This document discusses skin and wound infections. It begins by describing the normal protective functions of skin and how wounds disrupt this, allowing microbial proliferation. Common skin infections like impetigo and cellulitis caused by bacteria like Staphylococcus aureus and Streptococcus pyogenes are described. Obligate anaerobic bacteria often cause polymicrobial wound infections. Predisposing factors that can lead to wound infection like poor blood flow and the critical microbial load required are also outlined. The major pathogens involved in wound infections including S. aureus, S. pyogenes, Pseudomonas aeruginosa, and various anaerobic bacteria are identified.

1. Skin & Wound Infections
Dr. Ihsan Edan Alsaimary
Assistant Professor
Department Of Microbiology
College Of Medicine
University Of Basrah

2. • Normal function of skin: prevent
colonization and invasion of underlying
tissue by potential microbial pathogens
• Loss of skin integrity (wound) provides
moist and nutritious environment for
microbial proliferation
• Presence of foreign material and necrotic
tissue facilitates microbial proliferation
(“dirty” wound)

3. SKIN AND WOUND INFECTIONS
Entry
Skin (pores, hair follicles)
Wounds (scratches, cuts, burns)
Insect & animal bites
Diseases
Localized infections with local and/or systemic effect
Systemic infections
Multiplication
Extracellular
Intracellular (??)
Damage
Toxins, exoenzymes
Host immune response

4. DEFENSES
• Dry :
usual infection sites are wet areas, skin folds,
armpit, groin
• Acidic (pH 5.0)
• Temperature less than 37oC:
Some pathogens grow best <37oC
• Lysozyme and toxic lipids:
pore, hair follicles, sweat gland
• Resident microflora : mainly Gr+
• Skin-associated lymphoid tissue (SALT)

5. • COMMON SKIN INFECTIONS
Local infections
• Impetigo: epidermis
• Folliculitis: infection of hair follicles
• Furuncles (boil): extension of folliculitis
• Carbuncles: furuncles coalesce and extend to the
deeper subcutaneous tissue
fever and chills
• Skin abscesses :
• Localized infection of dermis
and subcutaneous tissue
• Erysipelas - dermal lymphatics
• Cellulitis - subcutaneous fat layer
Systemic effect
• Toxic shock
• Sepsis

6. Cellulitis
• Acute, spreading infectious process affecting
epidermis and dermis
• Inflammation with little or no necrosis, edema
• Lymphatic involvement
• Fever, chills, leukocytosis
• Bacteremia up to 30% of cases
• Complications:
• Abscess and osteomyelitis
• S. aureus
• Streptococcus pyogenes (group A
streptococcus)

7. Staphylococcus aureus
Local skin infections:
Impetigo, Folliculitis, Furuncles (boil),
Carbuncles
Systemic infections:
Staphylococcal Scalded skin syndrom (SSSS)
• Toxic shock syndrom control After 12h incubation with S. aureus
TSST-1
Sepsis
Endocarditis
Osteomyelitis

8. Pathogenesis
1. Encounter : Broken skin or mucous membrane – humans only
2. Spread : can spread hematogenously ANYWHERE to cause abscesses or osteomyelitis or septic arthritis
3. can also cause toxigenic infections --- toxin spreads
4. Multiplication : grows well anywhere
5. Avoid Host Immune Response :􀂋 coagulase, catalase, leukotoxins, Fc binding protein, adhesins
6. Damage : inflammation, superantigens, degradative enzymes
7. Transmission :􀂋 contact from fomites or direct person-person contact
Major defenses against S. aureus
1. C3b
2. Activated by Staph cell wall fragments
3. Opsonizes the bacteria
4. Enhances phagocytosis
5. Chemotaxis – attracts neutrophils
6. Neutrophils – engulf the bacteria
7. Intracellular killing by O2 radicals
Conditions predispose to Staph infections
1. C3 hypercatabolism
2. Neutropenia
3. Chemotherapy
4. Cyclic neutropenia
5. Chronic granulomatous disease
6. reduced production of H2O2 and O2
7. “Lazy leukocyte” (chemotaxis deficiency)
Virulence Factors
1- Fibronectin binding protein
2- Toxins
• Superantigen (TSST-1)
• Dermonecrotic toxin
• Enterotoxin
3- Enzymes
• Hyaluronidase, collagenase
• Lipase
• Coagulase
• Hemolysin, leukocidin
• Nuclease, protease
4- Fc binding protein

