Lecture 11- Streptococci and disease Flashcards
Describe the general characteristics of streptococci?
- Gram positive cocci
- Grow in chains
- Some strains produce capsules
- Susceptible to penicillin(no B lactamase found)
Classify streptococci
Group A= S.PYOGENES Group B= S.Agalactiae Group C= S.Disgalactiae. S.equi Group D= S. bovis, Enterococcus spp Group F= S. intermedius No lancefield antigen: S.pneumoniae, viridans group
Describe diagnostic lab test to identify Streptococci?
Catalase test should give no bubbles for streptococci. Distinguish from S. aureus which gives bibbles
Describe diagnostic lab tests to classify streptococci?
If hemolysis on blood agar= B hemolytic Streptococci
If no hemolysis: 3 tests can be performed
Test 1- Bacitracin. If bacitracin susceptible then GAS species. If bacitracin resistant then GBS species of streptococci
Test 2- Bile esculin agar. If hydrolyse esculin then it is GDS Enterococcus. If no hydrolysis of esculin then do further tests
Test 3- antibiotic test with OPTOCHIN. If optochin susceptible then S.pneumoniae. If optochin resistant then viridans strep
Describe S. pyogenes epidemiology
- Asymptomatic colonisation
- can survive on dry surfaces
- spread from wound infections or respiratory droplets from person to person(crowding)
- Higher infection rates in maori and pacific island populations
List the streptococcal virulence factors of Group A S. Pyogenes- adhesins?
1) MSCRAMMS
- cell wall attached proteins
- bind to host extracellular matrix proteins
- Examples= M protein(antiphagocytic), F protein(binds fibronectin) and Cpa(collagen binding protein)
2) Pili
- Long hair like structures
- cell wall anchored
- tip protein functions as adhesin
- Cell aggregation(biofilm formation)
List the streptococcal virulence factors of Group A S. Pyogenes- cytolysins?
- Streptolysin O (SLO)= oxygen liable-forms pores in host cell membrane
- Streptolysin S(SLS)= oxygen-stable- lyses red blood cells(B-hemolysis), antigenic, anti SLO antibodies test for acute rheumatic fever
List the streptococcal virulence factors of Group A S. Pyogenes- spreading factors?
Same as S. aureus except Staphylokinase is called Streptokinase
What are the superantigens associated with S.pyogenes?
Streptococcal pyogenic exotoxins(SPE-A)
Streptococcal mitogenic exotoxin Z(SMEZ)
Structurally and functionally related to S. aureus superatigens ie result over-response and inflammation. However they do not cause food poisoning like S. aureus
List the immune invasion factors for S.pyogenes?
Capsule
- Hyaluronic acid coat= inhibits phagocytosis
-M protein= adhesin and functions to prevent complement factor C3b from opsonising(antiphagocytic).
C5a peptidase= cleaves complement factor C5a preventing neutrophil migration to the site of infection.
What are the three types of streptococcal diseases?
Non-invasive disease - pharyngitis, tonsillitis - pyoderma (impetigo) - cellulitis Invasive disease - necrotising fasciitis (flesh-eating disease) - streptococcal toxic shock syndrome Post-streptococcal disease - acute rheumatic fever! - acute glomerulonephritis
Describe characteristics of pharyngitis and tonsillitus?
Develops 2-4 days after exposure to S. pyogenes (sore throat, fever, reddened pharynx, pus-filled vesicles on tonsils)
Describe characteristics of scarlet fever?
- complication of pharyngitis!
- production of pyrogenic exotoxin A (SPE-A) = “scarlet fever toxin” = superantigen!
- can develop into serious systemic disease!
- fever, sore throat, “strawberry tongue”, characteristic rash on chest
Describe characteristics of impetigo?
- purulent infection of the skin (“derma”)!
- S. pyogenes colonisation after contact with infected person or fomites!
- spread to subcutaneous tissue through break in skin (e.g. scratch)!
- highly communicable, often in hot/humid climate!
Describe characteristics of cellulitis?
