Paradigms in Bacterial Pathogenesis Flashcards
What enzyme distinguishes pathogenic species of Staphylococcus from those that are non-pathogenic?
coagulase: an enzyme complex produced by S. aureus. The cell-associated (bound) from react w fibrinogen in plasma and results in clumping of the blood
the free form activates prothrombin causing clotting of plasma
although commensal strains may not possess coagulase, it has emerged as significant opportunistic pathogens.
What are some clinically relevant strains of antibiotic resistant S. aureus?
- Methicillin-resistant Staphylococcus aureus (MRSA)
- evolved via HGT
- hospital aquired rates have massively declined in the UK - Vancomycin-resistant Staphylococcus aureus (VRSA)
- vancomycin is last resort to MRSA - Community-associated MRSA (CA-MRSA)
- relevant outside of hospitals
- dont seem to spread perhaps due to high fitness cost of gene activation - Linezolid-resistant Staphylococcus aureus (LRSA)
- linezolid used to treat MRSA - Daptomycin-resistant Staphylococcus aureus (DRSA)
- daptomycin used to treat MRSA
S. aureus is a versatile opportunistic pathogen. Give examples of serious diseases associated w infection.
endocarditis - infection of heart valves
pneumonia - infection of the lungs
What are the core and accessory genomes in bacteria?
core - found in every strain of species
accessory - only found in certain strains eg adaptation to a specific niche
together these make up the pangenome
What are the S. aureus pathogenicity islands?
array of islands in accessory genome that encode for antibiotic resistnace and virulence factors - in the form of mobile genetic elements
What are MSCRAMMs (S. aureus)
microbial surface components recognising adhesive matrix molecules
can produce ~20
single peptide at n terminal recognised by export apparatus,then is exported outside of cell and the signal is cleaved
A domain - ligand binding domain, binds to host receptor
B repeats that act as stalk to extend protein away from cell surface so can interact w receptor
has an R domain too but idk what it does, made up of repeats
M domain - LPXTG motif that is a cell wall anchor is covalently bonded to peptidoglycan through action of sortase
Give an example of an MSCRAMM (S. aureus)
fibronectin-binding protein (FnBP)
binds to major fibronecin-integrin receptor (α5β1)
forms a cross bridge which induces uptake of bacterial cell into host cell
confers antibiotic resistance as not all can enter host cells + immune evasion
How does SEIX in S. aureus inhibit phagocytosis?
SEIX can bind to human neutrophils and disrupt the IgG-mediated phagocytosis
SEIX can interact w cytoskeletal components to disrupt processes reliant on rearrangement ie phagocytosis
What is an example of an extracellular toxin produced by S. aureus?
alpha toxin
forms pores in erythrocytes by forming a complex which inserts itself into the membrane, causing an unrestricted flow of ions out leading to cell lysis
shows a recognisable phenotype on blood plate
What makes community-associated MRSA so deadly?
enhanced virulence - production of toxins at high concentrations due to accessory gene regulator (agr)
SEIX - a super antigen
What are host specific leucocidins? (S. aureus)
encoded by mobile genetic elements
present on prophages of host cell genome
v effective at lysing cells
PVL - human specific leucocodins that cause high tissue damage and the lysis of neutrophils
What are superantigens (SAg)?
- most encoded on mobile genetic elements
- small thermostabel proteins
- conserved 2 domain structure: contain MHC II and TCR binding domains
- large number of sequence variants
- disulphide loop (enterotoxigenicity)
bypass normal antigen presentation - bring together MHC class II and TCR
non-specific
can lead to activation of overcan lead to activation of over 50% of all T cells → production of huge amount of cytokines → loss of T cell function and sepsis?
Give some background info about Streptococcus.
- most species colonise the oral cavity or nasopharynx as part of the commensal flora
- many can function as opportunistic pathogens and cause disease by exploting weaknesses in the immune system resulting from age, dampened immune system, or co-infection
- can be catergorised by heamolytic capabilities (alpha, beta, or non-haemolytic)
- can be classed by what M protein they possess
What is the most medically important species of Streptococcus?
S. pneumoniae
What genes are contained within the core genome?
factors involved in colonisation, virulence, metabolism, and niche adaptation
What genes are contained within the accessory genome?
strain specific virulence attributes and antimicrobial resistance
What is the region of difference 2? (M. tuberculosis)
region of difference 2 (RD2) is an integrative conjugative element containing factors associated w adhesion and biofilm formation
How do mosaic genes arise?
result from recombination between two species within the same host eg altered penicillin binding proteins
a good example of this is influenza
What is meant by bacterial competence, and why is it important in Streptococcus?
