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Bacterial Skin Infections
Dr. Hani MasaadehMD, PhD
Lecture objectives
Normal Skin Flora
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Natural Defenses of the Skin
•
Keratin
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Skin sloughing
•
Sebum: low pH, high lipid
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Sweat: low pH, high salt, and
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Lysozyme, which digests peptidoglycan
Normal Flora of the Skin
Large numbers of microorganisms live on or in the skin
Numbers of bacteria are determined by location and
moisture content
Skin flora are opportunistic pathogens
Most skin flora can be categorized in three groups:
diphtheroids
staphylococci
yeasts
Normal Skin Flora
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Propionibacterium acnes
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Corynebacterium sp.
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Staphylococci
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Staphylococcus epidermidis
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Staphylococcus aureus
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Streptococci sp.
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Candida albicans (yeast)
Normal Flora of the Skin
Diphtheroids
Named for their resemblance to Corynebacterium
diphtheriae
Toxin inhibits elongation factor 2
Gram-positive bacteria with varied shape and low
virulence
Non-toxin producers like C. diphtheriae
Responsible for body odor
Odor caused by the bacterial break-down of sweat
Common diphtheroid is Propionibacterium acnes
Normal Flora of the Skin
Staphylococci
Gram-positive, salt-tolerant organism
Relatively avirulent
Can cause serious disease in immunocompromised
people
Principal species is Staphylococcus epidermidis
Functions on the skin to prevent colonization of
pathogenic flora
Maintains balance among microbial skin flora
Normal Flora of the Skin
Fungi (yeast)
Tiny lipophilic yeast universally found on normal skin
Usually from late childhood throughout life
Fungi shapes vary among strains
Usually round or oval; however, can be short rods
Fungi found on skin are generally harmless
Can cause skin conditions such as rash or dandruff
Acne
Microscopy
•
Propionibacterium acnes:
Gram + rod
•
Digests sebum
•
Attracts neutrophils
•
Neutrophil digestive
enzymes cause lesions,
“pus pockets”
•
Acne
Most common skin
•
disease in humans
•
Oil-based
cosmetics worsen
disease
•
No effects of diet
Acne Treatments
•
Benzoyl peroxide dries plugged
follicles, kills microbes
•
Tetracycline (antibiotic)
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Accutane – inhibits sebum formation
Hair Follicle Infections
Symptoms – Folliculitis
Presents as a small red
bump or pimple
Infection can spread from
infected follicle to adjacent
tissues
Causes localized redness,
swelling and tenderness
The lesion produced is called
a furuncle
Most are caused by Staph
aureus
Scalded Skin Syndrome
Staphylococcal
scalded skin
syndrome (SSSS)
Toxin-mediated
disease
Occurs primarily in
infants
Potentially fatal
Scalded Skin Syndrome
Symptoms
Skin appears to be burned (scalded)
Begins as generalized redness
Other symptoms include malaise, irritability, fever
Nose, mouth and genitalia may be painful before other
indicators become apparent
Within 48 hours of infection, symptoms manifest
Skin becomes red and wrinkled
Large fluid-filled blisters appear
Skin is tender to the touch and may feel like sandpaper
Scalded Skin Syndrome
Causative Agent
Bacterial agent is Staphylococcus aureus
Disease is due to the production of toxins
produced by S. aureus
Toxins are call exfoliatins
Exfoliatins destroy integral layers of the outer
epidermis
Toxins are coded either by plasmid or on the
bacterial chromosome
Scalded Skin Syndrome
Epidemiology
5% of S. aureus strains produce exfoliatins
Disease can appear in any age group
Most frequently seen in infants, the elderly and
immunocompromised
Transmission is generally person-to-person
Disease is usually isolated; however, small
epidemics can occur in nurseries
Scalded Skin Syndrome
Prevention and Treatment
Only preventative measure is patient isolation
Patients are in protective isolation
Helps limit spread of bacterial agent
Limits patient exposure to potential secondary
pathogens
Treatment includes bactericidal antibiotics
Antistaphylococcals such as penicillinase-resistant
penicillin
Treatment also includes removal of dead skin to
prevent secondary infection
Streptococcal Impetigo
Pyoderma infection
Characterized by pus
production
Pyodermas can result from
insect bites, burns and
scrapes
Such injuries can be so slight
that they miss detection
Impetigo is most common
type of pyoderma
Streptococcal Impetigo
Causative Agent
Many cases including epidemics are caused by
Streptococcus pyogenes
S. aureus is also implicated as a causative
agent
S. pyogenes is a Gram-positive, β hemolytic cocci
Often referred to as Group A
Due to presence of group A cell wall
polysaccharide
