Skin and Soft Tissue Infections Flashcards
List 6 reasons skin is intrinsically resistant to infection
Low water content Low pH Low temperature High salt Fats and fatty acids Microbiota
What makes up the skin microbiota?
Mainly bacteria and some yeasts
List 7 microbes commonly found on the skin
Staphylococcus epidermidis
Staphylococcus aureus
Diptheroids (any corynebacterium besides Corynebacterium diphtheriae)
Streptococci
Gram negative bacilli (e.g. Pseudomonas)
Anaerobes (Gram - and +, e.g. Propionibacterium acnes)
Yeasts (e.g. Candida spp)
What are some common sites for anaerobes to reside?
In hair follicles and glands due to the lower O2
What areas of the skin have more bacteria and why?
Axilla, perineum, soles of feet and between toes, due to higher moisture
How many bacteria can be found on normal dry skin?
~1000 bacteria/cm2 (amount and composition of skin microbiota varies at different sites)
Give 5 examples of cutaneous manifestations of systemic infections
Enteric fever "rose spots" Petechiae in septicaemia Rash in secondary syphilis Scarlet fever Toxic shock syndrome
What organism causes scarlet fever?
GAS
What organism causes toxic shock syndrome?
Staphylococci, streptococci (any bacteria producing toxic shock toxins)
Give 2 examples of fungi causing SSTI
Yeasts (e.g. Candida)
Filamentous fungi
Give 3 examples of bacteria causing SSTI. Which is most common?
S. aureus (most common cause of SSTI)
S. pyogenes
Clostridia
Give 3 examples of parasites causing SSTI
Leishmania
Schistosomes
Hookworms
Give 8 examples of viruses causing SSTI
HSV VZV HPV Measles Rubella Enteroviruses Parvovirus B19 Molluscum contagiosum
What type of infection typically results in a compromised patient?
Polymicrobial infection with primary and opportunistic pathogens
What are the 3 types of SSTI? Give examples of each
Localised infections: folliculitis, abscess
Spreading infections: impetigo, cellulitis
Necrotising infections: fasciitis
What is folliculitis?
Infection of the hair follicle
What most commonly causes folliculitis? What is 1 other possible cause?
Most often due to blockage
May also result from direct inoculation of bacteria into hair follicle
What causes an abscess?
Progression of folliculitis to abscess (same original causes as folliculitis)
What is the most common causative organism in a case of folliculitis?
S. aureus
What is furunculosis?
Furuncle = abscess/boil
What is S. saprophyticus?
Urinary tract pathogen
What is the “definition” of S. aureus?
Coagulase +
Why does infection with S. aureus often recur?
Adaptive immune response is weak
How does staphylococcus enter damaged tissues?
Binds to cells and matrix via adhesins
How does staphylococcus evade the immune system and persist?
Inhibits chemotaxis
Inhibits phagocytosis
Resists killing if ingested by PMNs
Forms biofilm on biotic and abiotic surfaces (especially S. epidermidis)
How does staphylococcus inhibit chemotaxis and what effect does this have on the inflammatory process?
Via CHIPS, which blocks the complement receptor on cells and other proteins
Overall effect slows wound healing
How does staphylococcus inhibit phagocytosis?
Capsule Protein A Staphylokinase Complement inhibitors (SCIN) Haemolysins Leukocidins
How does protein A inhibit phagocytosis?
Binds Fc of Abs and prevents them activating host cells
What is the role of haemolysins and leukocidins?
Inhibits phagocytosis by killing neutrophils
Give an example of an important leukocidin
Panton-Valentine leukocidin
Describe the resistance profile of S. aureus
~90% are resistant to penicillin
Increasing % are resistant to methicillin and other antimicrobials (MRSA)
What is the difference between CA-MRSA and HA-MRSA?
Differences in genes, e.g. PV leukocidin
CA-MRSA more virulent
Different spectrum of disease (CA-MRSA causes aggressive abscesses which can invade to produce pneumonia, affects young people)
What is impetigo?
An infection of the epidermis characterised by bullous (blistered), crusted or pustular lesions
What is the causative organism in impetigo?
S. aureus and/or GAS (often both)
What organism causes bullous impetigo?
S. aureus
What is erysipelas?
A rapidly spreading erythematous infection (usually of face, legs or feet) with well-defined border, plus pain and fever
Potentially serious
What is cellulitis?
Similar to erysipelas but involves subcutaenous fat
Potentially serious
What is the causative organism in erysipelas/cellulitis?
Most often GAS
Cellulitis has wider range of causative organisms including S. aureus and other environmental organisms (e.g. Vibrio spp but not cholerae)
When does anaerobic cellulitis occur?
When tissue is devitalised
What is necrotising fasciitis?
Rapidly spreading infection along fascial planes which disrupts the blood supply, leading to necrosis (as well as myonecrosis and gangrene in severe cases)
What are some common causes of necrotising fasciitis?
GAS
Anaerobes (including Clostridium spp)
What is the most common cause of gas gangrene?
