Skin and Soft Tissue Infections Flashcards
What is a macule?
Flat, red inflammatory response to a microbe or toxin
What is a papule?
Raised, red bump with more marked inflammation
What is a vesicle?
A blister
What is an ulcer? What is its developmental progression?
Rupture of epithelium
Macule -> Papule -> Vesicle -> Ulcer
What is impetigo?
A bullous, crusted, or pustular eruption.
Generally caused by S. pyogenes, called pyoderm, especially when honey-colored
What is a furuncle vs carbuncles?
Furuncle - boil
Carbuncle - collection of boils under the skin
What is erysipelas?
A well-defined, spreading inflammation of dermal lymphatics very diagnostic of S. pyogenes
What is cellulitis?
An acute, serious inflammation due to infection of subcutaneous fat
What is necrotizing fasciitis?
Inflammatory response in soft tissue below site of infection
Give the three virulence factors associated with S. aureus infection?
- Alpha-toxin
- Toxic shock syndrome toxin
- Exfoliative toxins
What is alpha toxin?
A pore-forming cytolysin which kills RBCs (hemolysin) and leukocytes
Works like complement
What is TSST?
Toxic shock syndrome toxin -> pyrogenic exotoxin which is a superantigen, crosslinking TCR to MHC2, causing cytokine release and shock
(rash, fever, hypotension, very similar to endotoxic shock from LPS)
How do exfoliative toxins work and what do they cause if all over the body or locally?
Induce intercellular splitting at desmosome, between stratum spinosum and granulosum
All over body: Staphylococcal Scalded Skin Syndrome, common on babies but gets better because it’s very superficial
Locally: Bullous impetigo
How is S. aureus generally transmitted?
Skin and nasal carriage, also fomites
No acquired immunity
What are the skin lesions which can be caused by S. aureus?
Furuncle -> folliculitis followed by coagulation of fibrin around lesion leads to boil
Carbuncle - focal abscess which can lead to entry of organism into blood via lymphatics
Scalded skin syndrome (bullous exfoliation) or bullous impetigo
Where are two places where TSS is caused?
- Vagina
2. Wound infection
What is the most common mechanism of S. aureus bacteremia and what are some clinical results of this?
Entry via wound contamination, spread to blood stream via lymphatics
Cause: Endocarditis, meningitis, pulmonary infection, osteomyelitis
What is the gram stain, catalase, and coagulase for Staphylococcus aureus?
Gram stain + clusters
Catalase positive (will cause O2 bubbles when H2O2 added to culture)
Coagulase positive (will cause coagulation when put in plasma, versus other staphylococcal infections which won’t)
How is antibiotic susceptibility testing done?
Spread the culture all the way across the agar with an Abx strip of descending concentration. The concentration where it starts growing is MIC
Need to be able to reasonably achieve MIC in vitro
What are four virulence factors for Streptococcus pyogenes?
- M protein
- Streptolysin O
- Streptococcal pyrogenic exotoxins (Spe A-C)
- Hydrolytic enzymes (Streptokinase)
What is the purpose of M protein? What is it responsible for?
Antiphagocytic, does molecular mimicry with antigen variation via N terminus (>80 serotypes)
Mediates binding to epidermis
Responsible for post-streptococcal sequelae, including acute glomerulonephritis, rheumatic heart disease
What blood marker is very diagnostic of S. pyogenes infection?
Antibodies to SLO, which mediate self-attack and augment cell lysis by fixing complement
What is the mechanism of action of Streptolysin O (SLO)?
Cytolysin, attacking cell membranes and forming large pores. It is oxygen-stable, and causes beta-hemolysis on blood agar plates
What produces SpeA?
Lysogenized (bacteriophage-infected) Group A Strept.
What are Spe’s?
Superantigens which are similar to TSST, inducing cytokine release causing shock and endotoxin sensitivity
What do hydrolytic enzymes like streptokinase cause?
They dissolve fibrin to facilitate spread of bacteria, and are responsible for runny, thin pus.
