Bacterial pathogenesis and infectious disease Flashcards
Process of gram staining
- Fixation
- Crystal violet
- Iodine treatment
- Decolonisation
- Counter stain (safranin)
Types of bacteria shapes
- Cocci
- Rods(bacilli)
- Spirals
- Vibrio
Gram positive bacteria membrane structure
- Thick peptidoglycans layer
- Lipoteichoic and teichoic acid
Gram negative bacteria membrane structure
- Outer membrane(lipopolysaccharide, proteins and pores)
- Thin peptidoglycans
- Inner membrane
S.aureus - type of growth
- Aerobic
- Use O2 as final electron acceptor (very efficient)
Clostridium spp - type of growth
- Anaerobic
- Fermentation - yields final electron acceptor is organic molecule
- Ok when substrates are plentiful
- Oxygen usually toxic to anaerobic bacteria
What are facultative anaerobes
- Can switch between aerobic and anaerobic metabolism
- E.coli
Terms used to classify streptococci
- alpha haemolytic ‘viridians’ streptococci - these cause partial haemolysis of blood agar and a greenish colour
- Beta haemolytic streptococci - these cause complete haemolysis making the blood agar translucent
- gamma(non)-haemolytic streptococci
Streptococci typing
Relatedness of strains within a species e.g:
- E. coli 0157:H7
- (sero-) group A N.meningitidis
- M3T3 S. pyogenes
Primarily serological types
- antibodies to expressed antigens
Now frequently correlating genotypes eg: S. pyogenes emmtype = M type
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
What determines whether an organism is a pathogen or a commensal
- The immune status of the patient
- The site/sample in question
- The disease causing properties of the bacteria(virulence)
Virulence of S. aureus
Coagulase
Adhesins: Bind host proteins
Protein A
Coagulase
- Stimulates clotting
- Role in immune evasion
- Not expressed by less virulent ‘coagulase negative’ staphs
Adhesins
Bind host proteins
- Tissue adherence
- Colonisation
- Deep infections
- Immune evasion ‘cloaking’
Protein A
- An adhesin
- Binds the Fc portion of IgG
What can S.aureus sepsis present with on a chest x-ray
- Lung abscesses
- Also splenic abscesses (not on an x-ray tho lol)
Staphylococcal toxins
- Cytotoxins
- Exfoliative toxins
- Enterotoxins(superantigens)
- Complement inhibitors
Many of these are encoded on mobile genetic elements (only present on a proportion of strains)
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
Panton valentine leukocidin (PVL)
- Neutrophil pore-forming lysin
- Present in around 2% of S.aureus strains
- Family and community outbreaks
What is PVL disease associated with
- Associated with community onset MRSA cases in USA - not yet in Europe
- Severe, purulent, necrotizing skin infections
Scalded skin syndrome
- Ritter’s disease
- Exfoliative toxins (ET)A, B etc
(Serine proteases - specific for desmoglein I) - Outbreaks in nurseries (ET+ve strains, no immunity)
- Local infection - eg. umbilicus
- Distant bullae
- Sheet-like desquamation
Toxic shock syndrome
- Historically most notorious as “Tampon shock”
- Caused by bacterial toxins
Symptoms of toxic shock syndrome
- Rash
- Renal failure
- Septic shock
- Multiorgan failure
- Skin desquamation on recovery
S. aureus food poisoning
- S. aureus superantigens = enterotoxins
- Ingestion –> rapid brief illness
- Vomiting
- Minimal diarrhoea
Contamination of food
- Manufacture
- Preparation
S. aureus infections
- Normal commensal
- Pathogen in skin/soft tissue infections
- Vascular line related infections
- Bacteraemia - commonest cause
- Surgical site infections
- Toxin mediated
S.aureus bacteraemia
- Endocarditis
- Osteomyelitis
- Septic arthritis
- Almost anywhere else
S.aureus toxin mediated
- Toxic shock
- Scalded skin
- Food poisoning
Coagulase negative staphylococci
- Gram positive cocci in clusters (like S. aureus)
- Don’t make coagulase
(less virulent, differentiated in the lab) - Includes several species (S.epidermis most commonly)
S. epidermis
- 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 are streptococci
- A huge family of gram positive bacteria which colonise the GI tract and skin
Ways of classifying streptococci
1) Appearance on blood agar(haemolysis) e.g ‘beta haemolytic strep
2) Lancefield groups(A, B, C etc)
- Surface carbohydrate antigens
- e. ‘group A strep’