Clostridia (especially C. perfringens)
How are streptococci classified?
According to type of haemolysis they produce on blood agar
Further classified into Lancefield groups (A-T)
What is GAS?
S. pyogenes
What is GBS?
S. agalactiae
What are the Lancefield groups which contain pathogens?
A, B, C, D, G
What is the basis of Lancefield grouping?
Dependent on type of CHO in cell wall
List 5 structural virulence determinants of GAS
M-protein Capsule Lipoteichoic acid Cell wall CHO Other surface Ags
What is the M-protein? What is the clinical signifiance of the M-protein?
A structural virulence determinant of GAS; can be used to classify GAS by M-type (low numbered are generally more virulent)
Immunity can be M-type specific (can get multiple infections with GAS of different M-types)
List 5 enzymes which act as virulence determinants for GAS. What are their roles?
Streptokinase (fibrinolysin) Hyaluronidase (spreading) C5a peptidase SpeB (protease) DNAses
List 3 toxins which act as virulence determinants for GAS. What are their roles?
Streptolysins (haemolysins)
Leukocidins
Superantigens (e.g. SpeA)
What virulence factors are involved in the adhesion and colonisation of GAS?
Lipoteichoic acid
Fibronectin-binding proteins
M-protein
How do GAS invade?
Mechanism unknown
Invade into or between cells
List 4 ways GAS evades innate immunity
M-protein and capsule are anti-phagocytic
Leukocidal toxins kills phagocytes
DNAse overcomes NETS
C5a peptidase
What is NETS?
Neutrophil Extracellular Traps: fibres (composed of fibrin and other granular material) extrude and form a net around the neutrophil to trap microorganisms
List 3 mechanisms by which GAS cause tissue damage
Directly via cytolethal toxins and enzymes
Superantigens (causing toxic shock syndrome)
Activation of autoimmunity
Describe the morphology and classification of Clostridia
Gram+ rods
Anaerobes
What makes Clostridia so difficult to remove from the environment?
Forms hardy spores (especially C. tetani - spores can be seen under the microscope; C. perfringens will only form spores if the going is very tough)
What is the typical mechanism of wound colonisation in the case of clostridial infection?
Spores (usually) or vegetative bacteria from environment, gut or vaginal microbiota colonise wound
Under what conditions does clostridial infection take hold?
Clostridia germinates and replicates under anaerobic conditions, and therefore occurs more readily in devitalised (damaged) tissue
How does clostridial infection cause tissue damage?
Many different toxins and enzymes are secreted which lead to gas production, tissue damage and further spread
What test is used to identify C. perfringens?
Litmus milk test (positive litmus occurs when gas is produced which blasts apart the milk)
What kind of pathogen is Pseudomonas?
Opportunistic
What is a possible complication of diabetic foot ulcer?
Osteomyelitis
What is a hallmark symptom of infection with Pseudomonas?
Blue-green pus
Why is diabetic foot ulcer a common complication of DM?
Due to peripheral neuropathy (cannot detect damage)
List 2 types of fungi which can cause SSTI, and give examples of each
Dermatophytes (e.g. Epidermophyton, Trichophyton, Microsporum)
Yeasts (e.g. Candida, Malassezia)
What does Malassezia cause?
Pityriasis versicolor (local unsightly skin discolouration)
When does Candida infection usually occur?
With damaged, moist skin and lowered host resistance
Also associated with some forms of primary immunodeficiency (e.g. defect in AIRE) or aggressive autoimmunity
What kind of presentation can a severe haemorrhagic varicella infection produce?
Pneumonitis
ARDS
Typically patient is immunocompromised
List 4 methods which may be used in diagnosis of SSTI
NAT (for viruses)
Microscopy (Gram stain or other)
Culture and identification
Antimicrobial susceptibility testing
What are the 5 main principles of treating a wound infection?
Use strict aseptic technique
Remove damaged tissue and foreign material
Use topical disinfectant/wound dressing
Consider oral antibiotic (if disseminated) e.g. co-amoxyclav
Consider tetanus prophylaxis
What strategies are used to treat an abscess?
Drain
Consider oral antibiotic e.g. flucloxacillin (not penicillin - almost all S. aureus are resistant), may have to change if CA-MRSA
How is impetigo treated?
Soap and water with an anti-staphylococcal topical antimicrobial (mupirocin) for a mild infection
Add oral antibiotics (flucloxacillin or dicloxacillin - unless S. pyogenes alone, in which case penicillin can be used) for a more severe infection
How is cellulitis treated?
Flucloxacillin or dicloxacillin (unless S. pyogenes alone, in which case penicillin can be used)
How is gas gangrene treated?
Surgery
Penicillin G (good for Clostridia, S. pyogenes)
+/- hyperbaric oxygen to oxygenate tissues
How are diabetic foot infections treated?
Typically polymicrobial, so give co-amoxyclav and metronidazole (good for anaerobes)
What antibiotics can be used to treat Pseudomonas?
Dependent on antibiotic susceptibility testing (highly resistant organism)
