Infection 3: Bacterial pathogenesis and infectious disease Flashcards
Gram positive bacteria
Thick peptidoglycans layer
Lipoteichoic and teichoic acid
Gram negative bateria
Outer membrane
- lipopolysaccharide
- proteins and pores
Thin peptidoglycans
Inner membrane
Bacterial growth- atmosphere
Some are aerobes e.g. S. aureus
- use O2 as final electron acceptor
Some are anaerobes e.g. clostridium Spp
- fermentation- yields final electron acceptor is organic molecules
- ok when substrates are plentiful
- oxygen usually toxic to anaerobic bacteria
Many are ‘facultative anaerobes’ e.g. E.coli
- can switch between aerobic and anaerobic metabolism
Bacterial growth on agar
Morphology, requirements etc
- used in diagnostic laboratory
Some terms used clinically e.g. in classification of Streptococci
Haemolytic streptococci
- alpha, beta and gamma
Alpha haemolytic streptococci
Cause partial haemolysis of blood agar and a greensih colour
Beta haemolytic streptococci
These cause complete haemolysis making the blood agar translucent
Commensal
Something which is probably not causing disease when identified from a clinical sample
Pathogen
Something which is probably causing disease when identified from a clinical sample
Gram positive cocci
Staphylococci
- S. aureus
- coagulase negative staphylococci
Streptococci
- group A (S. pyogenes)
- group B
- group C and G
- group D (enterococcus)
- S. pneumoniae
- viridans type
Peptococcus/ peptostreptococcus
Gram negative cocci
Neisseria
- N. meningitidis
- N. gonorrhoea
Moraxella ctarrhalis
Gram positive rodes (bacilli)
Bacillus
- B. cerus
- B athracis
Corynebacterium
Propionibacterium
Listeria monocytogens
Clostridium
- C. difficile
Gram negative rods
Haemophilus influenza
Enterobacteriaciae
- salmonella and shigella
- Eschericia coli
- klebsiella and enterobacter
- proteus
Pseudomonas
- P. aruginosa
Bacterioles
- B. fragilis
Vibrio cholerae
Bordatella pertussis
Legionella
Staphylococcus aureus
Commensal of nose (60%)
G+ve cocci in clusters
- furunculosis
- staph abscess
- impetigo
Virulence of S.aureus: coagulase
Stimulates clotting
Role in immune evasion
Not expressed by less virulent ‘coagulase negative’ staphs
Virulence of S.aureus: adhesins
Bind host proteins
Tissue adherence
Colonisation
Deep infections
Immune evasion ‘cloaking’’
Virulence of S.aureus: protein A
An adhesin
Binds the Fc portion of IgG
Staphylococcal toxins
Cytotoxins
Exfoliative toxins
Enterotoxins
Cytotoxins
Pore forming toxins, lyse host cells
Panton- valentine leukocidin- lyses polymorphs
Exfoliative toxins
Proteases
Target epidermal structural proteins
Enterotoxins
Stimulate massive T cell activation
? Immune evasion
Scalded skin syndrome
Ritter’s disease
Exfoliative toxins
Outbreaks in nurseries
- ET+ve strains
- no immunity
Local infection e.g. umbillicus
Distant bullae
Sheet like desquamation
Toxic shock sydrome
Superangtigen exotoxins
- rash
- renal failure
- septic shock
- multiorgan failure
- skin desquamation on recovery
S.aureus food poisoning
S.aureus superantigen
- enterotoxins
Ingestion –>
- rapid brief illness
- vomiting
- minimal diarrhoea
S.aureus pneumonia
Rare except with influenza
- impaired ciliary function
- lost mucosal integrity
- specific immunocompromise
Associated with panton valentine leukocidin
S.aureus infections
Normal commensal
Pathogen in skin/ soft tissue infections
Vascular line related
Bactaraemia (endocarditis, osteoylelitis, septic arthritis)
Surgical site indections
Toxic mediated
Coagulsae negative staphyloccocus
Gram positive cocci in clusters
Don’t make coagulase
- less virulent
- differentiated in the lab
Includes several species
- S. epidermidis most commonly
S. epidermidis
Lives on the skin
Frequently contaminates blood culture
Central venous line sepsis
- endocarditis (prosthetic valves)
- orthopaedic surgical infections
- foreign material in ‘sterile’ place
Three ways to classify streptococci
Appearance on blood agar
Lancefield groups
- surface carbohydrate antigens
True species names
Classification of streptococci by haemolysis
Alpha- partial (green colour)
- common commensals of the mouth
Beta- complete
Gamma- none
Necrotising fasciitis
Caused by S.pyogenes of deep tissues
Production of tissue- destructive enzymes by organisms in stationary phase
- pain out of proportion to physical signs
- bruising and blistering
- generalised toxaemia
- renal impairment
- very high inflammatory response
- raised creatine kinase
Superficial S.pyogenes infections
Pharyngitis
Cellulitis
Deep S.pyogenes infections
Severe soft tissue infection
Myositis
Necrotising fasciitis
Autoimmune sequelae of S.pyogenes infections
Rheumatic fever- a major cause of heart disease
Glomerulonephritis
S.pyogenes exotoxins
Superantigens
Streptococcal inhibitor of complement
Haemolysins
DNAses
Hylauronidase
Streptokinase
Streptococcal 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 mimicry
Molecular mimicry and autoimmune sequelae
M protein- alpha helical coiled coil structure
Homology with
- cardiac myosin
- glomerular basement membrane
Recurrent childhood infection associated with cross reactive anti-self responses
Rheumatic fever
Post streptococcal glomerulonephritis
Enterobacteriaciae
Commensals of the gut
Not enterococcus
GNRs including
- klebsiella
- enterobacter
- citrobacter
Opportunistic infections where organisms gain access to sterile sites
Specific syndrome of infections associated with specific virulence mechanisms
E.coli
Hugely diverse species
Hundreds of serotypes
Cell wall ‘O’ flagella ‘H’ and capsular ‘K’ antigens
Three major human diseases
- UTI
- enteric
- meningitis
Four main virulence mechanisms
E.coli adhesins
P fimbria
- P= RBC P antigen
- also binds uroepithelial antigen
- also known as pylenonephritis associated adhesin
Non-fimral GI adhesins
- diarrhoeal diseases EPEC, EIEC
E.coli siderophores
Iron chelation
E.coli capsule
K1 capsular serotype
Protects from complement mediated responses
Important early in life
E.coli exotoxins
Cytolysins (like gram positive streptolysins)
Enterotoxins (upper GI food poisoning) ETEC
Verotoxins
- associated with haemolytic uraemic syndrome
- enterohaemorrhagic E.coli (EHEC)
E.coli disease
Commonest cause of urosepsis
Major contributor to GI related biliary sepsis
Major cause of nosocomial infections
- wounds
- devices
- pneumonia
Commonest pathogen grown in blood
