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Streptococci Lecture 5

Streptococci Lecture 5. Medical Microbiology SBM 2044. a -haemolytic. non-haemolytic. b -haemolytic. Streptococcus. Gram-positive. Grow in chains. Non-motile. Facultative anaerobes. 3 types of streptococci classification:.

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Streptococci Lecture 5

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  1. StreptococciLecture 5 Medical Microbiology SBM 2044

  2. a-haemolytic • non-haemolytic • b-haemolytic Streptococcus • Gram-positive • Grow in chains • Non-motile • Facultative anaerobes • 3 types of streptococci classification: • Early studies distinguished 3 broad groups on blood agar • Group-specific antigens (Lancefield classification) – by serological reactivity of extracted cell wall antigens (A  U) • Species – biochemical tests

  3. M-type specific antigen was sensitive to proteases M proteins Streptococcus pyogenes • Natural habitat: Humans • Strains distinguished by M serotyping • Devised by Lancefield in 1920s, using panels of absorbed sera • to hot-HCl extracted antigen, she called “M antigen” • > 100 distinct M types of GAS distinguished since then • - called M1, M2, M3, M4,……..etc. • Highly versatile pathogen • Suppurative infections • Toxinogenic diseases • Immunologically-mediated diseases

  4. GAS diseases – changing patterns • Changes in virulence of prevailing GAS strains ? • Changes in social conditions – less crowding? • Reemergence of severe invasive infections • Sporadic cases since mid-1980s – new virulent strains? • Streptococcal toxic shock • Some cases associated with obviously severe tissue infections • Many others – shock following mild or unapparent infections • Sporadic – implies predisposing factors

  5. Group A Streptococci Principle sites of infection: Invasive infections Local spread (e.g.) Other tissues Pharyngitis, tonsillitis, otitis media, sinusitis. Pharynx Bacteraemia or septicaemia Skin pyroderma, erysipelas Occasionally Streptococcal Toxic Shock Extensive necrosis (necrotizing fasciitis) Deep-seated tissues • Puerperal fever (childbirth fever) in women – major killer in past

  6. Streptococcus pyogenes Tonsillitis Follicular tonsillitis

  7. Streptococcus pyogenes Impetigo Erysipleas Cellulitis

  8. Streptococcus pyogenes Necrotizing fasciitis (< 24 hours post surgery)

  9. Streptococcus pyogenes Scarlet fever

  10. Group A Streptococci • Encounter • Carriers appear asymptomatic • Person-to-person spread is mediated by respiratory droplets or by direct contact to skin • Entry • For pyodermal infections, streptococci need to gain entry into deeper layers of skin • In pharyngeal infections, to prevent from being swept away, GAS must adhere to lipoteichoic acid (LTA), protein F and/or M protein

  11. M protein • Important for cell adherence to keratinocytes • Prevent opsonization by complement • bind to fibrinogen and interferes with the alternative pathway • bind with host complement control proteins and inhibit opsonins formation • Hypervariable regions of M protein are antigenic, but there are > 100 M protein serotypes

  12. EM showing the M Proteins (hair-like surface structures) of S. pyogenes

  13. Group A Streptococci • Spread and multiplication • Most GAS remain localised to the site of initial infection • In pharynx and tonsils, may result in erythema and exudate associated with strep throat • Peritonsillar abscess (quinsy) or spread to adjacent structures (mastoid and middle ear) • Impetigo in skin • Erysipelas and cellulitis in deeper layers of skin • GAS may spread laterally in deep tissues, by secreting enzymes • necrotizing fasciitis and myositis

  14. GAS virulence factors – Excreted products • Both lyse wide range of cells, including PMNs • suppuration and/or necrosis • Cytolytic toxins • Streptolyxin O (SLO) Produced by all strains • Streptolysin S (SLS) • Various subtle effects at sub-lytic concentrations • SLO – ‘sensitive’ to O2 • SLS – stable in O2 b-haemolysis “Thiol-activated” toxin