9. Staphylococcal scalded skin syndrom(SSSS)
• Dermonecrotic toxin (exfoliative toxin)
• Bullous exfoliative dermatitis
Toxic shock syndrome
• Toxic shock syndrome toxin(TSST-1)
• Super antigen
• Produced by 5-25% isolates
• Tampon or infected wound :
• Fever
• Rash
• Exfoliation of skin
• Shock (death rate 3%)
TSST-1
Structure : 22 KD secreted protein
Mechanism of action
1. Interacts with MHC-II and T-cell receptors
2. Stimulate the release of cytokines (IL-1, IL-2 and TNF)
Treatments
1. Synthetic antibodies and peptides: Block interaction with MHC-II,
preventing T cell activation
2. Immunosuppressants : Prevent T-cell activation and cytokine releases
3. Corticosteroids : Reduce inflammatory effects

10. Streptococcus pyogenes
Local infections
1. Impetigo
2. Erysipelas
3. Cellulitis
4. Necrotizing fasciitis (flesh-eating bacterium)
Systemic effect
1. Streptococcal toxic shock-like syndrome (STSS)
2. Spe (similar to TSS by S. aureus)
3. Scarlet fever (pyrogenic toxin by lysogenized φ)
4. Post-infection
5. Rheumatic fever (associated with pharyngitis)
6. Glomerulonephritis
Virulence factors
1- Adhesins
1. M protein (fibrillar Ag)
2. Fibronectin binding proteins (Protein F)
3. Lipoteichoic acid (LTA)
2- Hyaluronic acid capsule
3-Invasins
1. Streptolysins (S & O)
2. Hyaluronidase
3. Streptokinases : activates blood clot dissolving protein-plasminogen (human specific)
4. Dnase
4-Exotoxins
Pyrogenic (erythrogenic) toxin - Spe
Scarlet fever
Toxic shock syndrome

11. Clostridial infections (soil)
Anaerobic Gr+, spore-forming
1- Wound botulism (Clostridium botulinum)
Deep wounds:
1. anaerobic environment
2. Botulinum toxin (BT) leaks into blood
BT enters neurons
1. Inhibits acetylcholine release in neuromuscular junction
2. prevent muscle contraction : flaccid paralysis
Therapy
1. Antibiotics
2. Antitoxin
3. Supportive therapy
Botulinum toxin
Structure : A & B Chains (50/100 KD) by disulfide bond
Mechanism of action
Protease
1. Cleave fusion protein (SNAP-25) at neuromuscular junction
2. Prevent vesicles from anchoring to the membrane
3. Inhibit acetylcholine release
Application in the cosmetics : Botox and Dysport
Distinguish
“food botulinum” : Premade toxin (canned beans)
“infant botulinum” : Spore in foods (honey)

12. 2- Tetanus (Clostridium tetani)
Local infection with systemic effect
Early symptoms
1. Locked jaw
2. Stiffness in the neck and abdomen
3. Difficulty swallowing
Late symptoms
1. Fever
2. Elevated blood pressure
3. Severe muscle spasm
Tetanospasmin (A & B toxin)
Released when cell lyses
A domain – a zinc endopeptidase, attacks the vesicleassociated membrane protein
(VAMP), degrading synaptobrevin, thus stop the release of inhibitory
neurotransmitters GABA and glycine
B domain – binds disialogangliosides (GD2 and GD1b) on the neuron membrane
Acts in spinal cord, prevents the inhibition of contraction of opposing
sets of muscles – hence, tetanic or spastic paralysis.
Vaccine
1. Tetanus toxoid
2. DTaP for <7 yr, Td for >7 yr

13. 3- Gas gangrene (Clostridium perfringens)
Alpha toxin (phospholipase C): Zinc metallophospholipase
hemolysis and bleeding
Gas formation
Mechanism (by host cell)
Content: N2 (74.5%), O2 (16.1%),
CO2 (3.4%), H2 (5.9%)
Myonecrosis, shock, renal failure and death
Alpha toxin
Structure : Single 40KD protein
Mechanism of action
1- C-terminal domain binds phospholipid bilayer
2- N-terminal domain has phospholipase activity
3- The hydrolysis of phosphatidyl choline produces diacylglycerol which activates a
variety of second messenger pathways. The end result includes edema due to
increased vascular permeability.
Treatment
1. Debridement and excision
2. Antibiotics (prevent further spreading)
3. Hyperbaric oxygen therapy : Inhibit or kill the anaerobic bacteria

14. Cutaneous Anthrax Bacilllus anthracis
Gr+ and spore forming
Farm animals are major reservoir
Inhalation, GI, cutaneous
Virulence factors:
Capsules
Edema factor (A) + Protective antigen (B)
Lethal factor (A) + Protective antigen (B)
Vaccine
Toxoid (protective antigen)
Effective in short term but not long term

