Bacterial Diseases of the Skin

Streptococcal Skin Infections

Streptococci are gram-positive, catalase negative cocci that often grow in chains.

 

Streptococci are classified according to their hemolytic enzymes and cell wall antigens.

 

Types of Streptococcus

Alpha - partial hemolysis - green zone -due to incomplete lysis of red blood cells

Beta - complete hemolysis -clear zone - complete lysis - due to streptolysin O and streptolysin S

Gamma - no hemolysis

Groups of Streptococcus (A-U)

Classified by cell wall antigens (Lancefield groupings)

Streptococcus pyogenes

 

Group A beta-hemolytic streptococci (serological types of S. pyogenes) produce a number of virulence factors: M protein, erythrogenic toxin, deoxyribonuclease, streptokinases, and hyaluronidase.

 

Classified into two types by the type of M protein (Antiphagocytic, 80 serotypes, antibodies to M protein provides type-specific immunity)

Rheumatogenic - causes rheumatic fever

Nephritogenic - causes acute glomerulonephritis

The M Protein

Erysipelas (reddish patches) and impetigo (isolated pustules) are skin infections caused by S. pyogenes.

 

Erysipelas

Infection of the dermal layer

Reddish patches with raised margins

Infection often begins on the face

Often preceded by streptococcal pharyngitis although it is considered an endogenous infection

High fever

Treatment is penicillin

Erysipelas

Impetigo

Isolated pustules that rupture and become crusted

Incidence in children

Probable mode of spread is contact

Penicillin preferred for treatment (for streptococcal infections, not S. aureus)

Impetigo

Invasive group A beta-hemolytic streptococci cause severe and rapid tissue destruction.

Cellulitis - destruction of solid tissue

Myositis - destruction of muscle

Necrotizing Fascititis - destruction of fascia (connective tissue covering of muscle)

Mode of transmission is direct contact

Treatment is surgical removal of tissue and penicillin

Necrotizing Fascititis

Infections by Pseudomonads

Pseudomonads are gram-negative rods. They are aerobes found primarily in soil and water that are resistant to many disinfectants and antibiotics.

Pseudomonas aeruginosa produces an endotoxin and several exotoxins.

Diseases caused by P. aeruginosa include otitis externa, dermatitis, burn infections, urinary tract infections, and respiratory infections.

Infections have a characteristic blue-green pus caused by the pigment pyocyanin.

Fluoroquinolones, carbenicillin, gentamicin, and beta-lactams are useful in treating P. aeruginosa infections, although Pseudomonas dermitis is usually self-limiting.

Acne

Propionibacterium acnes can metabolize sebum trapped in hair follicles.

Metabolic end-products (fatty acids) cause inflammatory acne.

Tretinoin, benzoyl peroxide, erythromycin, and Accutane are used to treat acne.