  15. SLO- and SLS-defective mutants • murine s.c. model - weight loss at 24h post infection Sterile ~3 x 108 cfu ~3 x 106 cfu + 0.5 ~3 x 107 cfu ~3 x 105 cfu ~3 x 109 cfu 0.0 - 0.5 - 1.0 Weight gain (grams) - 1.5 - 2.0 - 2.5 sloΔ1- sagBΔ1 sagBΔ1 sloΔ1 WT PBS

  16. Hyaluronic acid capsule • Antiphagocytic structure on bacterial surface • Hyaluronic acid is abundant in human connective tissue - hence GAS can camouflaged themselves • But capsule may interfere with the adherence of GAS to epithelial cells • so GAS shed the capsule during the early stages of infection using hyaluronidase

  17. Damage • GAS can evoke an intense inflammatory responses in tissues • Streptococcal pyrogenic exotoxins (SPE) • SPE A, B and C cause rash, a characteristic of scarlet fever • SPE A and C are bacterial superantigens that activate a large subset of T cells • Immunologically mediated disease (nonsuppurative sequelae) • acute rheumatic fever (ARF) • acute post-streptococcal glomerulonephritis

  18. Toxic Shock • In past – probably linked to scarlet fever • Since mid-1980s – associated with ‘new’ highly virulent strains - rapidly fulminating • some cases in previously healthy young adults, no obvious • predisposing factors • Associated with production of superantigenic toxins, but other factors also involved

  19. Acute rheumatic fever (ARF) • autoimmune disease - triggered only by GAS pharyngitis • associated with strong immune response to GAS. • antibodies and/or T cells X-react with host antigens? (later) • Symptoms arise > 10days + after GAS infection • responsible GAS strain already ‘cleared’ • Symptoms may include: • inflammation multiple sites, starting with major joints (arthritis) • neurological disorders (Sydenhams chorea) • rheumatic heart disease (RHD) (ca 50% cases) • - damage to heart valves, permanent scaring in survivors

  20. Acute rheumatic fever (ARF) • Initial attack rates low (3% in untreated pharyngitis) • High recurrence (up to 50%) - increasing in severity Widespread prescription of penicillin for ‘sore throats’ Remarkably, GAS have not (yet ?) developed resistance to penicillins

  21. GAS infections - complications Post-streptococcal acute glomerulonephritis (PSGN): • Common, but rarely life-treating - some GAS infections • of either pharynx or skin. • Symptoms arise some 10 days after infection • reflect kidney dysfunction, probably involving inflammation • of glomeruli • Pathogenesis: • Most probably entrapment of GAS antigen-host antibody • complexes at basement membranes of glomeruli • might also involve an ‘autoimmune’ response

  22. Normal glomerulus Glomerulonephritis Mesangial cell Mesangial cell intrusion Endothelial cell, has 100 nm pores PMN Basement membrane Inflammation Too much large immune complex entrapment ? Small complexes diffuse thro’ basement membrane into urine, but the occasional larger complex can’t & is normally removed by mesangial cell Example: Sequel of some S. pyogenes infections

  23. Treatment and Prevention • Penicillin – 10 day oral therapy • Erythromycin or other macrolide antibiotics for individuals allergic to penicillin

  24. Group B streptococci • Streptococcusagalactiae are aerobic G+ diplococci that are β-haemolytic on blood agar plates • found in lower GIT and female genital tracts • GBS is a leading cause of neonatal sepsis and meningitis • prevent opsonization and phagocytosis with a polysaccharide capsule

  25. Enterococci • Enterococcusfaecalis cause UTI, wound infections, endocarditis, intraabdominal abscesses and bacteremia. • Normal flora of GIT and GUT • resistant to bile and high salt concentrations • nosocomial infections • resistance to many antibiotics, often bacteriostatic • bacterial killing must use a combination treatment of a β-lactam and an aminoglycoside

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