15. Other Skin and Mucus Membrane Infections
Staphylococcus epidermidis
Catheters and prostheses
Vibrio vulnificus
From shellfish and salt water
Obligate anaerobes
Puncture wounds
Deep wounds
Impaired blood supply
Gram negative bacteria
Decubitus ulcer (bed sores)
After intestinal “spill”
Pseudomonas aeruginosa
Catheters and prostheses
Burns
Surgical wounds

16. • Fungal infection (dermatophytes)
• Tinea(pedis, corporis, cruris, manuum, facei,
capitis, unguium)
• Id reaction : allergic dermatophytids
• Dermatoplytosiscomplex : mixed fungal and
bacterial infection of toe web

17. • Laboratory diagnosis –Dermatophyte infection
• Skin scraping –choose the active margin, collect scale with colourpaper, (dry up the
oily or wet skin with alcohol or ether in the old days before scraping), can send to
laboratory by post
• Hair plugging (NOT cutting) –choose the short broken hair (mostly likely found in the
margin of bald patch), can be guided by Wood’s light; sample with specific brush;
sample the pets
• Nail clipping –scrape the sub ungual hyperkeratosis as proximal as possible,
maximum amount of disease nail material, may use a small punch biopsy needle to
sample in proximal subungualonychomycosis, scraping of the nail surface in
superficial white onychomycosis
• Wet mount –dissolve the keratin material with KOH, direct examination with or
without stain (e.g. Parker stain) [Sn12% & Sp 93%]
• Culture with Sabouraudagar (if mould [non-dermatophytefilamentous fungi] infection
is suspected, culture without cycloheximideis required)
• Nail clipping for histology (with special stain) can be performed in those persistent
culture negative cases (with clinical features of fungal infection)

18. Fungal infection
(yeast & Pityrosporum/Malassezia)
• Candidiasis: intertriginousarea, napkin area,
chronic paronychia, chronic
mucocutaneouscandidiasis, balanitis, oral
thrush
• Pityrosporum: pityriasisversicolor,
pityrosporumfolliculitis(seborrhoeicdermatitis,
pityriasiscapitis)
• Trichosporosis: pied

19. Viral skin infections
• HSV: cold sore, genital herpes
• VZV: Chicken pox,zoster
• HPV: wart
• Pox virus : molluscumcontagiosum
• Viral exanthemata :measle, rubella,roseola
infantum, erythemainfectiosum,
• Others: Hand foot mouth disease; (EBV, HIV)

20. Parasitic infestation
• Mite: Sarcoptes scabei
• Lice: head lice, body lice, pubic lice
• Others: cutaneous larva migran, amoeba,
myiasis, etc

21. wound infections
• Pathophysiology
• Acute wounds: external damage to intact
skin (surgical wounds, bites, burns,
gunshots, minor cuts and abrasions)
• Chronic wounds: endogenous mechanisms
compromising epidermal and dermal tissue
(impaired arterial supply or venous
drainage, diabetes mellitus, poor nutrition,
immunosuppression, sustained external
skin pressure)

22. Pathophysiology of wound
infection
• Microbial colonization precedes wound
infection
• If tissue devitalized and/or host immunity
compromised, conditions optimal for
microbial growth and invasion follows
colonization
• Source of microorganisms: exogenous
(environmental), surrounding skin, and
endogenous (mucous membranes of
gastrointestinal tract and genitourinary tract,
oropharyngeal cavity)

23. Predisposing factors for wound
infection
• Poor blood perfusion with hypoxia (pO2
< 20 mm Hg) inhibits granulation tissue
response and wound repair
• Cell death and tissue necrosis due to
hypoxia creates ideal growth
conditions for wound microflora
• Hypoxia compromises oxygen radical
dependent killing of bacteria by
polymorphonuclear neutrophils

24. Pathophysiology of wound
infection
• Density of microorganisms the critical
factor determining whether or not a
wound heals
• Presence of specific microbial
pathogens of primary significance in
delayed wound healing
• Most likely both factors important in
delayed wound healing due to infection

25. Pathophysiology of wound
infection
• Healing of decubitus ulcers occurs only when
bacterial load <106 cfu/ml of wound fluid
• Acute and chronic wound infection occurs with
microbial load >104 (complex extremity wounds) or
>105 cfu/g of wound tissue
• Single microorganism on Gram’s stain reliably
predicts >105 cfu’s/g of wound tissue
• Presence of bacterial cells on Gram’s stain of burn
wounds consistently correlates with >106 organisms
per swab specimen
• Critical microbial load for wound infection appears
to be 104-106cfu/g wound tissue or ml wound fluid,
and 106cfu/wound swab specimen

