Streptococcus pneumoniae Lecture 33 PDF

Streptococcus pneumoniae - Respiratory Infections MIIM30011 Dr Stephanie Neville “The Pneumococcus” First discovered in 1881 by Louis Pasteur Later identified by Carl Friedländer & Albert Fränkel (1884) Strictly human-adapted No environmental reservoir Can reside asymptomatica...

Streptococcus pneumoniae - Respiratory Infections MIIM30011 Dr Stephanie Neville “The Pneumococcus” First discovered in 1881 by Louis Pasteur Later identified by Carl Friedländer & Albert Fränkel (1884) Strictly human-adapted No environmental reservoir Can reside asymptomatically in the upper respiratory tract Highly successful pathogen “Pneumococcal pneumonia as the most widespread and fatal of all acute diseases” The “pneumococcus as the Captain of the Men of Death” -Sir William Osler, 1918 Global burden of disease Estimated to cause ~1.6 million deaths annually One of the leading causes of mortality globally Annually in the US alone, >2 million infections, >150,000 hospitalisations, and ~6,000 deaths >USD$4 billion in costs (healthcare and loss of productivity) WHO-designated priority pathogen Leading cause of community acquired bacterial pneumonia (up to 50% of cases) Ever increasing rates of antibiotic resistance Brooks and Mias, 2018 https://doi.org/10.3389/fimmu.2018.01366 S. pneumoniae features Features: Gram-positive, encapsulated, Non-motile diplococcus Size: 0.5 – 1.5 µm Cell wall composed of: Peptidoglycan N-acetylglucosamine (GlcNac) N-acetylmuramic (MurNac) acid teichoic acid (TA) lipoteichoic acid (LTA) Most strains also encapsulated Capsule determines the serotype A human specialist Relatively small genome (2.0 – 2.1 Mbp) Little over 2000 genes Host-mediated genome decay Nutritionally fastidious Will scavenge nutrients from the host wherever possible Auxotrophic for numerous amino acids Scavenges catalase by lysing host RBCs (α-haemolysis) Multiple incomplete biosynthetic pathways e.g. Glutathione Can adopt different ‘lifestyles’ Carriage Disease Pneumococcal carriage Carried asymptomatically in the nasopharynx ~60% of children, ~5% adults Primary source of host-to-host transmission Carriage without dissemination often associated with specific serotypes Colonisation is necessary for disease Dissemination or invasion into deeper tissues Pneumococcal invasion Middle ear – otitis media Inflammation and pain in the middle ear Can result in perforated ear drums and deafness Acute, chronic or recurring Lungs – pneumonia Inflammation of the resp. tract and lung tissue Results in cough, shortness of breath, fluid accumulation in lungs Blood – bacteraemia Also called blood poisoning Results in systemic inflammatory response Invasive Pneumococcal Disease Pre-cursor to meningitis if not treated quickly (IPD) Brain – meningitis Severe infection of the CNS Mortality rates up to 37%, long-term or permanent morbidity in 30-50% of surviving patients Pneumococcal pneumonia Pneumonia is responsible for > 1M deaths annually (~7% of global deaths) Risk is greatest in the young (70 years) Both community acquired and hospital acquired CABP leading cause of sepsis Can be initiated by viral infection Requires antibiotic intervention How to be a successful pathogen a.k.a. Pneumococcal virulence factors 1) Polysaccharide capsule 2) Phase Variation 3) Natural competence 4) Toxins (Pneumolysin) 1) Pneumococcal capsule – what is it? Extracellular polysaccharide that coats the bacterial cell wall Made up of repeating sugar molecules (saccharides; often 2-8) of varying complexity Poorly immunogenic Currently ~100 different serotypes Type of sugars (i.e. glucose, galactose, glucuronic acid) Sugar modifications (o-acetyl, phosphoglycerol, pyruvyl acetal) Types of linkages Charge (anionic, zwitterionic, no charge) Solubility Serotype designation complicated by frequent recombination events at the capsule biosynthesis locus (cps) Calix et al., 2012 https://doi.org/10.1074/jbc.M112.380451 1) Pneumococcal capsule – what is it? Serotype 3 Serotype 11A Geno et al., 2015 https://doi.org/10.1128/CMR.00024-15 “From the point of view of the adaptive immune system, each serotype of S. pneumoniae represents a distinct organism” - Charles Janeway, Jr. Pneumococcal capsule – what does it do? Primary virulence determinant in S. pneumoniae Systemic Colonisation Invasion Dissemination Reduce mucociliary Prevents complement Facilitates survival in the clearance deposition on the cell blood Facilitate non-specific surface Allows escape from NETs interactions with host Prevents phagocytosis by Prevents opsonisation by cells macrophages antibodies Prevents host recognition of highly immunogenic surface receptors Amerighi et al., 2015 DOI: 10.1093/infdis/jiv461 Less capsule More capsule Pneumococcal capsule – impact on disease Different serotypes (generally) have different disease outcomes Anatomical niches Age of the patient (susceptibility and severity) Invasiveness Only 20-30 of the ~100 serotypes show significant invasiveness Patient-dependent (co-morbidities) Will be recognised differently by the immune system Pneumococcal capsule – impact on disease Serotype 1 Serotype 2 Serotype 19F Serotype 11A Highly invasive Highly invasive Associated with One of the least carriage and invasive serotypes Not often