Week 11 2023-2024 - Microbial mechanisms of pathogenicity - Section 3 - QM+.pptx

MD117 – The Microbial World & Huma Microbial Mechanisms of Pathogenicity Section 3 Dr. Olivier Marchès Learning objectives: section 3 • Describe how pathogens damage the host • Compare and contrast exotoxins and endotoxins • Define the cytopathic effect of viruses • Explain the mechanism of acti...

MD117 – The Microbial World & Huma Microbial Mechanisms of Pathogenicity Section 3 Dr. Olivier Marchès Learning objectives: section 3 • Describe how pathogens damage the host • Compare and contrast exotoxins and endotoxins • Define the cytopathic effect of viruses • Explain the mechanism of action of different types of toxin How pathogens damage the host • Direct damage • bacterial growth in tissues: invasion, host cells nutrients • intracellular growth can cause host cells to lyse, spreading pathogen to new cells • Toxins • primary mechanism of host damage • Around 250 toxins known: produce fever, cardiovascular problems, diarrhea, shock, death • capacity of organisms to produce toxins is termed toxigenicity • Exotoxins and Endotoxins Produced inside cell and transported outside Part of the cell wall of Gram-negative bacteria Exotoxins • Proteins; many are enzymes with specific activity, can destroy parts of host’s cells or inhibit essential functions • Typically active in very low amounts – 1 mg of botulinum toxin could kill 1 million guinea pigs • LD50: Lethal dose (of a toxin) for 50% of the test population • Soluble in body fluids so can travel around body in blood • Toxin often responsible for disease symptoms, not bacterium itself. Ingestion of toxin can be sufficient eg. botulism, staphylococcal food poisoning - intoxications Host response to exotoxins • Body produces antitoxins - Antibodies • Exotoxins can be heat-inactivated but still provoke body into producing antitoxins • Inactive exotoxins are termed toxoids and can be used as vaccines, e.g. Diphteria and tetanus vaccines Exotoxin nomenclature • Exotoxins are named in various ways type of host cell target • neurotoxins – nerve cells • enterotoxins - cells lining gut • cardiotoxins – heart cells disease caused • diphtheria toxin • tetanus toxin bacterium producing toxin • botulinum toxin – Clostridium botulinum • Cholera toxin – Vibrio cholerae Types of exotoxins by mode of action • Three main types of exotoxins: • A-B toxins • e.g. diphtheria toxin • Membrane-disrupting toxins • Clostridium perfringens toxin • Superantigens • Staphylococcal toxic shock toxin (TSST-1) A-B toxins: A toxic part – B binding part A-B toxin example: Shiga toxin (EHEC) AB5 subunit structure • B binding part binds host cell glycolipid in large intestine (Gb3) and on kidney • A domain internalized via receptormediated endocytosis: causes irreversible inactivation of the 28S ribosomal RNA subunit Shiga toxin blocks protein synthesis mples of disease caused by A-B exoto  Diphtheria – caused by Corynebacterium diphtheriae – Toxin inhibits protein synthesis in heart muscle & other cells  Tetanus – caused by Clostridium tetani – Toxin affects neuromuscular junctions → acts at the level of CNS inhibitory motor neurones→ blocks release of inhibitory neurotransmitters (GABA)→ irreversible contraction of muscle and spastic paralysis  Botulism – caused by Clostridium botulinum – Toxin affects neuromuscular junctions → prevents release of excitatory neurotransmitter in peripheral nerves (acetyl choline) → lack of stimulus to muscle Membrane-disrupting toxins • Can damage membrane by enzymatic action e.g. phospholipase activity of C. perfringens a-toxin Clostridium perfringens : Gas gangren • Can form pores in host cell membranes, e.g. Streptococcus pneumoniae pneumolysin Pore forming toxins (PFT) • Damage and kill host cells particularly phagocytes Superantigens • Provoke intense immune response • proliferation of T cells • T cells release excessive amounts of cytokines which damage host • fever, nausea, vomiting, diarrhoea, shock • Examples of Superantigens: Staphylococcus aureus enterotoxin Toxic shock syndrome – caused by staphylococcal toxic shock toxin (TSST-1) Endotoxins • Gram-negative bacteria have an additional structure in their cell wall, compared to Gram-positives – the outer membrane • Outer membrane is composed of lipoproteins, phospholipids and lipopolysaccharides (LPS) • The lipid portion of of LPS, lipid A, is the endotoxin Lipopolysaccharide is composed of lipid A, a core glycolipid, and an O-specific polysaccharide side chain. Lipid A is the toxic component that promotes inflammation and fever. Endotoxins and the pyrogenic response Endotoxins: Endotoxic shock • Endotoxins are released when bacteria multiply or die • Antibiotic treatment can cause endotoxin release • In high quantities and if release in blood, Endotoxin stimulates macrophages to release cytokines at high levels • Causes chills, fever, weakness, muscle ache, shock. Can be fatal • Cause endotoxic shock Comparison of exotoxins and endotoxins Exotoxins Endotoxins Chemistry Proteins Lipid portion of LPS Action Specific General Toxicity High Generally low Vaccine potential Can be converted to toxoid for vaccine Not neutralised by antitoxin Pathogenicity of viruses • Obligate parasites – have to replicate within host cells • Pathogenic properties • gain access to host • evade host defences • cause damage to host and kill host cell when replicating Viral mechanisms for evading host defence • Have attachment sites for receptors on host cells • Virus can bind and penetrate cell • Sometimes use mimicry, e.g. rabies virus attachment site resembles neurotransmitter acetylcholine. Virus can enter cell along with neurotransmitter • HIV hides its attachment sites from immune system and actively attacks immune system components HIV binds to CD4 positive cells, mainly T cells. CD4 proteins are long and bind to molecules on HIV surface. Antibodies cannot gain full access to these molecules HIV Cytopathic effects of viruses • Effects of virus infection on appearance of host cells • Include • cell lysis • formation of inclusion bodies – viral particles under assembly (use for diagnosis: eg. rabies, measles, smallpox) • syncitium formation – fusion of adjacent cells (eg. measles, common cold) • Determination of cytopathic effect in laboratory cell lines is used in diagnosis of viral infections Cytopathic Effects of Viruses Cytoplasmic inclusion body in brain tissue from a person who died of rabies Inclusion bodies are found in the cytoplasm or nucleus of some infected cells. Portion of syncitium (giant cell) formed in infected cells by measles virus. SARS-CoV-2-induced lymphocyte loss via syncytia-mediated cell-in-cell formation From: Nature volume 594, pages 88–93 (2021) and Cell Death & Differentiation volume 28, pages Portals of exit • Respiratory tract • Coughing and sneezing: respiratory diseases microbes • Gastrointestinal tract • Feces and saliva: Salmonellosis, Cholera • Genitourinary tract • Urine and vaginal secretions: STMs, Typhoid fever • Skin • Squames shed into atmosphere: impetigo, Herpes • Blood • Biting arthropods and needles or syringes: Malaria, HIV How Microbes Cause Disease Essential reading Tortora GJ, et al. 2015. Microbiology; an introduction. Pearson education ltd. Chapter 15.