Chapter 33 Skin

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Chapter 33 Skin Soft-Tissue Infections

• MLAB 2434 Clinical Microbiology
• Keri Brophy-Martinez

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Usual Skin Flora

• Skin flora consists of those microbes able to
  adapt to the high salt concentration and drying
  effects of the skin
• Coagulase-negative Staphylococcus is permanent
  resident
• Staphylococcus aureus is transient colonizer

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Usual Skin Flora (contd)

• Diphtheroids
• Propionibacterium acnes
• Corynebacterium xerosis
• Yeasts
• Candida
• Pityrosporum

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Bacterial Skin Infections

• Primary Pyodermas
• Pyoderma skin infection characterized by
  necrosis in tissue
• Impetigo
• Most often caused by Streptococcus pyogenes/
  Group A
• Less than 10 caused by S. aureus
• Group B Streptococcus in newborns of infected
  mothers
• Vesicles rupture, creating a thick, yellow,
  encrusted appearance

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Bacterial Skin Infections (contd)
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Bacterial Skin Infections (contd)

• Impetigo (contd)
• Lesions are superficial and painless but pruritic
  and easily spread by scratching
• Bullous impetigo caused by strain of S. aureus
  that produces exfoliative toxin

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Bacterial Skin Infections (contd)

• Impetigo (contd)
• May be complicated by complex-related acute
  glomerulonephritis
• Highly contagious
• Folliculitis
• Inflammation infection of hair follicles.
  Usually found in areas of points of friction
• S. aureus most common agent, but P. aeruginosa
  implicated from swimming pools and whirlpools

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Bacterial Skin Infections (contd)

• Folliculitis (contd)

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Bacterial Skin Infections (contd)

• Furuncles and Carbuncles
• Furuncle deep inflammatory nodule
• Carbuncles abscess extending more deeply into
  subcutaneous fat and may have multiple draining
  sites occur most frequently at nape of neck and
  back of thighs
• Known as boils
• Most caused by S. aureus
• Most require surgical drainage in addition to
  oral, IM, or IV antibiotics

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Bacterial Skin Infections (contd)

• Furuncles and Carbuncles

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Bacterial Skin Infections (contd)

• Cellulitis
• Diffuse inflammation and infection of superficial
  skin layers
• Localized, mildly painful, swelling with poorly
  demarcated margins

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Bacterial Skin Infections (contd)

• Erysipelas
• Deeper form of cellulitis that infects underlying
  dermis and lymphatic channels
• Painful, raised, crimson color with sharp
  demarcated border
• Usually affects face and lower extremities

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Bacterial Skin Infections (contd)

• Cellulitis and erysipelas caused primarily by
  Streptococcus pyogenes roughly 10 caused by S.
  aureus
• Myonecrosis infection of muscle
• Tissue death occuring from a drop in pH due to
  the reduction of pyruvate to lactate from trauma
  or surgery
• Severe pain, edema, cellulitis, production of
  gas, foul-smelling discharge
• Clostridial myonecrosis is called gas gangrene
• Most infections are polymicrobial, with mixed
  aerobic-anaerobic flora
• Flesh-eating disease caused by Group A Strep

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Bacterial Skin Infections (contd)

• Paronychia
• Infection of the cuticle surrounding the nail bed
• S. aureus is usual cause

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Secondary Bacterial Skin Infections

• Cutaneous manifestations of systemic bacterial
  infections
• Some cases of bacteremia will result in various
  morphologic rashes or lesions
• Virbrio vulnificus wounds exposed to marine
  water or shellfish
• P. aeruginosa
• S. aureus
• S. pneumoniae
• H. influenzae

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Secondary Bacterial Skin Infections

• Cutaneous Manifestations from Toxin Production
• Certain organisms are capable of producing toxins
  that affect the skin
• Stapylococcal Scalded Skin Syndrome
• Toxic Shock Syndrome
• Streptococcal Toxic Shock Syndrome
• Scarlet Fever

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Laboratory Diagnosis of Skin Soft-Tissue
Infections

• Cultures should be taken from as deep in the
  wound as possible to avoid contamination with
  skin flora
• Direct gram-stain should be performed on all
  cultures
• If wound exhibits signs of anaerobic organism(s),
  the culture must be cultured anaerobically

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Other Causative Organisms for Skin Infection

• Mycobacterial
• Spirochetal
• Viral
• Warts, herpes simplex, rubella, roseola,
  rubeola/measles
• Fungal
• Dermatophytoses, Candidiasis
• Parasites