Microbiology Flashcards
What are the two most common causes of skin and soft tissue infections as they appear on a Gram stain?
- Staphylococci: Gram+ cocci in clusters
- Streptococci: Gram+ cocci in chains
–> Each have thick murein layers, thus the Gram+ stain
What are the 3 sources of skin and soft tissue infections?
- Exogenous: direct invasion of microbe from external environment
- Endogenous: invasion of microbe from an internal source, such as blood or infected organ
- Toxin: mediated manifestations from an infection at a distant site
What are the most frequent transient skin flora found on humans?
- Staphylococcus aureus
- Streptococcus pyogenes (Group A strep)
What are the most frequent resident skin flora found on humans?
- staphylococcus epidermidis
- propionibacterium acnes
What types of skin and soft tissue infections are often caused by staphylococci?
Folliculitis
Carbuncles, furuncles
Impetigo
Cellulitis
What types of skin and soft tissue infections are often caused by streptococci?
Impetigo
Erysipelas
Cellulitis
Synergistic cellulitis
necrotizing fasciitis
What are the characteristics of pyogenic cocci?
Streptococci are:
Invasive
Produce purulent lesions
obligate extracelluar bacteria
have anti-phagocytic virulence factors
Facultative anaerobes (live w/ or w/o oxygen)
What diagnostic criteria need to be met to confirm streptococci infection?
- Gram+ stain in pairs or chains
- Catalase negative
What diagnostic criteria is used to classify streptococci?
- Hemolytic pattern on blood agar
- Physiological traits and biochemical reaction
- antigenic composition
What are the three hemolytic patterns found around streptococcal blood agar colonies?
- alpha-hemolytic: greenish hemolysis
- beta-hemolytic:complete/clear hemolysis (bad)
- gamma-hemolytic: no hemolysis (good)
What kind of hemolytic pattern does Group A strep (S. pyogenes) have?
beta-hemolysis: complete hemolysis
What are diagnostic characteristics of Group A strep?
Group A strep = Strep. pyogenes:
- Gram+ cocci in chains
- beta-hemolytic
- catalase negative
- PYR positive
- bacitracin sensitive (shown by bacitracin disk in blood culture plate)
Describe the rapid test used to detect Streptococci bacteria in a patient.
Control antigen and test capture antibody are fixed to test strip.
Gold-labeled (red) antibody is diffuse and not fixed to paper.
Paper is dipped in specipen w/suspect antigen and if capture antibody and control antigen show, test is positive
What are virulence factors for S. pyogenes (Group A Strep)?
M Protein
Hyaluronic acid capsule
Pyrogenic exotoxins
Streptolysin O
DNAases
Streptokinase
C5a peptidase
What are components of the streptococcal cell wall?

What is M protein?
A virulence factor of Group A Streptococci:
Involved in binding to epidermis and is anti-phagocytic.
- Antibody to M protein is type specific; there are ~90 types
- Share sequence homology with some mammalian proteins (i.e. tropomyosin, human heart)
Why are vaccines to M proteins dangerous?
If vaccine was not thoroughly screened, it could lead to autoimmunity to those vaccinated b/c of M proteins molecular mimicry of host components
What is a hyaluronic acid capsule?
Group A Strep virulence factor:
identical to component in normal tissue, anti-phagocytic, antibody to capsule not protective to host
What is pyrogenic exotoxins?
Virulence factor of Group A Strep:
- Fever producing toxin
- Super antigens stimulate production of cytokins
- Involved in pathogenesis of scarlet fever and toxic shock-like syndrome
- Depends on lysogeny of bacterium with temperate phage
How do superantigens work?
Bind TCR to MHC; no specific peptid required
- leads to wide-spread Tcell activation:
Fever
Confusion
Hypotension
Headache and vomiting
Diarrhea
Abdominal pain/weakness
What is streptolysin O?
Virulence factor for Group A Strep (S. pyogenes):
- O2 labile, antigenic, hemolytic, toxic to wide variety of cells
- antibodies to toxin basis of anti-streptolysin O test
What are DNAases?
Strep. pyogenes (Group A strep) virulence factor:
- depolymerizes cell-free DNA in purulent lesions
- measure antibodies to these proteins as aid to diagnosis of recent disease
What is streptokinase?
Group A Strep (S. pyogenes) virulence factor:
- lyses blood clots
- helps bacteria spread
- can be used commercially to clean wounds (surgical enzymatic debridgement)
What are clinical presentations of suppurative streptococcal disease?
-Pharyngitis (uncommonly w/Scarlet Fever)
- Pyoderma: Impetigo
Ersipelas
Cellulitis
Necrotizing faciitis - Streptococcal toxic shock syndrome
