1. CUTANEOUS INFECTIONS Dr. Nancy Cornish Director of Microbiology
Methodist and Children’s Hospitals

2. WHAT IS COVERED? Superficial Erythematous lesions Ulcers, Nodules
Sporotrichoid lesions
Sinus tracts
Burn wounds
Simple post-operative wound infections
Complicated wound infections
Crepitant cellulitis
Necrotizing fascistic
Gas gangrene with myonecrosis
Bite wounds

3. What is not covered? Systemic diseases with skin manifestations
Chicken pox
Rocky mountain spotted fever
Others
Toxin mediated disease
Toxic shock syndrome, others
Parasitic infestations
Scabies, others
Viral diseases
Orf (pox virus), others

6. SKIN INFECTIONS Primary - de novo, no obvious portal of entry
Secondary - secondary to injury
Localized or extensive
Mono microbial or poly microbial

7. SKIN INFECTIONS
Acute (staph folliculitis)
Chronic (fungal infections)

8. Superficial Erythematous Lesions
Erysipelas
Painful, red, edematous, indurated
KEY - sharply demarcated raised border
Fever, chills, regional lymphadenopathy
Causes: Group A strep (rare; Group C or G)
Involves dermis, superficial lymphatics
Diagnosis (Dx) is usually clinical
Treatment (Tx) is penicillin

10. Superficial Erythematous Lesions
Cellulitis
Diffuse spreading infection of loose connective tissue of deeper layers of dermis
KEY - ill defined margins
Causes:
Group A Strep
Penicillin
Staph aureus
Dicloxacillin
Rarely, Aeromonas, Vibrio from swimming in fresh, sea water

13. Cellulitis LAB DX, if needed Disinfect skin
Aspiration of advancing margin of erythema
If necessary, inject 0.1 to 0.5 ml sterile, non bacteriostatic, isotonic saline subcutaneously and aspirate back
Plant blood, chocolate and broth 37°C

16. Cellulitis Success of Cultures at Diagnosis Blood cultures - 4%
Soft tissue aspirate - 10%
Skin punch biopsies - 20%

17. Superficial Erythematous Lesions
Erysipeloid cellulitis
Erysipelothrix rhusiopathiae (gram positive rod, catalase negative, H2S positive)
Occupational disease
Fisherman, butchers, abattoir workers, cooks
Hands, fingers
Painful purplish inflammation with red margin and central pallor
Tx is penicillin

19. Erysipeloid LAB DX Gram stain/culture usually negative
Full-thickness skin biopsy at margin of lesion after decontamination of skin surface
Place in broth, 35°C, 5 days
Subculture daily onto blood agar until positive

20. Superficial Erythematous Lesions
Impetigo
Small erythematous papules →vesicles → thick honey colored crusts
Bullous form (associated with S. aureus)
Causes - Group A strep, Staph aureus and community acquired methicillin resistant Staph aureus (CA-MRSA)
Superficial intra-epidermal infection
Tx is topical antibiotic or dicloxacillin
Ecthyma is deeper form, leads to scarring

26. Impetigo LAB DX Usually clinical If bacterial cultures needed
Clean lesion with alcohol
Remove crust, culture base of lesion
Bullous lesion - aspirate fluid
Plant on blood agar, 37°C

27. Folliculitis, Furunculosis, Carbuncles
Localized cutaneous abscesses
Distinguished by size and extent of subcutaneous involvement

28. Folliculitis
Dome shaped papules, pustules around a hair with rim of erythema

30. Folliculitis CAUSES Staph aureus
Enterobacteriaceae (patients on long term antibiotics)
Proteus
Pseudomonas aeruginosa
Associated with contaminated whirlpools, swimming pools, hot tubs
Itchy rash, buttocks, hips, thighs, axillae
apocrine sweat glands, occluded by bathing suit
Other symptoms
Low grade fever
Malaise
Earache (otitis externa)
Painful breasts (mastitis)

31. Folliculitis LAB DX Treatment Usually not necessary and often negative
Usually local
Topical antibiotics
Warm compresses

32. Furunculosis (Boil)
Abscess destroys tissue around hair follicle

35. Carbuncles
Deep, extensive abscess involving several hair follicles/sebaceous glands
Systemic symptoms
Fever
Malaise
Cellulitis
Bacteremia

38. Furuncles/Carbuncles
Occur where hair follicles are present and subject to friction and perspiration
Staph aureus most common pathogen

39. Furuncles/Carbuncles
Treatment
Warm compresses
Incision and drainage (I & D)
Antibiotics, if necessary
LAB DX
Aspirated pus, collected after sterilization of skin
Plant blood/MacConkey agar, 35°C

40. Paronychia Superficial infection of nail fold Acute; Staph aureus
Drainage, warm compresses
Chronic; Candida species
Frequent immersion of hands in water
Remove cause, topical antifungal medication

42. Paronychia Usually not necessary
LAB DX:
Usually not necessary
Decontaminate skin surface with 70% alcohol collect aspirated/drainage pus
Plant on blood agar, 35°C

43. Whitlow
Purulent lesion on distal phalanx of finger often caused by a puncture wound
Staph aureus
Herpetic Whitlow is due to Herpes simplex often mistaken for bacterial paronychia
How to distinguish herpetic Whitlow from pyogenic cause (bacterial)
Medical, paramedical, dental occupation
Satellite vesicles
Pain disproportionate to physical findings
Non tense pulp space
Serous drainage (not purulent)
HSV lesions, mouth or genitals

44. Staphylococcus aureus - Virulence Factors
Catalase; enzyme attacks H2O2 cell defense system
Fibrinolysins; breaks down fibrin clots
Hyaluronidase; dissolves connective tissue
Lipases; dissolves fat
 lactamases; breaks down  lactam antibiotics “penicillin” and others

45. Staphylococcus aureus - Virulence Factors Contd.
Hemolysins; kills PMN’s, lysis of RBC’s
Exotoxins; kills neutrophils
Exfoliatins; causes shedding of skin
Enterotoxins; causes food poisoning, vomiting, diarrhea
Toxic shock syndrome toxin 1 (TSST-1)
Panton-Valentine leukocidin→punches holes in WBCs (Mostly found in community acquired MRSA)

46. Streptococcus pyogenes (Group A Strep) - Virulence factors
Capsule of hyaluronic acid; prevents phagocytosis
M protein; helps prevent phagocytosis and intracellular killing
Streptolysin O and S; toxic to a variety of cell types (also responsible for hemolysis on blood agar)
Exotoxins; cause rash of scarlet fever, TSS
Hyaluronidase; breaks down connective tissue
Streptokinases; lysis fibrin clots

49. Whitlow LAB DX - HSV Decontaminate skin surface
Firmly sample base of lesion, aspirate vesicles
Submit in viral transport media
Direct fluorescent antibody rapid test (DFA)
Culture
Treatment - HSV
Acyclovir
LAB DX - Bacterial
Usually not needed
Culture aspirate of pus on blood agar, 37°C
Treatment - Bacterial
Dicloxacillin or Clindamycin

50. Cutaneous Fungal Infections (Superficial Mycoses)
Involve the keratinized layer of the epidermis, hair or nails
Causes
Dermatophytes
Candida spp.
Malassezia spp.