Skin & Soft-Tissue
Infections
MLAB 2434 – Microbiology
Keri Brophy-Martinez
Usual Skin Flora
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Skin flora consists of those microbes
able to adapt to the high salt
concentration and drying effects of
the skin
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Permanent
Transient
Offers protection from pathogens
and UV rays
Types of Usual Skin Flora
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Coagulase-negative Staphylococcus is
permanent resident
Staphylococcus aureus is transient colonizer
Diphtheroids
 Propionibacterium acnes
 Corynebacterium xerosis
Yeasts
 Candida
 Pityrosporum
Gram negative rods
Bacterial Skin Infections
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Primary Pyodermas
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Pyoderma: skin infection characterized by
inflammation and pus.
Common forms
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Impetigo
Erysipelas
Cellulitis
Folliculitis
Furuncles
Carbuncles
Paronychia
Impetigo
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Most often caused by:
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Streptococcus pyogenes/ Group A
S. aureus- Exfoliative toxin
Vesicles rupture, creating a
thick, yellow, encrusted
appearance
Lesions are superficial and
painless but pruritic and easily
spread by scratching
Highly contagious
Erysipelas
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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
Cellulitis
Diffuse inflammation and infection
of superficial skin layers
 Localized, mildly painful, swelling
with poorly demarcated margins
 Streptococcus pyogenes &
Staphylococcus aureus common
pathogens
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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
Furuncles and Carbuncles
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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
Furuncles and Carbuncles
Paronychia
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Infection of the cuticle surrounding
the nail bed
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S. aureus & Candida are usual
pathogens
Bacterial Skin & Soft
Tissue Infections (cont’d)
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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
Bacterial Skin & Soft
Tissue Infections (cont’d)
Burn wounds
 Bites
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Secondary Bacterial Skin
Infections
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Cutaneous Manifestations from
Toxin Production
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Certain organisms are capable of
producing toxins that affect the
skin
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Stapylococcal Scalded Skin Syndrome
Toxic Shock Syndrome
Streptococcal Toxic Shock Syndrome
Scarlet Fever
Other Causative Organisms
for Skin Infection
Mycobacterial
 Spirochetal
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Viral
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Warts, herpes simplex, rubella,
roseola, rubeola/measles
Fungal
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Treponema
Dermatophytoses, Candidiasis
Parasites
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Filaria, Hookworm, Leishmania
Laboratory Diagnosis of Skin
& Soft-Tissue Infections
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Cultures should be taken from as deep in the
wound as possible to avoid contamination with
skin flora
 Aspirate/ tissue specimen of choice
Direct gram-stain should be performed on all
cultures
Inoculate blood, MacConkey, enrichment
broth
If wound exhibits signs of anaerobic
organism(s), the culture must be cultured
anaerobically
Laboratory Diagnosis of Skin &
Soft-Tissue Infections
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Interpretation of Cultures
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Initial read at 24 hours
• No growth
• Re-incubate for an additional 24 hours
• Growth
• Consider gram stain report
• Consider ratio of normal flora to potential
pathogen
• Work-up pathogen
References
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Kiser, K. M., Payne, W. C., & Taff, T. A. (2011). Clinical
Laboratory Microbiology: A Practical Approach . Upper
Saddle River, NJ: Pearson Education.
Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011).
Textbook of Diagnostic Microbiology (4th ed.).
Maryland Heights, MO: Saunders.