Can be used therapeutically to dissolve blood clots
What is the pathogenesis / spread of pyoderma / impetigo?
Minor trauma such as insect bite on face or lower extremities causes a vesicle to rupture, leading to superficial spread especially between young children with poor hygiene.
Spread easily by fomites and exacerbated by staph aureus lesions (bullous impetigo)
What do post-streptococcal sequelae typically cause?
Acute glomerulonephritis, especially following skin infection (rarely respiratory)
Hematuria, hypertension, proteinuria due to kidney damage. Rare in the US.
This is a type 3 hypersensitivity
When does erysipelas occur and what does it progress to? What are the associated symptoms?
Occurs on face, especially following strept throat. Can rapidly spread infection to deeper layers and cause necrosis + septicemia.
Associated with edema, fever, and lymphadenopathy
What causes cellulitis?
Deep skin infection or wound
Caused by Group A Strept / S. aureus
What causes Toxic shock-like syndrome (TSLS)? Necrotizing fasciitis?
Highly invasive S. pyogenes strains producing SpeA, due to superantigen
Often caused by wound infection involving S. pyogenes
Same thing causes necrotizing fasciitis
How is Streptococcus pyogenes indentified in the lab?
Gram positive cocci in chains Beta-hemolytic, pyogenic Lancefield group A (carbohydrate antigen) Catalase negative Bacitracin sensitive
What bacteria causes acne?
Propionibacterium acnes
What does Clostridium perfringens cause?
Gram positive anaerobic rod causing Gas gangrene from wounds
How does Candida albicans attach and invade?
Adheres vie fibronectin
Invades via pseudohyphae / germ tubes which extend across mucosal barriers, mediated by proteases / elastases
How does our immune system protect us from yeast infection and what does Candida infection indicate?
Neutrophils are the first line of defense, Chronic candidiasis indicates T cell deficiency
Where does Candida like to grow? Who does it affect most?
Hair follicles (folliculitis) Intertrigo (skin folds), especially wet skin folds. Thus, effects dishwashers on hands and infants (diaper rash)
How is Candida identified?
Erythematous papules are scraped and seen under microscope with germ-tube pseudohyphae (they are unicellular)
What causes Sporotrichosis?
Subcutaneous infection via thornprick and inoculation of conidia (spores) of Sporothrix schenckii
Affects gardeners and farmers the most
When can Sporotrichosis get really bad?
The initial stage is a painless papule week post-inoculation. Can get really bad if it ulcerates and becomes chronic, draining lymph channels.
Can spread to CNS, bones, eyes, and lungs in rare cases
What are the dimorphic growth phases of Sporothrix?
Yeast form - in humans at 37 degrees C
Mold form - with thin hyphae and conidial spores at 25 C, very ubiquitous
How do the tinea infections live (virulence factors)?
They are adapted to living in keratinized tissue of nails, hair, and stratum corneum of skin
Arthroconidia will invade hair shafts plugging root, causing ring-shaped hair loss
What organisms cause the Tinea infections?
Dermatophytes, like Epidermophyton, Tricophyton, and Microsporum
What is ringworm?
Tinea infection, especially of the body (Tinea corporis, on nonhairy skin) or scalp (Tinea capitis)
What is tinea pedis called? Where is it?
Athlete’s foot, interdigital spaces of feet
What is tinea cruris called? Where is it?
Jock itch (groin area)
What is tinea unguim? What does it cause?
Tinea of nails, causing discoloration and growth malformation
What environments do tinea infections typically occur in?
Moist skin folds, and maceration promotes infection
How is tinea passed?
They have low infectivity and virulence, typically passed via person-person transmission via very close contact with infected area, or infection from animal or soil.
What limits the spread of tinea infections? What do chronic infections mean?
Rapid shedding of keratinized layers due to advanced skin growth which is induced by infection.
Cell mediated immunity is important and delayed-type (Type 4) hypersensitivity will result
Chronic infections: impaired T cell and lack of DTH reaction
How is tinea identified in the lab?
Scraping from the edge of the lesion or infected hairs.
Some species fluoresce under UV light
