Lecture 34: Pathogenicity Factors (2024) - Tobi PDF

Vaccination wrap-up  Placebo-controlled clinical trials x 1000 x 1000 x 1000 x 1000 Wait for infection + vaccine + placebo (saline) + vaccine + placebo...

Vaccination wrap-up  Placebo-controlled clinical trials x 1000 x 1000 x 1000 x 1000 Wait for infection + vaccine + placebo (saline) + vaccine + placebo (saline) Efficacy = number of infected people without vaccine vs. number of infected people who received the vaccine (Moderna: 15 k people each received vaccine vs. vehicle: 185 people in saline group developed Covid-19, 11 in the vaccine group) = % unvaccinated infected - % vaccinated infected / % unvaccinated infected Herd Immunity Partial immunization of a population can protect the whole population including those who can’t be vaccinated http://www.vaccines.gov/basics/protection/ Basic Reproduction Number (R0) and Herd Immunity Thresholds Disease Ro Herd Immunity Diphtherias 6-7 85% Measlesa 12-18 83-94% Mumpsa 4-7 75-86% Pertussisa 12-17 92-94% Poliof 5-7 80-86% Rubellaa 6-7 83-85% Smallpoxc 5-7 80-85% Mode of transmission of the disease-causing agent: s - saliva; a - airborne; f - fecal-oral; c - casual social contact Ro – the expected number of people infected by a single person with disease  Summary Vaccines exploit the memory function of B- cells and T-cells Generally, antigens from a pathogen are used to prime the immune system for subsequent exposure The antigen can be derived from a whole cell, but can also be a pathogen fragment (e.g. spike protein of SARS-CoV-2 34 – Pathogenicity and Virulence Factors  Parasite or Pathogen or Predator? Pathogen –disease-causing organism, sometimes a toxin is involved. (pathos (“suffering”) + genesis (“creation”) Pathogenicity - the ability of an organism to cause disease (tissue damage or injury that impairs host activity) Parasite - long term relationship, the most successful parasites don’t kill the host. Term usually used in reference to eukaryotes (tapeworms, Plasmodium, Giardia). Usually no toxin involved. Predator – brief interaction leading to the rapid death of prey. (e.g. Myxococcus wolf pack attack on other bacteria) Opportunistic pathogen – causes disease when there is a breach in normal host barriers (e.g. Pseudomonas aeruginosa) Obligate pathogen – requires a host to live  Virulence A measure of an organism’s pathogenicity LD50 “lethal dose 50%” – number of cells needed to kill 50% of the infected test population ID50 “infectious dose 50%” – number of pathogen cells needed to colonize 50% of test subjects (e.g. Vibrio cholerae) 50 5000  109 V. cholerae cells in water 10 E. coli cells on lettuce.  Where do pathogens come from? Food/water: Pathogenic E. coli, Salmonella, Listeria, Shigella, Vibrio cholerae Inhalation: Legionella, Klebsiella etc. Wounds: Staphylococcus, Clostridium perfringens  Zoonotic diseases – transfer between animals and humans - Salmonella can be acquired by coming in contact with raw chicken meat, but also by hugging chickens! - Pasteurella, Campylobacter, Brucella…can be acquired from dogs https://www.cdc.gov/media/dpk/food-safety/live-poultry-salmonella/live-poultry-salmonella.html Copyright by Facundo Torres To cause disease, a typical pathogen… - Attaches to host cells - overcomes a barrier (cuts/ingestion/inhalation/immunity) - Finds food/multiplies (often means killing host cells) - Disseminates (e.g., diarrhea)  Virulence Factors - Adherence Adherence this is the enhanced ability of a microorganism to stick to a body surface. Adherence is mediated by: – Cell secretions (glycocalyx, biofilm EPS, capsules and slime layers - also help hide from phagocytic cells) – External structures (pili, fimbriae, flagella, adhesins or other surface proteins that assist in binding) review previous lectures  Generalized vs. specialized virulence factors: Adhesins Intimin is a surface protein General adhesion to surfaces made by some pathogenic E. is promoted by bacterial coli that enables adhesion to surface structures called pili the gut epithelium or fimbriae. However, some bacteria have specialized adhesion factors to allow them to stick to host cells. Gut epithelium Other example: Influenza virus - hemagglutinin  Capsule as a virulence factor From W.H. Taylor and E. Juni, “Pathways for Biosynthesis of a Bacterial Capsular Polysaccharide,” Journal of Bacteriology (May 1961) Acinetobacter spp (shown above), Haemophilus influenzae, Klebsiella pneumoniae, Streptococci…all make capsules. Pathogenic E. coli Vibrio cholerae  Virulence Factors - Invasion & Infection Invasion – entering cells Infection – a foreign or tissues of the host, microbe is growing in or allowing the microbe to on the body but not spread necessarily causing disease (yet!)  