Chapter 16 Bacterial infections

Resident flora of the skin The surface of the skin teems with micro-organisms, which are most numerous in moist hairy areas, rich in sebaceous glands. Organisms are found, in clusters Mixture of harmless and poorly classified Staphylococci: Staphylococcus epidermidis, NOT aureus Micrococci Diphtheroids: Aerobic diphtheroids predominate on the surface Anaerobic diphtheroids (Propionibacteria sp.) deep in the hair follicles. Several species of lipophilic yeasts also exist on the skin.

Erythrasma Some diphtheroid members of the skin flora produce porphyrins. Symptom-free macular wrinkled Slightly scaly pink Brown or macerated white areas Found in the armpits, groins or between the toes. In diabetics, larger areas of the trunk may be involved. Diagnosis (Wood’s light  coral pink). DDx: tinea pedis Treatment: Topical fusidic acid Topical antifungal Miconazole

Wood’s light  coral pink Macular wrinkled Slightly scaly pink Wood’s light  coral pink

Staphylococcal infections S.aureus is not part of the resident flora Carried in nostrils, perineum or armpits Multiply on areas of diseased skin such as eczema Cause: Impetigo Ecthyma Furunculosis (boils) Carbuncle Scalded skin syndrome Toxic shock syndrome

Impetigo Erosions in the stratum corneum Caused by Staphylococci (bullous type) Streptococci (crusted ulcerated) Both S. aureus produce Exfoliative toxins, which cleave the cell adhesion molecule desmoglein, If the toxin Localized  blisters of bullous impetigo Generalized  widespread blistering as in the staphylococcal scalded skin syndrome.

A thin-walled flaccid clear blister May become pustular If rupture leave Extending area of exudation Yellowish varnish-like crusting. Multiple Particularly around the face. More obviously bullous in infants. A follicular type of impetigo (superficial folliculitis) is also common. Fig. 16.2 Widespread impetigo due to Staph. aureus with erosions and cruising.

Course: Spread rapidly through a family or class. Clear even without treatment. Complications: Acute glomerulonephritis (Streptococcal impetigo). DDx: Herpes simplex Eczema. Scalp lice (esp. in recurrent impetigo of head & neck) Investigation: Clinically. Swab & culture Treatmant: (after investigation) Systemic antibiotics (flucloxacillin, erythromycin or cefalexin) penicillin V (nephritogenic strain of streptococcus) Removal of crusts by compressing them & application of a topical antibiotic (neomycin, fusidic acid, mupirocin or bacitracin)

Ecthyma Full thickness ulcer  Heals with scarring Ulcers forming under a crusted surface infection. The site may have been that of Insect bite Neglected minor trauma

Furunculosis (boils) Acute pustular infection of a hair follicle Usually with Staphylococcus aureus Adolescent boys are especially susceptible to them. Tender, red nodule enlarges, may discharge pus its central ‘core’ before healing to leave a scar Fever and enlarged draining nodes are rare. Most patients have 1-2 boils only, and then clear. The sudden appearance of many furuncles suggests a virulent staphylococcus including strains of community-aquired MRSA. A few unfortunate persons experience a tiresome sequence of boils (chronic furunculosis).

Tender Red Nodule enlarges May discharge pus Enlarged swollen mass with purulent material

Fig. 16.3 Chronic furunculosis.

Complications: Cavernous sinus thrombosis (boils in central face). Septicaemia DDx: Hidradenitis suppurativa (groin and axillae). Investigations in chronic furunculosis General examination: skin disease (scabies, pediculosis, eczema). Test the urine for sugar. Full blood count. Culture swabs from lesions and carrier sites. Immunological evaluation only Treatment Acute episodes: incision & drainage. Systemic antibiotics (fever or immunosuppressed). Chronic furunculosis topical antiseptic or antibiotic (treat carrier sites twice daily for 6 weeks). Treat family carriers in the same way. Stubborn cases systemic antibiotic (for 6 weeks) Daily bath using an antiseptic soap. Improve hygiene and nutritional state, if faulty.

Carbuncle A group of adjacent hair follicles becomes deeply infected Swollen painful suppurating area Discharging pus from several points. More severe than boil. Diabetes must be excluded. Treatment: Topical and systemic antibiotics. Incision and drainage (speed up healing) DDx: fungal kerion in unresponsive carbuncles.

Swollen painful Discharging pus

Scalded skin syndrome Cause Exfoliative toxins by staphylococcal infection, that cleave the superficial skin adhesion molecule desmoglein 1  acantholysis. Loosening of large areas of overlying epidermis, cause: Erythema Tenderness Affects children and patients with renal failure Most adults have antibodies to the toxin. Treatment: systemic antibiotics DDx: toxic epidermal necrolysis Investigation: ski biopsy (to exclude toxic epidermal necrolysis).

Fig. 16.4 Staphylococcal scalded skin syndrome in a child. The overlying epidermis is loosening in the red areas.

Toxic shock syndrome Cause: staphylococcal toxin Fever Rash (widespread erythema) Sometimes circulatory collapse Most marked on Fingers and hands. Vagina of women using tampons Treatment: Systemic antibiotics Irrigation Generalised erythema

Streptococcal infections Erysipelas Cellulitis Necrotizing fasciitis (surgical emergency)

Erysipelas Acute skin infection Split in the skin (perhaps a minor tinea pedis) Between the toes Under an ear lobe Start as malaise, shivering & fever. After a few hours  red plaques Well-defined Blisters may develop on the red plaques Treatment: Untreated, the condition can even be fatal Systemic penicillin (sometimes given IV). Recurrences: Can affect the same area repeatedly, so lead to persistent lymphoedema. Treated by low dosage long-term oral penicillin V.

Fig. 16.5 Erysipelas – note sharp spreading edge, here demarcated with a ballpoint pen.

Cellulitis Inflammation of the skin. Cause (streptococci, staphylococci or other organisms) Occurs at a deeper level than erysipelas. The subcutaneous tissues are involved The area is more raised and swollen The erythema less marginated than in erysipelas. Cellulitis often follows an injury Favours areas of hypostatic oedema. Treatment Elevation Rest Systemic antibiotics (sometimes given IV)

Cellulitis Spreading inflammation of subcutaneous or connective tissue