3) True species names(S. pyogenes, S. pneumoniae etc) e.g S.pyogenes = group A strep and is a beta haemolytic strep
Classification of streptococci by haemolysis
- alpha haemolysis = partial
- beta haemolysis = complete
- gamma haemolysis = none
What are organisms which cause alpha haemolysis sometimes called
- Viridians-type streptococci
- Common commensals of the mouth
- S. milleri, S. mitis, S. Sanguis
- Also S. pneumoniae
Which lancefield groups of streptococci are beta haemolytic
- A, B, C, G
- V similar organisms
- Site and species adapted(S. pyogenes - pharynx of man, S. equi, S.canis etc, same disease in other species)
- All can cause pharyngitis and cellulitis
What is necrotising fasciitis caused by
- Caused by S.pyogenes infection of deep tissues
- Production of tissue-destructive enzymes by organisms in stationary phase
Effects of necrotising fasciitis
- Pain out of proportion to physical signs
- Bruising and blistering
- Generalised toxaemia
- Renal impairment
- Very high inflammatory response (CRP)
- Raised creatine kinase
- Diagnosis by surgical exploration
S.pyogenes - superficial infections
- Pharyngitis
- Cellulitis
S.pyogenes - deep infections
- Severe soft tissue infection
- Myositis
- Necrotising fasciitis
S.pyogenes - autoimmune sequelae(v.rare in UK)
- Rheumatic fever - a major cause of heart disease
- Glomerulonephritis
S.pyogenes structue and virulence
- Fibronectin-binding proteins
- Collagen-binding protein
- C5a peptidase
- M protein
- Capsule
- Wall
- Membrane
Exotoxins released by s.pyogenes
- Superantigens
- Streptococcal inhibitor of complement
- Haemolysins
- DNAses
- Hyaluronidase
- Streptockinase
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 mimickry
Molecular mimicry and autoimmune sequelae
M protein - alpha helical coiled coil structure
- Homology with cardiac myosin/trophomyosin, glomerular basement membrane
- Recurrent childhood infection associated with cross-reactive anti-self responses
- Rheumatic fever
- Post-streptococcal glomerulonephritis
What is molecular mimicry also implicated in aetiology of
- Guillain-barre syndrome
- HLA B27-associated spondyloarthropathies
Enterobacteriaciae(E.coli etc)
- Commensals of the gut (hence entero)
- Note NOT enterococcus (a gram positive coccus)
- GNRs including klebisella, enterobacter, citrobacter
- Opportunistic infections where organisms gain access to sterile sites(wound infections, biliary infections, hospital acquired pneumonia)
- Specific syndromes 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 disease caused E.coli
- UTI
- Enteric
- Meningitis(neonatal, elderly, immunocompromised)
E.coli virulence mechanisms
- Adhesins (fimbrae = pili)
- Siderophores
- Capsule
- Toxins
E.coli - Adhesins
P fimbria
- P = RBC P antigen
- Also binds uroepithelial antigen
- Also known as pyelonephritis associated adhesin
Non-fimbral GI adhesins
- Diarrhoeal diseases EPC, EIEC
E.coli - siderophores
- Enterobactin
- Powerful iron chelators
- Essential for survival in tissues(very low free fe3+ environment)
E.coli - capsule
- Protects from complement mediated responses
- Important early in life (lack of antibody)
- Some associated with particular manifestations e.g. K1 capsular serotype and neonatal meningitis (mechanism? adhesin interactions with blood brain barrier)
E.coli - toxins
- Endotoxin
- Exotoxins positive streptolysins)
- Enterotoxins
- Verotoxins
Exotoxins
- cytolysins(like gram positive streptolysins)
Enterotoxins - e.coli
- Very different from staphyloccocal enterotoxins - cause fluid leak in the GI tract
Verotoxins
- Disrupt ribosomal protein synthesis
- Verotoxin producing strains cause haemorrhagic diarrhoea
- Enterohaemorrhagic e.coli (ehec)
- Associated with haemolytic uraemic syndrome (HUS)
What does e.coli illustrate
- Gram negative bacteria can make exotoxins too
- Within a species, very different pathogens can exist
- Usual virulence mechanisms (adhesion, toxins)
- Also, specific ‘fimbral’ adhesins, iron chelation
E.coli disease
- Commonest cause of urosepsis
- Major contributor to GI-related and biliary sepsis
- Major cause of nonsocomial infections(wounds, devices, pneumonia)
- Commonest pathogen grown in blood
Two big changes in E.coli disease going on
- Invasive infection(bacteraemia) is becoming much more common
- Strains are increasingly resistant to first-line antibiotics
