S11 Streptococci Flashcards
What type of gram stain is Streptococci?
Gram positive cocci (in chains)
How are Streptococci classified by haemolysis?
- alpha haemolysis e.g. Strep. pneumoniae (partial breakdown - greenish zone)
- beta haemolysis e.g. Strep. pyogenes (full breakdown - clear zone)
(* non-haemolytic gamma e.g. Enterococcus faecalis)
What is the Lancefield (1993) classification of Streptococci?
Serological classification of beta-haemolytic streptococci - based on cell wall antigens
What does Sherman (1937) classify Streptococci based on?
4 classes:
- pyogenic
- viridans (alpha-haemolytic)
- enterococcal
- lactic streptococci
What is the Lancefield antigen for S. pyogenes?
For S. agalactiae?
For S. dysglactiae?
A
B
C
How do most people experience S. pyogenes?
Sore throat - tonsillitis
Necrotising fasciitis
How do most people experience S. galactiae?
Usually symptom free - often carried as part of woman’s normal vaginal flora - so can cause infections in newborns - bacteria into vagina and into amniotic cavity, causing infection whilst baby is still in-utero or as baby passes through birth canal - becomes colonised with bacteria which can then invade
How do most people experience S. dysgalactiae?
Sore throats and skin infections
How do most people experience S. pneumoniae?
Pneumonia
If capsulated:
Meningitis
Endocarditis
Peritonitis
What is the S. milleri group main characteristic? What Sherman group is it?
Cause abscesses
Viridans
You have Lancefield, Sherman and haemolysis classifications for Strep. bacteria, what is the last and most recent?
DNAsequencing (16S ribosomal RNA sequences)
What are the virulence factors for Streptococcus pyogenes?
- hyaluronic acid capsule
- M protein
- adhesins
- streptolysins O and S
- Dnases A, B, C and D
- hyaluronidase
- streptokinase
- streptococcal pyogenic exotoxins
What is the action of the hyaluronic acid capsule?
Streptococcus pyogenes
Inhibits phagocytosis by neutrophils and macrophages (has similarities to human CT so not much immune response to it)
What is the action of M protein? (Streptococcus pyogenes)
Resistance to phagocytosis by inhibiting activation of alternative complement pathways on bacterial cell surface.
There are many different antigenically different stereotypes of M protein
(Fuzzy around cell)
What is the action of adhesins? (Streptococcus pyogenes)
Helps with adherence (which is the first step in colonisation/infection)
E.g. lipoteichoic acid, M protein, fibronectin binding proteins
What is the action of streptolysins O and S? (Streptococcus pyogenes)
Lysis of erythrocytes, neutrophils and platelets
What is the action of Dnases A, B, C and D? (Streptococcus pyogenes)
Degradation of DNA
What is the action of hyaluronidase? (Streptococcus pyogenes)
Degradation of hyaluronic acid in CT
What is the action of streptokinase? (Streptococcus pyogenes)
Break down of clots through conversion of plasminogen to plasmin
What is the action of streptococcal pyrogenic exotoxins? (Streptococcus pyogenes)
Cleaves IgG bound to Group A strep
Fever producing toxin
What is Streptococcal pharyngitis - what is it caused by, who does it occur most in, how is it spread, why is there an argument to leave untreated?
Caused by Streptococcus pyogenes
Mostly occurs in children (5-15 years)
Spread by droplets e.g. coughing
Associated with overcrowding
untreated patients develop an M protein specific antibody - if treated they don’t have an immune response
What are the clinical features of Streptococcal pharyngitis?
- abrupt onset sore throat
- malaise, fever, headache
- lymphoid hyperplasia
- tonsillopharyngeal exudates
A throat swab will show Group A strep
What are the complications of streptococcal pharyngitis?
- scarlet fever - if have streptococcal pyrogenic exotoxin, local/haematogenous spread , high fever, sepsis, arthritis and jaundice
- suppurative (pus) complications - peritonsillar cellulitis/abscess, retropharyngeal abscess, mastoiditis, sinusitis, otitis media, meningitis and brain abscess
- acute rheumatic fever - inflammation of heart, joints and CNS
- acute post-streptococcal glomerulonephritis - acute inflammation of renal glomerulus (specific M types lead to this)
What are the possible mechanisms of acute rheumatic fever (following pharyngitis)?
- autoimmune
- serum sickness
- binding of M protein to collagen - certain types of M protein are linked to rheumatic fever
- ASO, ASS induces tissue injury
What are 4 types of streptococcus pyogenes skin infections?
- impetigo
- erysipelas
- cellulitis
- necrotising fasciitis
What is impetigo?
A childhood infection (2-5 years)
Initial skin colonisation followed by intradermal inoculation
Common cause of glomerulonephritis
What is erysipelas?
A dermis infection with lymphatic involvement, usually occurs on face and lower limbs
On the face - usually preceded by pharyngitis
On the lower limbs - usually secondly to invasion of skin via trauma, skin disease or a local fungal infection
What is cellulitis?
A skin and subcutaneous tissue infection
Risk factors of cellulitis are: impaired lymphatic drainage and illicit injecting drug use
What is necrotising fasciitis?
An infection of the deeper subcutaneous tissues and fascia - it involves rapid extensive necrosis
It’s usually secondary to a skin break
Results in severe pain even before there are any clinical changes, high fever and has a high mortality rate
What is streptococcal toxic shock syndrome, clinically and pathologically?
A deep tissue infection with Strep. pyogenes
And bacteraemia, vascular collapse and organ failure
(Health to death in hours)
It involves the entry of group A strep into deeper tissues and the bloodstream - strep pyrogenic exotoxins stimulate T-cells by binding to MHC class II APCs, this induces cytokines and lymphokines. M-proteins bind to fibrinogen forming a complex.
Degranulation, respiratory burst and endothelial damage occur
