1. Chapter 16
Bacterial infections

2. 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.

3. 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

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

5. 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

6. 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.

7. 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 Widespread impetigo due to Staph. aureus with erosions and cruising.

8. 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)

10. 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

12. 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).

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

14. Fig. 16.3 Chronic furunculosis.

15. 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.

16. 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.

17. Swollen painful
Discharging pus

18. 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).

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

20. 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

21. Streptococcal infections
Erysipelas
Cellulitis
Necrotizing fasciitis (surgical emergency)

22. 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.

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

24. 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)

25. Cellulitis Spreading inflammation of subcutaneous or connective tissue