- infection of skin that involves subcutaneous tissue!
* acute, rapidly spreading infection (hyaluronidase, DNAse)!
Describe characteristics of Necrotising fasciitis(flesh eating disease)?
• deep infection of the skin that involves destruction of muscles
• S. pyogenes is introduced through e.g. minor cuts, trauma, burn
surgery or vesicular viral infection
• deep tissue infection is supported by spreading factors!
(DNAses, proteases, hyaluronidase)
• often development into severe systemic disease with high mortality
Describe characteristics of streptococcal toxic shock syndrome(STSS)?
• deep infection of the skin that involves destruction of muscles
• S. pyogenes is introduced through e.g. minor cuts, trauma, burn,!
surgery or vesicular viral infection.!
• deep tissue infection is supported by spreading factors!
(DNAses, proteases, hyaluronidase)!
• often development into severe systemic disease with high mortality
Describe rheumatic fever and rheumatic heart disease?
- Develops after untreated/chronic sore throats due to GAS
- inflammation of endocardium, myocardium, pericardium resulting in thickened and deformed valves and granulomas in myocardium
- inflammatory changes in joints (arthritis), blood vessels!
- Autoimmune disease, NOT infection !!
Mechanism: a conserved region in GAS protein structurally resembles certain host antigens (‘molecular mimicry’). Molecular mimicry is when different proteins share antibody epitope. So M. protein from S.pyogenes amd a host protein share a small section which is similar and this results in a antibody cross reaction. Hypersensitivity reaction- inflammation
Describe S. agalactiae- group B streptococcus physiology, epidemiology and disease?
Physiology
• Gram-positive cocci in long chains
• -hemolytic or non-hemolytic
• carry Lancefield group B specific carbohydrate
Epidemiology
• asymptomatic colonisation of upper respiratory & genitourinary tract
• most infections in newborns acquired from mother during pregnancy, at time of birth or the first week after birth
Disease
• neonatal disease: pneumonia, bacteremia, sepsis, meningitis
• Urinary tract infections in pregnant women
Describe physiology, epidemiology and disease caused by Viridans Streptococci?
Physiology
• hemolytic or non-hemolytic
• carry no specific Lancefield group antigen
Epidemiology
• asymptomatic colonisation of oropharynx, gastrointestinal tract and genitourinary tract
• commensales of mouth flora (S. mitis, S. mutants, …)
Virulence Factors
• less virulent than S. pyogenes (less immune evasion toxins)
• adhesins/pili for binding to teeth, biofilm (dental plaque), S. mutants
Disease!
• dental caries
• subacute endocarditis
• septic shock in immuno-compromised patients
Describe physiology, epidemiology and virulence by Streptococcus pneumoniae?
Physiology
• ‘lancet-shaped’ diplococci or short chains
• most strains with outer capsule
Epidemiology
• endogenous spread from colonised pharynx to lungs, sinuses, ears,..
• conditions that interfere with bacterial clearance are risk factors
e.g. recent viral lung infection, chronic pulmonary disease, diabetes
Virulence
• antiphagocytic polysaccharide capsule
• pneumolysin: destroys ciliated eptithelial cells
Describe diseases caused by Streptococcus pneumoniae?
• Pneumonia (60% of bacterial pneumonia)
- after aspiration, bacteria multiply in alveolar spaces
- infiltration of neutrophils and alveolar macrophages
- inflammation, most damage caused by immune response
- Symptoms: Fever, yellowish sputum, chest pain
• Meningitis: headache, fever, sepsis, high mortality
children and elderly have increased risk
• Bacteremia: more common in patients with meningitis
• Sinusitis and Otitis Media: usually after virus infection
Describe Enterococcus spp?
- formerly known as group D streptococcus
- colonise the gastrointestinal tract (very common)
- can spread to other mucosal surfaces when normal gut flora is suppressed by broad-spectrum antibiotics
- increased risk in patients with prolonged hospitalisation
- UTIs, wound infection, bacteremia and subacute endocarditis