Streptococcus species also form the
characteristic chain formation
Rocky Mountain Spotted Fever
First recognized in
Rocky Mountain
region of United
States
Representative of a
group of rickettsial
diseases
Transmitted by ticks
Rocky Mountain Spotted Fever
Symptoms
Distinguished by initial rash of faint pink spots
Appears first on palms, wrists, ankles and soles of
feet
Rash eventually spreads to other parts of the body
Spots become raised bumps and are hemorrhagic
Shock or death can occur when certain body systems
become involved
Especially the heart and kidney
Rocky Mountain Spotted Fever
Causative Agent
–
Rickettsia rickettsii
Obligate, intracellular
bacterium
Requires host organism
for survival
Gram-negative, nonmotile, coccobacillus
Bacteria are very small
and often difficult to see
in gram stain
Rocky Mountain Spotted Fever
Epidemiology
Zoonotic disease
Occurs in areas in the United States,
Canada and Mexico
Highest incidence in US is in south
Atlantic and south-central United States
Maintained in several species in nature
Primarily in ticks and certain mammals
Main vectors include wood tick,
Dermacentor andersoni and the dog
tick, Dermacentor variabilis
Tick vectors remain infected for life
Rocky Mountain Spotted Fever
Prevention
No vaccine currently available
Prevention should be directed towards:
Avoiding tick-infested areas
Using protective clothing
Using tick repellents containing DEET
Carefully inspect body
Especially dark, moist areas
Remove attached ticks carefully
Avoid crushing and contaminating bite area
Treatment
Antibiotics are highly effective in treatment if given early
Doxycycline and chloramphenicol used most often
Without treatment, overall mortality reaches approximately 20%
With early diagnosis and treatment, mortality rates drop to less than 5%
Why does skin get infected?
• There are multiple types of bacteria which are
normally present on the skin.
• For example: Staphylococcus epidermidis and
yeasts
• The presence of bacteria does not automatically
lead to a skin infection
What is the difference between colonisation and
infections???
• Colonisation: Bacteria are present, but causing no harm
• Infection: Bacteria are present and causing harm.
A break in the epidermal integrity can allow organisms to enter and
become pathogenic. This can occur as a result of trauma,
ulceration, fungal infection, skin disease such as eczema
Impetigo
• A highly infectious skin disease, which commonly
occurs in children.
• What is the likely causative organism?
• The causative organism is usually Staphylococcus
Aureus (>90% cases1), but less often can be strep
pyogenes.
• Begins as a vesicle, which may enlarge into a bulla.
• Weeping, exudative area with characteristic honey
coloured or golden, gummy crusts, which leave
denuded red areas when removed.
• May present as macules, vesicles, bullae or pustules
• Bullae are more prominent in staphylococcal infection
and in infants
• What is the treatment?
Impetigo
Treatment:
Mild localised cases - use topical antibiotic Polyfax
 Widespread or more severe infections – use systemic
antibiotics, such as flucloxacillin (or erythromycin if penicillin
allergic)
Folliculitis
• Inflammation of the
hair follicle.
• Presents as itchy or
tender papules and
pustules at the follicular
openings.
• Complications include
abscess formation and
cavernous sinus
thrombosis if upper lip,
nose or eye affected.
What is the causative organism?
• Most common cause is Staph Aureus.
• Other organisms to consider include:
• Gram negative bacteria – usually in patients
with acne who are on broad spec antibiotics
• Pseudomonas (“Hot tub folliculitis”)
• Yeasts (candida and pityrosporum)
What is the treatment?
Folliculitis treatment
• Topical antiseptics such as Chlorhexidine
• Topical antibiotics, such as Fusidic acid or Mupirocin
• More resistant cases may need oral antibioics such as
Flucloxacillin
• Hot tub folliculitis – ciprofloxacin2
• Gram negative – trimethoprim
Cellulitis
• Infection of the deep subcutaneous
layer of the skin
• Presents as a hot, tender area of
confluent erythema of the skin
• Can cause systemic infection with
fever, headache and vomiting.
• Erysipelas is more superficial and
has a more well demarcated border
Erysipelas
• What is the most common causative organsism?
Cellulitis
• Streptococcus – Group A Strep Pyogenes.
• Others include Group B, C, D strep, Staphylococcus Aureus,
haemophilus influenzae (children) and anaerobic bacteria (e.g
Pasteurella spp. After animal bites)
eatment of cellulitis
• Oral Flucloxacillin or erythromycin if allergic
• Co-amoxiclav in facial cellulitis
• If severe systemic upset, may require admission for IV
antibiotics.
• After the acute attack has settled, especially in
recurrent episodes – consider the underlying cause
Orbital cellulitis – refer urgently
Furunculosis (boils) and carbuncles
• Deeper Staphylococcal abscess of the hair follicle
• Coalescence of boils leads to the formation of a carbuncle
• Treatment is with systemic antibiotics and may need incision
and drainage.
• Consider looking for underlying causes, such as diabetes