Streptococcus pyogenes
S. pyogenes, or group A Strep (GAS), is the most common cause of pharyngitis. It is also responsible for more serious diseases such as necrotozing fasciitis, Toxic shock syndrome, scarlet fever, and rheumatic fever. It can also cause various soft tissue infections.
- epidemiology
- classification and characteristics
- virulence factors
- transmission and infection
- clinical manifestations
- diagnosis
- treatment
Epidemiology
- asymptomatic colonization of upper respiratory tract and transient colonization of skin
Classification and Characteristics
- gram positive coccus
- grows in chains
- beta hemolytic
Virulence Factors
- hyaluronic acid capsule: evades immune response via molecular mimicry
- protein F: mediates adhesion to fibronectin
- M protein: in fimbriae; prevents phagocytosis
- streptokinase: cleaves plasminogen to activate plasmin, dissolving clots
- hyaluronidase
- streptolysin A and O: induces leukocyte and RBC lysis
- anti-streptolysin O titres (ASOT) used to diagnose recentstreptococcal infection
- streptodornase: degrades extracellular DNA used in neutrophil traps
- C5a peptidase
- streptococcal chemokine protease: degrades neutrophil chemotaxin
- toxins:
- lipoteichoic acid: released during lysis; triggers cytokine storm and potentially endotoxic shock
- streptococcal pyrogenic exotoxins A,B,C: act as superantigens by binding non-specifically to MHCII, responsible for toxic shock-like syndrome
Transmission and Infection
- respiratory droplets
- through breaks in skin after direct contact with infected person, fomite, or insect vector (bastards)
Clinical Manifesations
pharyngitis (strep throat)
- develops 2-4 days after exposure
- abrupt onset of sore throat, fever, malaise, headache
- posterior pharynx is red, with exudate and cervical lymphadenopathy
necrotizing fasciitis
cellulitis
- poorly demarcated
- deeper than erysipelas
- staph or strep
toxic shock
erysipelas
- well demarkated
- more superficial
- canse by beta hemolytic strep in over 98% of people
scarlet fever
- complication of primary pharyngitis, usually in age 3-8, as older have abs. Can also be due to skin infection, surgical wounds, or peri-anal infections.
- bacteriophages lyse S. pyogenes and induce it to produce pyrogenic and eryhtrogenic exotoxin
- causing a sandpaper-like rash to spread across upper chest and extremities
- strawberry tongue
- desquamation
rheumatic fever
- occurs approximately 20 days after Strep. pyogenes infection, especially pharyngitis
- involves inflammatory changes to heart, vasculature, joints, and subcutaneous tissues
- carditis, mitral stenosis, polyarthritis, subcutaneous skin nodules, chorea, erythema marginatum
- M protein is considered rheumatogenic due to its antigenic similarity to cardiac epitopes
- erysipelas
- acute infection of skin, raised and distinctly diffenrentiated from uninvolved tissue; systemic symptoms
- usually preceded by resipratory or skin infections
impetigo (pyoderma): epidermal infection
- more commonly Staph aureus alone
- follows colonization and break in skin
- vesicles develop into pustules that rupture and crust over
- cellulitis
- poorly demarcated
- deeper than erysipelas
- staph or strep
glomerular nephritis
- induced by immune complex deposition in glomerular capillaries
- complement induces mast cell degranulation and frustrated phagocytosis
- platelets lead to clots
Diagnosis
- ASOT can be used to deomostrate antibodies to streptolysin O; anti-DNase more useful in epidermal infections
- patients with glomerular nephritis have decreased urine output but increased protein excretion; RBC casts form as they clump together
Treatment
- good personal hygeine decreases skin colonization
- S pyogenes is very sensitive to penicillin
- Erythromycin or an oral cephalosporin is useful in patients with penicillin allergy