Progresses from detected in the NP lungs to brain in otitis media Encodes O- ~24 h (mice) Can cause acetyltransferase Zwitterionic (wcjE) – synthesises capsule – Less prevalent in severe and recurrent OM in the ligand for ficolin-2 associated with human disease (opsonin) abscess formation children Very well Results in phagocytosis High morbidity, Can become described in the low mortality In vivo mutation of hypervirulent literature through Very high rates wcjE – ficolin-2 can no inactivation of of antibiotic longer bind genes during resistance Evolution of new, infection (spxB) highly invasive serotype 11E  2) Phase Variation Random, high frequency, reversible switching of gene expression Alters global gene expression though differential methylation of the Seib et al., 2020. Annu. Rev. Microbiol. “Phasevarion” (phase variable regulon) Allows for highly variable expression of surface antigens & virulence factors (including the capsule!) Opaque Transparent Mediated by Restriction-Modification (R-M) systems (SpnD39III) hsdR – restriction endonuclease hsdM – DNA methyltransferase hsdS – specificity subunit Li et al., 2016. PLoS Pathog.  SpnD39III – Host evasion level: expert Highly variable expression of: Capsule Adhesins Invasion factors Immunogenic surface antigens -Increased adhesion -Increased invasion -Highly resistant to copper stress -Completely attenuated in vivo Modified from Seib et al., 2020. Annu. Rev. Microbiol. 3) Natural Competence (and fratricide) 1928 – Frederick Griffith discovers natural competence for transformation for the first time in S. pneumoniae. Unlike other mechanisms of DNA acquisition, the pneumococcus can take up dsDNA un- aided. Provides a rapid mechanism for genetic diversity Induced by a peptide pheromone – controlled by quorum sensing and the competence operon (com genes) When population numbers reach critical mass, cells are induced as either: Donors Recipients Recipients are primed to take up DNA, donors are instructed to autolyse (fratricide) Vecteezy Autolysis Autolysis (fratricide) mediated by competent cells (recipients) Hydrolases (bind to phosphorylchloine in the TA and LTA) Bacteriocins (antimicrobial peptides) Competent cells protected from lysis by immunity proteins Regulated by com genes Numerous benefits for the pneumococcus: Increases genetic diversity, allows recombination of genes and loci (cps locus) Promotes immune evasion and survival Facilitates release of toxins 4) Pneumolysin (Ply) Cholesterol-dependent cytolysins Binds to cholesterol on host cells, causing lysis Intracellular protein Release by pneumococcal lysis Many variants described Monomers (42) oligomerise to form a pore Marshall et al, 2015 https://doi.org/10.1038/srep13293 4) Pneumolysin (Ply) Van Pee et al., 2017 https://doi.org/10.7554/eLife.23644.001 Therapeutics – Vaccines Vaccine options: Pneumovax-23 (Merck) Prevenar 13 (Pfizer) -Polysaccharide vaccine (PPV) -Polysaccharide conjugate vaccine (PCV) -Contains 23 serotypes -Contains 13 serotypes -Directly immunises with CPS -Uses CPS conjugated to diphtheria toxin -Poorly immunogenic in children (Dtx) -Recommended for adults -Elicits strong immune response in infants -Serotypes covered (1, 2, 3, 4, 5, 6B, 7F, -Given at 2, 4 and 12 months 8, 9N, 9V, 10A, 11A, 12F, 14, 15B, 17F, 18C, 19F, 19A, 20, 22F, 23F, and 33F) -Serotypes covered (1, 3, 4, 5, 6A, 6B, 7F, 9V, 14, 18C, 19A, 19F, 23F) Vaccine escape The PCV 7 vaccine was highly effective in reducing incidence of IPD …from those 7 serotypes only Incidence of IPD from other serotypes was unchanged or increased PCV 10 offered increased protection (except against serotype 3) Serotype 3 sheds its capsule during invasive disease Carriage of vaccine serotypes reduced, but: Replaced with rarely seen serotypes (altered epidemiology) Incidence of NT strains dramatically increased, accounting for 15-18% of isolates Selective pressure leads to increased genetic diversity = emergence of new serotypes Serotype replacement Therapeutics – Antibiotics β-lactams (e.g. penicillin or amoxycillin) - mortality resulting from pneumococcal bacteraemia decreased from 80% in untreated patients to 17% among patients who received treatment Resistance reported up to 80%(!) in some countries Macrolides (e.g. azithromycin, clarithromycin or erythromycin), tetracyclines (doxycycline) and cephalosporins are also used Multi-drug resistance in up to 46% of isolates Both monotherapy and combination therapies are used in community and hospital settings Treatment failure occurs in up to 30% of cases due to antibiotic resistance So what can we do…? We need novel therapeutics! àVaccines àNon-serotype dependent àƔ-irradiated pneumococci, highly-conserved surface antigens, mRNA àNew antimicrobials àTarget highly conserved & constitutively expressed pathways/proteins àAcquisition of essential nutrients, fundamental cellular processes àAdjunct therapies àCombinations of therapeutics given simultaneously  Summary Streptococcus pneumoniae is a VERY good human pathogen Despite our best therapeutic efforts, still kills more people per year than AIDS, TB and malaria combined. This pathogenicity is possible due to specific evolutionary pressures exerted by the human host This may also be the Achilles' heel of the pneumococcus