Infection 3: Bacterial pathogenesis and infectious disease

Question 1

Gram positive bacteria

Answer 1

Thick peptidoglycans layer

Lipoteichoic and teichoic acid

Question 2

Gram negative bateria

Answer 2

Outer membrane

• lipopolysaccharide
• proteins and pores

Thin peptidoglycans

Inner membrane

Question 3

Bacterial growth- atmosphere

Answer 3

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

Question 4

Bacterial growth on agar

Answer 4

Morphology, requirements etc
- used in diagnostic laboratory

Some terms used clinically e.g. in classification of Streptococci

Haemolytic streptococci
- alpha, beta and gamma

Question 5

Alpha haemolytic streptococci

Answer 5

Cause partial haemolysis of blood agar and a greensih colour

Question 6

Beta haemolytic streptococci

Answer 6

These cause complete haemolysis making the blood agar translucent

Question 7

Commensal

Answer 7

Something which is probably not causing disease when identified from a clinical sample

Question 8

Pathogen

Answer 8

Something which is probably causing disease when identified from a clinical sample

Question 9

Gram positive cocci

Answer 9

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

Question 10

Gram negative cocci

Answer 10

Neisseria

• N. meningitidis
• N. gonorrhoea

Moraxella ctarrhalis

Question 11

Gram positive rodes (bacilli)

Answer 11

Bacillus

• B. cerus
• B athracis

Corynebacterium

Propionibacterium

Listeria monocytogens

Clostridium
- C. difficile

Question 12

Gram negative rods

Answer 12

Haemophilus influenza

Enterobacteriaciae

• salmonella and shigella
• Eschericia coli
• klebsiella and enterobacter
• proteus

Pseudomonas
- P. aruginosa

Bacterioles
- B. fragilis

Vibrio cholerae

Bordatella pertussis

Legionella

Question 13

Staphylococcus aureus

Answer 13

Commensal of nose (60%)

G+ve cocci in clusters

• furunculosis
• staph abscess
• impetigo

Question 14

Virulence of S.aureus: coagulase

Answer 14

Stimulates clotting

Role in immune evasion

Not expressed by less virulent ‘coagulase negative’ staphs

Question 15

Virulence of S.aureus: adhesins

Answer 15

Bind host proteins

Tissue adherence

Colonisation

Deep infections

Immune evasion ‘cloaking’’

Question 16

Virulence of S.aureus: protein A

Answer 16

An adhesin

Binds the Fc portion of IgG

Question 17

Staphylococcal toxins

Answer 17

Cytotoxins

Exfoliative toxins

Enterotoxins

Question 18

Cytotoxins

Answer 18

Pore forming toxins, lyse host cells

Panton- valentine leukocidin- lyses polymorphs

Question 19

Exfoliative toxins

Answer 19

Proteases

Target epidermal structural proteins

Question 20

Enterotoxins

Answer 20

Stimulate massive T cell activation

? Immune evasion

Question 21

Scalded skin syndrome

Answer 21

Ritter’s disease

Exfoliative toxins

Outbreaks in nurseries

• ET+ve strains
• no immunity

Local infection e.g. umbillicus

Distant bullae

Sheet like desquamation

Question 22

Toxic shock sydrome

Answer 22

Superangtigen exotoxins

• rash
• renal failure
• septic shock
• multiorgan failure
• skin desquamation on recovery

Question 23

S.aureus food poisoning

Answer 23

S.aureus superantigen
- enterotoxins

Ingestion –>

• rapid brief illness
• vomiting
• minimal diarrhoea

Question 24

S.aureus pneumonia

Answer 24

Rare except with influenza

• impaired ciliary function
• lost mucosal integrity
• specific immunocompromise

Associated with panton valentine leukocidin

Question 25

S.aureus infections

Answer 25

Normal commensal Pathogen in skin/ soft tissue infections Vascular line related Bactaraemia (endocarditis, osteoylelitis, septic arthritis) Surgical site indections Toxic mediated

Question 26

Coagulsae negative staphyloccocus

Answer 26

Gram positive cocci in clusters Don't make coagulase - less virulent - differentiated in the lab Includes several species - S. epidermidis most commonly

Question 27

S. epidermidis

Answer 27

Lives on the skin Frequently contaminates blood culture Central venous line sepsis - endocarditis (prosthetic valves) - orthopaedic surgical infections - foreign material in 'sterile' place

Question 28

Three ways to classify streptococci

Answer 28

Appearance on blood agar Lancefield groups - surface carbohydrate antigens True species names

Question 29

Classification of streptococci by haemolysis

Answer 29

Alpha- partial (green colour) - common commensals of the mouth Beta- complete Gamma- none

Question 30

Necrotising fasciitis

Answer 30

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

Question 31

Superficial S.pyogenes infections

Answer 31

Pharyngitis Cellulitis

Question 32

Deep S.pyogenes infections

Answer 32

Severe soft tissue infection Myositis Necrotising fasciitis

Question 33

Autoimmune sequelae of S.pyogenes infections

Answer 33

Rheumatic fever- a major cause of heart disease Glomerulonephritis

Question 34

S.pyogenes exotoxins

Answer 34

Superantigens Streptococcal inhibitor of complement Haemolysins DNAses Hylauronidase Streptokinase

Question 35

Streptococcal M protein

Answer 35

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

Question 36

Molecular mimicry and autoimmune sequelae

Answer 36

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

Question 37

Enterobacteriaciae

Answer 37

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

Question 38

E.coli

Answer 38

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

Question 39

E.coli adhesins

Answer 39

P fimbria - P= RBC P antigen - also binds uroepithelial antigen - also known as pylenonephritis associated adhesin Non-fimral GI adhesins - diarrhoeal diseases EPEC, EIEC

Question 40

E.coli siderophores

Answer 40

Iron chelation

Question 41

E.coli capsule

Answer 41

K1 capsular serotype Protects from complement mediated responses Important early in life

Question 42

E.coli exotoxins

Answer 42

Cytolysins (like gram positive streptolysins) Enterotoxins (upper GI food poisoning) ETEC Verotoxins - associated with haemolytic uraemic syndrome - enterohaemorrhagic E.coli (EHEC)

Question 43

E.coli disease

Answer 43

Commonest cause of urosepsis Major contributor to GI related biliary sepsis Major cause of nosocomial infections - wounds - devices - pneumonia Commonest pathogen grown in blood