Virulence Factors - Invasion/Infection Enhance the ability of a microbe to invade tissues and establish an infection Enzymes that damage tissue structure – hyaluronidase, collagenases, other proteases, nucleases and lipases Coagulase (S. aureus) deposits host fibrin on S. a. to form a clot around S. a. which allows it to hide from immune surveillance  Virulence Factors - Toxins Toxicity is the ability of an organism to cause disease by the release of toxins that either immediately destroy cells or inhibit their normal functions, toxins often travel and affect sites in the body that are distant from the infection site. Exotoxins – proteins secreted by live pathogens Three major kinds of exotoxin: – Cytotoxins (e.g. Haemolysin) – AB toxins (e.g. cholera toxin, Shiga toxin) – enzymes (proteases/lipases) Cytotoxins Kills host cells e.g. alpha toxin produced by Staphylococcus. The toxin inserts into host cell membrane and forms of hole. Hemolysis often tested on blood agar plates  Pore forming cytotoxins release nutrients and promote host tissue penetration - Hemolysins - Some RTX-type toxins - leukocidins Hemolysins facilitate iron acquisition (e.g., streptolysin is genetically under control of an iron starvation response)  A-B(5) toxins A (alpha) and B (beta) proteins Beta binds cell and promotes entry Alpha is cleaved and enters the cell to subvert cell functions: e.g., protein synthesis, signal transduction, vesicle trafficking, production of messengers like cAMP (cholera toxin shown here) Adenylate cyclase  Nucleotide transfer is a common feature of bacterial toxins NAD ADP ribosylation is a posttranslational modification of host proteins. Examples: target protein - Pertussis toxin - Cholera toxin - Heat labile toxin of enterotoxigenic E. coli (ETEC) - Typhoid toxin? VopS of Vibrio parahaemolyticus AMPylates host proteins  Proteases and lipases in pathogenesis Many pathogens produce enzymes that digest host macromolecules like lipids, proteins, DNA in order to spread (tissue destruction) and/or acquire food (release of phosphate, amino acids, nucleotides…) Look away now if you are squeamish! Gangrene is massive tissue destruction that can be caused by bacterial proteases, collagenases, lipases (alpha toxin of Clostridium perfringens is a phospholipase) BoTox (botulinum toxin) is a Clostridial protease that degrades SNARE proteins Tetanus toxin is a Clostridial protease important for that cleaves a SNARE, synaptobrevin. neurotransmitter release in However, Tetanus toxin affects nerve cells. Without SNARE, inhibitory neurotransmitter of motor no neurotransmission à neurons and inhibition of its release paralysis thus causes muscle spasms. Clostridium botulinum Clostridium tetani Food-borne pathogen wound pathogen BoTox  Endotoxin Lipid A and LPS: released from dead bacterial cells (WHICH KIND?) – stimulates strong immune response  Food poisoning through bacterial toxins* Cereulide is a potent cytotoxin that attacks e.g. mitochondria (K+ ionophore, collapses membrane potential) *disclaimer: This is a very very very rare toxin. Most food poisoning is caused by other bacteria (Salmonella, E. coli)  Toxins overview – memorize this Endotoxin (LPS) Exotoxins Proteases, Lipases Botox, Tetanus toxin, Clostridial alpha toxin (Clostridium spp.) Cytotoxins A-B toxins (pore-forming toxins, destroy (modify the activity of host host cells for cell tissue proteins, after being taken up by penetration and to liberate the host cell) nutrients) Examples: Examples: - Cholera toxin (Vibrio cholerae) - Leucocidins (Staphylococcus - Shiga toxin (Enterohemorrhagic aureus) E. coli) - Haemolysins (many bacteria)  Discuss with your neighbor: Which of the toxins we just talked about is useless against cells that are defective in endocytosis (uptake of external molecules)?  Secretion systems in proteobacteria Tat – twin arginine transport system: export of proteins that have folded in the cytoplasm Type I - VI secretion systems: important for interactions with other cells (i.e. Pathogenicity) Also note OMV – outer membrane vesicle formation FEMS Microbiol Rev (2010) 34:107-133  8. Type III Secretion (T3SS) Used to deliver effector proteins from pathogen cytoplasm into the cytoplasm of a host cell, no need for special receptor or mechanism to get proteins into e.g., plant pathogen Pseudomonas syringae; host cell, also no dilution human pathogenic E. coli  Secretion systems and host cell reprogramming Hausner J et al, IAI, 2016 Effector proteins fulfil diverse functions, e.g. reorganization of target cell cytoskeleton, dampening the immune response, inhibiting phagocytosis etc. The number of effector proteins varies between species: - Pathogenic E. coli (Type III) ~ 25 – 50 - Legionella pneumophila (Type IV) ~ 330 - Vibrio parahaemolyticus (Type III) ~10 - Bordetella spp. (Type III/IV)~ 2  Summary When exposed to potential pathogens, the invader must first interact with our physical, chemical and microbiota barriers. One class of pathogenicity factors (PFs) enhances the ability of a pathogen to attach to human cells and a second class helps degrade physical barriers to invade cells and tissues. A third class of PFs includes toxins. Some cause host cell lysis, others alter normal cell function either locally or at a distance.