Skin & Soft-Tissue Infections MLAB 2434 – Microbiology Keri Brophy-Martinez

Usual Skin Flora Skin flora consists of those microbes able to adapt to the high salt concentration and drying effects of the skin Permanent Transient Offers protection from pathogens and UV rays

Types of Usual Skin Flora Coagulase-negative Staphylococcus is permanent resident Staphylococcus aureus is transient colonizer Diphtheroids Propionibacterium acnes Corynebacterium xerosis Yeasts Candida Pityrosporum Gram negative rod s

Bacterial Skin Infections Primary Pyodermas Pyoderma: skin infection characterized by inflammation and pus. Common forms Impetigo Erysipelas Cellulitis Folliculitis Furuncles Carbuncles Paronychia

Impetigo Most often caused by: 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 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

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 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 Infection of the cuticle surrounding the nail bed S. aureus & Candida are usual pathogens

Bacterial Skin & Soft Tissue Infections (cont’d) 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

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

Other Causative Organisms for Skin Infection Mycobacterial Spirochetal Treponema Viral Warts, herpes simplex, rubella, roseola, rubeola/measles Fungal Dermatophytoses, Candidiasis Parasites Filaria, Hookworm, Leishmania

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 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 Interpretation of Cultures 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 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.