- natural genetic transformation; uptake of DNA by one bacterium from another
- to take up DNA, specific genes must be expressed to allow the bacterium to enter a physiological state termed competence
- Competent bacteria acquire DNA from non-competent siblings via fracticide
- observed in S. pneumoniae, S. mitis, and S. oralis
- for S. pneumoniae promotes colonisation fitness, and lung infectivity
- homologous DNA allows recombinational repair of DNA damaged by oxidative stress
How are biofilm formation and competence linked? (Streptococcus)
both regulated by competence stimulating peptide (CSP), a pheromone
Describe the function of the com genes in Streptococcus.
comA - encodes ABC transporter for export and maintenance
comC - encodes CSP
comD - encodes CSP receptor, a histidine kinase
comE - encodes regulator of 2 component system
deletion of com genes abolishes biofilm formation and reduces virulence in tissue, enhances virulence in blood
Streptococcus has high genetic plasticity. What does this mean?
Its capacity for transformation means HGT rates 3-10x greater than DNApol mutation rates
only 50% of known genes in entire genus are present in 1 strain
there is a large distribution of genetic material
Revision: What are the 3 methods of gene transfer?
- transformation
DNA is bound at cell surface by a DNA binding proteins which pulls the DNA into the periplasm of gram- or through thick wall f gram+
either whole double strand is taken up or one strand is degraded by a nuclease
a competence-specific protein binds the donor DNA which protects the DNA from further nuclease attack
DNA is then integrated into the genome via recombination
2a. generalised transduction
a bacteriophage transfers DNA from one cell to another
DNA from any portion of the host genome is packaged inside the mature virion in place of the virus genome forming a transductant
happens during lytic cycle of bacteriophages
the resulting transducting particle DNA cannot replicate inside another bacteria it may integrate into the genome
frequency and specificity is low
2b. specialised transduction
a bacteriophage transfers DNA from one cell to another
DNA from a specific region of the host genome chromosome is integrated directly into the virus genome
occurs when phage DNA has been inserted into host genome is replicated to produce new virions but genes adjacent to the prophage have been picked up as well
high frequency, efficiency, and specificity
- conjugation
plasmid encoded mechanism that involves transfer of plasmids and other mobilised genetic material via the pili
uses rolling circle replication for DNA synthesis instead of the usual normal bidirectional replication
one strand is transferred to the recipient bacteria
as this transfer occurs, the rolling circle mechanism replaces the transferred strand in the donor while a complementary strand is made in the recipient
What is the Mga Regulon? (Streptococcus)
regulated by Mga Global transcriptional regulator
includes:
- emm gene: encodes M protein, M-like proteins, fibronectin
- scpA -> C5a peptidase
- sic -> a secreted inhibitor of complement
- speB -> Ig degrading cysteine protease
- has operon -> capsule biosynthesis
- Mga itself
Two Component Systems are common regulators of virulence factors in pathogenic strep. How do they work?
many variants but this is a basic model eg RR phosphorylation independent of HK
- membrane sensor of histidine kinase (HK) auto-phosphorylates a histidine residue
- transfers a phosphate to an aspartate residue on the response regulator (RR)
- leading to a conformational change and allows RR~P to bind to DNA
eg CovRS/CsrRS TCS
involved in transcriptional changes associated w transition from pharynx to invasion
What factors do pathogenic Strep strains possess that influence pathogenesis?
- pilus aids binding and can inhibit host defence
- polysaccharide capsule inhibits phagocytosis eg s. pyogenes hyaluronate capsule similar to human connective tissue
- biofilm formation limits phagocytosis and antimicrobial activity
What else must be acquired when picking up genetic material from competing bacteriocin?
the gene for the bacteriocin as well as the associated immunity factor
must possess immunity factor against own bacteriocin
What are examples of significant pneumococcal surface proteins?
- IgA protease (IAP)
- polymeric Ig receptor inhibits phagocytosis and aids attachment
- neuraminidase A & B enhance colonisation and invasion
What is the function of a capsule?
allows bacteria to move through mucous as well as additional recognition from complement system, preventing opsonisation and subsequent phagocytosis
How is the capsule formed?
Synthesis: Capsular polysaccharides are synthesized from sugar nucleotide building blocks in the bacterial cytoplasm.
Translocation: Synthesized polysaccharides are transported across the cytoplasmic membrane into the periplasm.
Assembly and Export: In the periplasm, polysaccharides are assembled into chains and exported out of the cell.
Attachment: Once outside the cell, polysaccharide chains attach to the cell surface, forming the capsule layer.
Regulation: Capsule production is regulated by the bacterial cell, often involving two-component systems and environmental signals.
What are S. pneumoniae biofilms made out of?
- pneumococcal serine rich repeat protein (PsrP), a pathogenicity island encoded adhesin
- large and heavily glycosylated
What is pneumolysin?
a streptolysin produced by pneumoniae
- forms pores on cholesterol rich membranes on which a conformational change occurs
- induces apoptosis
What is the difference between colonising vs invasive pneumococcal strains?
colonising
- more resistant to PMN mediated non-opsonic uptake
- thicker capsule
invasive
- adhesins (CbpA, PspA, PrsP) more accessible so bind and translocate across epithelium
- more resistant to macrophage ingestion and opsonic clearance
