Streptococci Flashcards
Streotococci comes from which greek word?
Streptos - meaning bent or twisted
Describe the bacteria streptococci
- gram positive
- coccus shaped
- grows in chains
- non-motile
- non-endospore
How are stretococci initially classified?
by their pattern of hemolysis on blood agar
α-hemolytic
- S. pneumoniae (pneumococcus) viridans group including: - S. viridans - endocarditis - S. mutans – tooth decay - S. thermophilus – dairy foods
- green colour du to oxidation of hemoglobin
β-hemolytic
- S. pyogenes (Group A Streptococcus)
- S. agalactiae (Group B Streptococcus)
- S. equi (Group C Streptococcus)
- “glowing” complete colouring
γ-hemolytic
- Enterococcus species (Group D Streptococcus)
- Lactococcus lactis (Group N Streptococcus)
- you dont have hemolysis
Streptococcus pyogenes
- an extracellular pathogen
- pyo = pus
- 5-15% asymptomatic carriage
what was Streptococcus pyogenes historically a major cause of?
mortaility due to scarlet fever, puerperal sepsis, wound infection in solidiers
What is Streptococcus pyogenes a common cause of, today?
pharyngitis and impetigo
- also causes invasive streptococcal disease and the streptococcal toxic shock syndrome (“flesh-eating” disease)
- an important cause of “post infection sequelae” including acute rheumatic fever
What is an asymptomatic carrier?
- has the bacteria, but displays no symptoms
- can still transmitt the disease
M - Protein
- an anti-phagocytic cell surface expressed protein
- binds “C4-binding protein” of the complement system
- C4-binding protein is a complement regulatory protein that protects self cells from complement
- > 100 M protein serotypes
hypervariable N-terminus (basis for M protein serotypes)
e.g. M1, M3 typically cause pharyngitis and invasive disease
e.g. M18 typically cause acute rheumatic fever - however, if you have antibodies to a particular M protein serotype you will opsonize and kill these bacteria
Hyaluronic Acid Capsule
- a polysaccharide
- hyaluronic acid is a major component of host tissues – bacteria “look like self”
- can also block opsonization through C3b (typical of other capsules)
S. pyogenes makes 2 hemolysins
Streptolysins (O and S)
Streptolysin S
produces B-hemolyssis
Streptolysin O
O2 sensitive
Superantigens
- secreted exotoxins
- Streptococcal pyrogenic exotoxins (Spe’s)
- function as potent activators of T cells and can result in a cytokine storm disease known as the toxic shock syndrome
- NOT emetic like the staphylococcal enterotoxins (SE’s)
Pharyngitis (Strep Throat)
- most common in school aged children and teenagers
- fever and severe sore throat
- typically absence of cough
- swollen cervical lymph nodes
- tonsillar exudate (pus)
- skin rash
Diagnosis - Is it Strep or viral?
- diagnosis by rapid strep test
positive test = strep throat, therefore treat with antibiotics
negative test, get throat culture
What type of antibiotics are used to treat strep?
- B-lactams (no documented resistance)
- erythromycin (resistant strains exist)
What can untreated pharyngitis lead to?
acute rheumatic fever
Impetigo
- most common among children
- also caused by S. aureus
- a superficial skin infection
- red sores that forms crusts, normally on the face
- highly contagious through direct contact
Scarlet Fever
- rash that develops typically during strep throat
- 5-15 years of age
- high fever, “strawberry tongue”
- rash – small red bumps
- normally on the chest and stomach
- can look like a sunburn
- rough sandpaper
- lasts 2-7 days
- toxin mediated - caused by the “scarlet fever toxins”
- same toxins as streptococcal pyrogenic exotoxins
Rheumatic Fever
- “post infection” sequele
- occurs 2-3 weeks after infection (e.g. strep throat or scarlet fever)
- typically occurs in children 5-15 years of age
- a form of autoimmunity caused by antibody cross-reactivity with the M protein
- rare in developed counties since 1960s but endemic in many developing countries
Acute Rheumatic Fever
the initital inflammation can cause swollen joints
Rheumatic heart disease
- damaged heart valves
- can lead to congestive heart failure
- patients are at increased risk for infective endocarditis by other pathogens
Invasive Streptococcal Disease
- rare in developed countries but very serious
- defined as isolation of S. pyogenes from a normally sterile site
- blood isolation –> bacteremia
FLESH EATING DISEASE:
- if soft tissue involved, leads to necrotizing fasciitis
if muscle involved, leads to necrotizing myositis
- streptococcal toxic shock syndrome (TSS)
Risk Factors for invasive streptococcal disease
- Tissue injury (penetrating and nonpenetrating)
- Prior use of nonsteroidal antiinflammatory agents
- Chicken pox in children (58-fold increased risk)
- Postpartum
- Lack of immunity to superantigens and M protein
- MHC class II haplotypes (ie. superantigen receptors)
Treatment for invasive streptococcal disease
- Antibiotics
- Supportive therapy
- Debridement/amputation
- Intravenous immunoglobulin (IVIG):
- neutralize superantigen
activity - opsonization of S.
pyogenes
- neutralize superantigen
Global Burden of Disease
- major burden of disease occurs in developing countries
-
Superantigen-mediated T cell activation
- leads to more T cells being activated (20% vs 0.01%)
Lancefield Classification
- classified on the basis of surface carbohydrate antigens