26. Microbiology of wound infection:
the Big Three1
• Streptococcus pyogenes (capable of
wound infection <105 cfu/g wound
tissue)2
• Staphylococcus aureus
• Pseudomonas aeruginosa
1Associated with monomicrobial and
polymicrobial wound infection
2And other pyogenic β-hemolytic
streptococci

27. Microbiology of wound infection:
Obligate Anaerobic Bacteria1
• Bacteroides
• Porphyromonas (pigmented)
• Prevotella (pigmented and non-pigmented)
• Fusobacterium
• Peptostreptococcus
• Clostridium2
1Primarily associated with polymicrobial aerobic and
anaerobic bacterial wound infection
2Monomicrobial infection by Clostridium perfringes in
myonecrosis (gas gangrene) (distinctive Gram’s
stain showing large “boxcar shaped” gram-positive
rods with a paucity of inflammatory leukocytes

28. Polymicrobial wound infection:
mechanisms
• Oxygen consumption by aerobic
bacteria induces tissue hypoxia and
favorable growth conditions for
anerobic bacteria
• Nutrients produced by one organism
supports growth of other fastidious
and potentially pathogenic organisms

29. Polymicrobial wound infection:
mechanisms
• Vitamin K production by
Staphylococcus aureus supports
growth of vitamin K-dependent
Prevotella melaninogenica
• Succinate produced by Klebsiella
pneumoniae a critical growth factor for
Prevotella melaninogenica

30. Microbiology of wound infection:
surgical wounds1,2
• Staphylococcus aureus (191 patients, 28.2%)
• Pseudomonas aeruginosa (170 patients,
25.2%)
• Escherichia coli (53 patients, 7.8%)
• Staphylococcus epidermidis (48 patients,
7.1%)
• Enterococcus faecalis (38 patients, 5.6%)
• Anaerobic bacteria (21 patients)
1n=672 surgery patients with wound infections
2Giacometti et al., JCM 38:918-922 (2000)

31. Microbiology of wound infection:
surgical wounds1
• Superficial wounds, surgical incisions: streptococci,
staphylococci
• Deep wounds: GI, female genital tract, and
oropharyngeal-streptococci, staphylococci, gram-
negative enterics, enterococci, Bacteroides, other
anaerobes; other-streptococci, staphylococci, gram-
negative enterics
• Gangrenous 24-48 hr after surgery: group A
streptococci, clostridia
• Necrotizing >4 days after surgery: polymicrobial
(aerobic and anaerobic)
1Nichols and Florman, CID 33(Suppl 2):S84-93 (2001)

32. Microbiology of wound infection:
diabetic foot infections1
• Cellulitis: β-hemolytic streptococci (A, B, C, G), Staphylococcus
aureus
• Infected ulcer (no antibiotic treatment): same as cellulitis, often
monomicrobial
• Infected ulcer that is chronic or with previous antibiotic: S. aureus, β-
hemolytic streptococci, Enterobacteriaceae (usually polymicrobial)
• Macerated ulcer due to soaking: Pseudomonas aeruginosa (usually
polymicrobial)
• Long-duration non-healing ulcers with prolonged, broad-spectrum
antibiotic treatment: S. aureus (MRSA), coagulase-negative
staphylococci, enterococci (VRE), diphtheroids, Enterobacteriaceae
(ESBL resistance), Pseudomonas, nonfermentative gram-negative’s,
possibly fungi (usually polymicrobial)
• “Fetid foot” with extensive necrosis, gangrene, and malodorous:
Mixed aerobic gram-positive cocci, Enterobacteriace, nonfermentative
gram-negative’s, obligate anaerobes
1Lipsky et al., CID 39:885-910 (2004)

33. Microbiology of wound infection:
burn wound infections1
• Staphylococcus aureus (22.9%)
• Pseudomonas aeruginosa (20.9%)
• Pseudomonas species (7.2%)
• Escherichia coli (6.7%)
• Group D Streptococcus (5.0%)
• Enterococcus faecalis (4.2%)
1Bacteria constituting >4% of organisms that
were recovered from 1,267 burn wound
infections during 1974-1978 (CDC, Mayhall,
CID 37:543-550 (2003)

