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
What is Scarlet fever?
Group A strep (S. pyogenes) infection characterized by:
- strawberry tongue
- circumoral pallor around lips
- desquamation that follows rash of S.F.
How does Streptococcal Toxic Shock Syndrome usually occur?
Infecting strains produce pyrogenic toxins
- clinically, like staphyloccocal toxic shock syndrome EXCEPT PATIENTS ARE BACTEREMIC and many have necrotizing faciitis
- high fatality rate
What are clinical presentations of non-suppurative streptococcal infection?
Rheumatic Fever
Acute glomerulonephritis
What is rheumatic fever?
- Presents after streptococcal throat infection of 6-15 year olds after infection with certain rheumatogenic M types of Group A strep
- Autoimmune mehcanisms likely; cross reaction between anti-M protein Abs and heart tissue
What is acute glomerulonephritis?
After throat or skin infection with certain nephritogenic M types of Streptococci.
- Renal injury due to deposition of antigen-antibody complexes on glomeruli (Type III hypersensitivity)
Why is it critical to always treat Group A Strep (S. pyogenes) infections?
In order to prevent nonsuppurative disease post-infection
- The earlier the treatment, the less time the patient’s immune system has to produce autoimmune antibodies resultant from infection
What is PANDAS Syndrome?
Pediatric Autoimmune neuropsychiatric disorder associated with group a Strep (PANDAS) characteristic of:
- Presence of OCD and/or tic disorder
- Pediatric onset
- Abrupt onset and episodic course of symptoms
- association with GAS infections
- Association with neurologic abnormalities (i.e. motoric hyperactivity, choreiform movements, tics)
What are characteristics of Staphylococci that can be determined by culture?
Gram+ cocci that form clusters
Facultatively anaerobic (can go w/ or w/o oxygen)
What factors are used to identify staphylococcus species?
What is the hallmark of staphylococcal infections?
pus (PMNs, dead tissue, bacteria)
What is hydradentis Suppurativa?
Staphylococcal infection infection of sweat glands
What is the primary treatment of cutaneous staphylococcal infections and why?
heat and drainage
- use of antibiotics only if invades subcutaneous layer b/c of antibiotic resistance
Why are antibiotics given to patients before surgery?
prophylaxis for hospital-acquired staph infections
What are some invasive, deep lesion infections caused by Staphylococcus aureus?
Osteomyelitis
septic arthritis
meningitis
pneumonia
What are some invaseive, bacteremia infections caused by Staphylococcus aureous?
endocarditis
meningitis
pneumonia
pyelonephritis
septicemia, septic shock
What are some risk factors for boils, furnuncles, carbuncles infections?
Diabetes mellitus, acne, occupational (wet conditions), poor hygiene
What are some risk factors for wound infections?
Diabetes mellitus, steroid therapy, obesity, malnutrition, prolonged surgery, foreign body
What is the #1 host defense against S. aureous?
Opsonophagocytosis –> granulocytopenic or C3b subunit deficient individuals are at high risk of infection
- Staph is resistant to PMN NETs because they are Gram+; so complement is best response
What are virulence factors of S. aureus?
- Protein A
- Catalase
- leukocidin
- Ribotechoic and techoic acid
- Coagulase
- Capsule
- Hyaluronidase
- Cytotoxins
What virulence factors of S. aureus defend against phagocytes?
- Protein A
- Catalase
- Leukocidin
What is Protein A?
Virulence factor for S. aureus
- binds to Fc receptors to prevent phagocytosis
What is catalase?
virulence factor for S. aureus
- prevents breakdown in phagosome
- converts H2O2 to H2O and O2
What is leukocidin?
Virulence factor of S. aureus:
- Punches holes in PMN to prevent phagocytosis and release bacteria
What is ribotechoic and techoic acid?
Virulence factors for S. aureus:
- bind fibronectin
- techoic acid induces shock that is similar to endotoxin-mediated shock
What is Coagulase?
Virulence factor for S. aureus:
- extracellular and suface-bound
- initiates conversion of fibrinogen to fibrin
How is a capsule useful to S. aureus?
A virulence factor, it is a polysccharide that prevents complement binding
- 8 different serological types
- maximum expression in vivo
How is hyaluronidase useful to S. aureus?
A virulence factor, it:
- actos on hyaluronic acids in connective tissue, which facilitates dissemination through subcutaneous tissues
What cytotoxins are used by S. aureus?
Alpha hemolysin: potent pore-former, toxic to many cell types
Beta toxin: AKA sphingomyelinase C; kills cells via hydrolysis of membrane phospholipids
Delta toxin: cytolitic for many cells; nonspecific detergent-like action
Gamma toxin: pore-forming toxin; lysis of neutrophils and macrophages
Panton-Valentine leukocidin: same as gamma toxin
What in S. aureus causes:
Bullous impetigo
scalded skin syndrome
staphylococcal scarlet fever
Exfoliatin A, B