arranged individually or in pairs.
Culture: flat colonies with green pigmentation and sweet, grape-like odor.
Oxidase test: positive.
Treatment and Prevention:
Culture and sensitivity testing.
Preventing contamination of sterile hospital equipment and cross-contamination of patients.
Clostridium perfringens
Gram-positive bacilli, anaerobic, rarely produce endospores.
Produces four lethal toxins (alpha, beta, iota, epsilon); alpha toxin is a lecithinase causing lysis of erythrocytes, platelets, and leukocytes, as well as massive hemolysis, bleeding, and tissue destruction.
Widely distributed in nature, associated with soil and water contaminated with feces.
Often seen with soil and water.
Mode of Transmission:
Colonization of skin following physical trauma or surgery.
Clinical Findings:
Soft tissue infections: cellulitis, suppurative myositis, and myonecrosis (gas gangrene).
Gas gangrene: life-threatening infection following physical trauma or surgery; massive tissue necrosis with gas formation, shock, renal failure, and death within two days of onset.
Laboratory Diagnosis:
Microscopic detection of gram-positive bacilli in pairs.
Growth in culture under anaerobic condition.
Treatment and Prevention:
Surgical wound debridement.
High-dose penicillin therapy.
Bacillus anthracis
Gram-positive bacilli arranged individually, in pairs, or long serpentine chains (