BACTERIAL INFECTIONS OF THE SKIN

Introduction

• Infections with pyogenic (pus forming) bacteria• usually Staphylococcus aureus and/or Streptococci

(usually Group A β-haemolytic Streptococci - GABS)

Factors in development of bacterial skin infection

1) the portal of entry 2) the host defences 3) the pathogenic properties of the organism

Classification

- primary infections (pyodermas) - secondary infections

Primary infections (pyodermas)

• infections that are produced by the invasion of normal skin by a single species of pathogenic bacteria

Secondary infections

• Infections after the integrity of the skin has been broken, or the local immune milieu is altered by the primary skin condition

AD, scabies, tinea, …• may show mixture of organisms

Staphylococcus - Commonly carried in nose(35%), perineum (20%), axillae and toe webs (5-10%) - Staphylococcus causes impetigo,

folliculitis, and carbuncles plus deeper infections. - Staph. toxins (epidermolytic) cause

bullous impetigo and SSSS( Staphylococcal

scalded skin syndrome).

Streptococcus - Rarely found on normal skin, often in

throat (10%), occasionally in nose - Main pathogenic type – Lancefield

Group A. - Causes Erysipelas, cellulitis,

lymphangitis, regional lymphadenitis - Post streptococcal state (1-3 weeks

later) can produce – acute GN, rheumatic fever,

rheumatism, erythema nodosum,

psoriasis.

Normal Flora

• organisms that characteristically survive and multiply in various ecologic niches of the skin

• S. epidermidis is the principal staphylococcal species

• Candida • Malasazia furfur , propionibacterium acne

Superficial Cutaneous infections

Impetigo

• infections in the epidermis• untreated pyodermas can extend to the dermis, resulting in ecthyma

Two clinical patterns

• Bullous impetigo and • Non-bullous impetigo

Non-bullous Impetigo70% of impetigo industrialized countries -- S. aureus and

less often by group A streptococcus in developing countries – group A streptococcus remains a common cause

Occurs in children of all ages and adultsusually spreads from nose to normal skinpruritis or soreness

Cutaneous Lesions• initially a transient vesicle or pustule honey- colored crusted plaque

• surrounding erythema• 90% of prolonged, untreated –

regional LAP• may progress to Ecthyma

Bullous Impetigo• by phage group II S. aureus• Three types of eruptions 1) bullous impetigo, 2) exfoliative disease( SSSS) 3) staphylococcal scarlet fever• Extracellular exfoliative toxins

("exfoliatin") types A and B

Cutaneous lesions • more common in new-borns and

infants• rapid progression of vesicles to flaccid

bullae• bullae arises on normal skin• fluid clear yellow- dark yellow –

turbid- collapse – may crust

Laboratory

• Gram stain• Culture• Histology

Treatment - good hygiene removal of crusts. - Antibiotics - topical if mild - mupirocin, fusidic

acid, - Systemic if severe, multiple lesions, - cloxacillin, Erythromycin,

amoxi+ clavulanic acid, cephalexin

Ecthyma usually a consequence of neglected impetigocharacterized by thickly crusted erosions or

ulcerationsCaused by Group A Strept and/or Staph

Commonest in children or debilitated adults, homeless and soldiers

• most commonly on the lower extremities• ulcer has a “punched out” appearance• Covered with dirty greyish-yellow crust• heals slowly

• Treat as impetigo

Folliculitis

• a pyoderma that begins within the hair follicle

• a small, fragile, dome-shaped pustule occurs at the infundibulum (ostium or opening) of a hair follicle

• Children – scalp• Adults - beard area, axillae,

extremities, and buttocks• Can complicate to Furuncles if

untreated

Furuncles – boil • deep-seated inflammatory nodule that

develops around a hair follicle• areas with friction, occlusion, and perspiration• usually from a preceding, more superficial

folliculitis

Cutaneous Lesions

• solitary or multiple • hard, tender, red folliculocentric nodule• undergoes abscess formation Ruptures

Carbunclemore extensive, deeper, communicating, lesion

that develops when multiple, closely set furuncles coalesce.

more serious inflammationred and indurated, and multiple pustules soon

appear on the surface, draining externally around multiple hair follicles scar

fever and malaise - ill

• beware of bacteremia from such lesions esp when appears on the face

• infection such as osteomyelitis, acute endocarditis, or brain abscess

• recurrent furunculosis

Treatment• a systemic antibiotic as impetigo for mild cases• severe infections or infections in a dangerous

areas - maximal antibiotic dosage by the parenteral route

• drain if abscess

Erysipelascaused by group A β- haemolytic

streptococcusacute infection of skin- level of part of

dermissuperficial cellulitis with marked dermal

lymphatic vessel involvementface or a lower extremitysuperficial erythema, edema with a sharply

defined margin to normal tissue

• there may be portal of entry• Recurrent erysipelas – tinea pedis,

lymphedema surgery • Can cause lymphedema

Cellulitisinfection extends deeper into the dermis and

subcutaneous tissueS. aureus and GAS – common causeslooks erysipelas but lack of distinct margins,

deeper edema, surface bulla/necrosis can go deep if untreated – fasciitis regional LAPportal of entry evident in half of cases

Treatment • Supportive - rest, immobilization, elevation, moist heat,

analgesia.• Dressings -cool sterile saline dressings for removal of

purulent exudates and necrotic tissue• Surgical - Drain abscess

• Antimicrobial Therapy

- against Strept in erysipelas - against staph in cellulitis + /- against Strept

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