Infection 3: Bacterial pathogenesis Flashcards
Describe the bacterial envelope structure of gram positive bacteria
Thick peptidoglycans layer
Lipoteichoic and teichoic acid
Describe the bacterial envelope structure of gram negative bacteria
Outer membrane
- Lipopolysaccharide
- Proteins and pores
Thin peptidoglycans
Inner membrane
Name a bacterial organism that is an aerobe
S. aureus
Name a bacterial organism that is an anaerobe
Clostridium Spp
Is E.coli an anaerobe or aerobe?
both
How can we classify Streptocci?
α haemolytic ‘viridans’ streptococci
β haemolytic streptococci
γ (non)- haemolytic streptococci
α haemolytic ‘viridans’ streptococci
These cause partial haemolysis of blood agar and a greenish colour
β haemolytic streptococci
These cause complete haemolysis making the blood agar translucent
Gram positive cocci
Staphylococci S. aureus Coagulase negative staphylococci Streptococci Group A streptococcus (S. pyogenes) Group B Group C and G Group D = enterococcus S. pneumoniae Viridans type streptococci Peptococcus / Peptostreptococcus
Gram negative cocci
Neisseria
N. meningitidis
N. gonorrhoea
Moraxella (Branhamella) catarrhalis
Gram positive rods (bacilli)
Bacillus e.g. B. cerus and B. anthracis Corynebacterium Propionibacterium Listeria monocytogenes Clostridium e.g. C. difficile
Gram negative rods
Haemophilus influenzae Enterobacteriaciae Salmonella & Shigella Eschericia coli Klebsiella and Enterobacter Proteus Pseudomonas e.g P. aeruginosa Bacteroides e.g. B. fragilis Campylobacter and Helicobacter Vibrio cholerae Bordetella pertussis Legionella
What is the significance of s aureus?
positive coagulase test would indicate the presence of S. aureus.
Coagulase normally stimulates clotting
Role in immune evasion
Not expressed by less virulent ‘coagulase negative’ staphs
In the context of s aureus, what are adhesins? Give an example
surface proteins which bind host proteins
S aureus can colonize host to initiate infection by adhering to components of the extracellular matrix.
Adherence is mediated by surface protein adhesins
E.g Protein A
- binds Fc portion of IgG
Name the toxins produced the strains of staph aureus
Cytotoxins
- Pore forming toxins, lyse host cells
- Panton-valentine leukocidin (PVL) – lyses polymorphs
Exfoliative toxins
- Proteases
- Target epidermal structural proteins
Enterotoxins (superantigens)
- Stimulate massive T cell activation ?immune evasion
Name 2 exotoxins produced by strains of Staphylococcus aureus
TSST-1 or Exfoliative toxin 1
What syndrome of infection is TSST-1 associated with?
Toxic shock
Superangtigen exotoxins
Mechanism- stimulate massive T cell activation
a. Rash
b. Renal failure
c. Septic shock
d. Multiorgan failure
e. Skin desquamation on recovery
What syndrome of infection is Exfoliative toxin 1 associated with?
Scalded skin syndrome
Mechanism:
Serine proteases - specific for desmoglein I (epidermal structural proteins)
Name a cytotoxin produced by strains of staph aureus.
Panton-valentine leukocidin (PVL) –
Mechanism- lyses polymorphs
Pore forming toxins, lyse host cells
Outline the mechanism by which s aureus causes food poisoning
Toxin mediated
specifically enterotoxins/ s aureus superantigens
Give an example of a Coagulase Negative Staphylococci
S epidermis
Staining is same as s.aurerus
Don’t make coagulase
Less virulent
Describe the virulence of S. epidermidis
- lives on the skin
- frequently contaminates blood culture
- Also a pathogen
- Central venous line infection
Endocarditis (prosthetic valve)
Orthopaedic surgical infections
Foreign material in a ‘sterile’ place
grafts, implants etc
What is the accurate name of s pyogenes?
group A strep and is a beta haemolytic strep
Outline the spectrum of disease caused by s pyogenes
Superficial infections
- Pharyngitis
- Cellulitis
Deep infections
- Severe soft tissue infection
- myositis
- necrotising fasciitis
Autoimmune sequelae (v. rare in UK)
- Rheumatic fever – a major cause of heart disease
- glomerulonephritis
Outline the virulence factor for s pyogenes
M protein
- A major antigenic determinant of S. pyogenes
Immunity is type specific. - A major virulence factor
Binds serum factor H – regulator of complement activation - Prevents opsonisation
Involved in pathogenicity - Has an alpha-helical ‘coiled-coil’ protein
- Molecular mimickry
Explain the term molecular mimicry in the context of post infective autoimmune disease and give an example and organism.
- structural similarity of microbial antigenic determinants and host proteins resulting in cross reactivity of antibody and T cell responses to microbial antigens with self antigens
Example is Rheumatic fever
S pyogenes
Describe the virulence mechanisms of E coli
- Adhesins eg pili
- Capsule
- Siderophores
Major diseases caused by E coli
UTI
Enteric
Meningitis (Neonatal, elderly, immunocompromised)
Virulence mechanism of E. coli
- Adhesins
- Adhesins eg pili
( E. coli adhesins are fimbrial, fibrillar and nonfimbrial.)
Colonization of host tissues mediated by adhesins
Virulence mechanism of E. coli
- Capsule
- Capsule- Protects from complement mediated responses
Important early in life (lack of antibody)
a/w particular disease manifestations e.g. K1 capsular serotype and neonatal meningitis
Mechanism? Adhesin interactions with blood brain barrier
Virulence mechanism of E. coli
- Sideropores
- Siderophores- Enterobactin
Powerful iron chelators
Essential for survival in tissues (very low free Fe3+ environment)
Virulence mechanism of E. coli
- Toxins
- Endotoxin (= …………………….)
- Exotoxins
- Cytolysins (like Gram positive streptolysins)
- Enterotoxins very different from staphylococcal enterotoxins - cause fluid leak in the GI tract
- Verotoxins
Disrupt ribosomal protein synthesis
Verotoxin producing strains cause haemorrhagic diarrhoea
Enterohaemorrhagic E. coli (EHEC) e.g. O157:H7
Associated with haemolytic uraemic syndrome (HUS)