34. Microbiology of wound infection:
burn wound infections1
• Staphylococcus aureus (23.0%)
• Pseudomonas aeruginosa (19.3%)
• Enterococcus species (11.0%)
• Enterobacter species (9.6%)
• Escherichia coli (7.2%)
• Coagulase-negative Staphylococcus (4.3%)
1Bacteria constituting >4% of organisms that
were recovered from 1,234 burn wound
infections during 1980-1998 (CDC, Mayhall,
CID 37:543-550 (2003)

35. Microbiology of wound infection:
human bite infections1,2
• Streptococcus (84%)
• Staphylococcus (54%)
• Prevotella (36%)
• Fusobacterium (34%)
• Eikenella corrodens (30%)
1Bacteria recovered from >30% of 50 patients
with infected human bite injuries.
2Talan et al., CID 37:1481-1489 (2003)

36. Microbiology of wound infection:
animal bite infections1,2
Same as human bites with the addition
of:
• Pasteurella multocida
• Neisseria weaveri
• Staphylococcus intermedius
1Goldstein, Mandell, Douglas, and
Bennett’s Principles and Practice of ID,
pp. 3552-3556 (2005)
2Capitini et al., CID 34:e74-74 (2002).

37. Clinical signs of wound infection
• Purulent discharge
• Painful spreading erythema
• Failure to heal

38. Wound Specimens
• Tissue
• Wound fluid (purulent exudate)
• Superficial swabs
• Basic principle of specimen collection:
Only wounds with clinical signs of
infection, are deteriorating, or fail to
heal should be sampled for Gram’s
stain and culture

39. Gram’s stain of wound
specimens
• Presence of bacteria by Gram’s stain
indicates 105 to 106 organisms/g wound
tissue or ml wound fluid
• Types of organisms present on Gram’s
stain should be correlated with culture
results to recognize predominant
organisms that don’t grow aerobically

40. Culture of wound specimens
• Facultative anaerobic and aerobic bacteria of
primary importance in wound infection
• Media for aerobic culture of wound
specimens include sheep blood, chocolate,
and MacConkey agar
• Media for anaerobic culture of wound
specimens include brucella blood agar, laked
blood with kanamycin and vancomycin,
Bacteroides bile esculin, and anaerobic
colistin-naladixic acid

41. Culture of wound specimens
• Correlate growth of facultative anaerobic and aerobic bacteria
with gram-stain morphotypes
• If Staphylococcus aureus, β-hemolytic Streptococcus, and/or
Pseudomonas aeruginosa present in any numbers, identify
with susceptibility testing
• If coagulase-negative Staphylococcus, Corynebacterium,
and/or Enterococcus present in moderate to many numbers,
and growth explains gram-stain results, report as genus and
full identification/susceptibility available by request.
• If Enterobacteriaceae, or non-fermenters other than
Pseudomonas aeruginosa present in moderate to many
numbers, and growth explains gram-stain results, identify with
susceptibility testing
• If >4 facultatively anaerobic or aerobic bacteria detected in
culture by these criteria, obtain a technical consult
• Report obligate anaerboic bacteria as polymicrobial flora

42. Culture of wound specimens
• If facultative anaerobic and aerobic bacteria
recovered in culture do not correlate with one or
more gram-stain morphotypes, review and repeat the
Gram’s stain
• If aerobic cultures still do not explain Gram’s stain
upon review and repeat, examine anaerobic cultures
• If obligate anaerobic bacteria in moderate to many
numbers correlate with gram-stain morphotype(s)
not explained by aerobic cultures, identify by genus
and report susceptibility available by physician
request
• If > 4 organisms detected in culture by these criteria,
obtain a technical consult

43. Genus identification of anaerobic
bacteria in wound culture
• Bacterioides: growth in 20% bile
• Porphyromonas: vancomycin susceptibile,
kanamycin resistant, colistin resistant1
• Prevotella: vancomycin resistant, kanamycin
resistant, colistin susceptibile1
• Fusobacterium: vancomycin resistant,
kanamycin susceptible, colistin susceptibile
1+/- pigmentation on laked blood

44. Genus identification of anaerobic
bacteria in wound culture
• Clostridium: large gram-positive rods
with sporulation
• Peptostreptococcus: kanamycin
resistant and sodium polyanethol
sulfonate susceptible, or kanamycin
susceptibile and sodium polyanethol
sulfonate resistant
• Perform aerotolerance on all anaerobic
isolates to be reported

45. Wound specimen negative by
direct Gram’s stain
• Review and repeat Gram’s stain. If
confirmed negative, proceed as
outlined below
• Identification and susceptibility testing
of Staphylococcus aureus, β-hemolytic
Streptococcus, and Pseudomonas
aeruginosa only
• Other culture isolates reported as
“polymicrobial flora”