micro lecture 12 Flashcards
which cocci looks like twisted chains?
Streptococci
what are the different schemes that Streptococci are classified into.
Haemolytic properties
Serologic grouping (Lancefield)
describe Haemolytic properties as a classification of streptococci
- Haemolytic properties
α– haemolytic streptococci
Chemical change in haemoglobin of red blood cells – not really lysis at all
Haemoglobin converted to biliverdin (green)
Green pigment around colony
β– haemolytic streptococci
Lysis of red blood cells
Clear ring around colony
γ– haemolytic streptococci
Not haemolytic; no colour changes
describe Serologic grouping (Lancefield) as a classification of streptococci
- Serologic grouping (Lancefield)
Antigen present in nearly all streptococcal cell walls, varying forms
haemolytic streptococci fall into
Groups A – U
Several other fall outside of these groups (including Streptococcus pneumoniae)
what are the characteristics of Group A β–haemolytic Streptococci
Most notable = Streptococcus pyogenes (pyo, pus + genes, generate/produce)
Nasopharyngeal carriage quite common
Doesn’t survive well in environment
Spread via respiratory droplets and skin contact
what are the Major virulence features of Group A β–haemolytic Streptococci
Capsule - hyaluronic acid
Mimics that found in human connective tissue
(so “hides” from immune system)
Cell wall fixed proteins
avoid/reduce phagocytosis
Help attachment to pharyngeal epithelium
Extracellular products
Secretes a range of exotoxins
what are the mild Clinical significance of Group A β–haemolytic Streptococci
Pharyngitis (“Strep throat”)
-Most common type of S. pyogenes infection
-Often mild, rarely progresses systemically
but can cause Rheumatic Fever, Scarlet Fever,
renal damage
Impetigo
-Typically affects children.
-Easily transmitted / extended by touch
-Topical treatment is sufficient when mild,
as this is a superficial infection
what are the MODERATE to RAPIDLY LETHAL Clinical significance of Group A β–haemolytic Streptococci?
Cellulitis
- Commonly S. aureus / GAS.
- Deep structures of skin/soft tissue.
- Often systemic symptoms and can lead to invasive GAS disease
Necrotising fasciitis (flesh-eating disease)
-Deep, rapidly invasive, life-threatening invasion of the
skin and underlying tissues
-Mediated by multiple toxins
-Often multibacterial but GAS common
what is the full meaning of (G.A.S. / GAS).
Group A β–haemolytic Streptococci
what are the MAJOR clinical significance of Group A β–haemolytic Streptococci?
Acute rheumatic fever -Autoimmune disease -Cross reaction of heart and joint tissue and antigens bacterial protein -Fever, rash, arthritis, carditis
Streptococcal toxic shock syndrome
-Exotoxins mediate immune response (TNFα,
cytokines, possibly direct T cell activation)
-Treatment must be initiated quickly
what are the treatment for Group A β–haemolytic Streptococci?
All infections require antibiotics
-penicillin G typically most effective but combinations
used for invasive infection
-penicillin G PLUS at least one of clindamycin or
gentamicin for necrotising fasciitis and toxic shock
(must turn off toxin production – often 3 or 4 agents
used as may also be multibacterial and “need to be
right”)
what is the full meaning of (G.B.S / GBS)
Group B β–haemolytic Streptococci
what are facts of Group B β–haemolytic Streptococci
Streptococcus agalactiae (of no milk, mastitis in cows)
Normal fluora of GI tract, vaginal tract, urethra
Transmission
Mother to newborn
Adult to adult – rarely pathogenic
Leading cause of meningitis and septicaemia in neonates - High mortality rate
what are facts of Streptococcus pneumonia (Pneumococcus)
NOT part of the Lancefield (group A/B/C … system)
α-haemolytic – “usually”
diplococcus (“chain” of two)
nasopharyngeal carriage
extremely sensitive to environment
commonest bacterial cause of
community-acquired lower respiratory tract infection
describe the major virulence factors of Streptococcus pneumoniae (Pneumococcus)
Capsule:
Most important factor
Composed of polysaccharide
Antiphagocytic and antigenic
