Microbial Virulence & Pathogenesis 2024-25 Student Notes PDF

Microbial Virulence & Pathogenesis Dr. Ronni Joji Dept. of Microbiology, Immunology and Infectious diseases (2024-25)  Definitions Morphology of bacteria Study of bacteria (Microscopy) Virulence factors/ determinants Determinants of bacterial pathogene...

Microbial Virulence & Pathogenesis Dr. Ronni Joji Dept. of Microbiology, Immunology and Infectious diseases (2024-25)  Definitions Morphology of bacteria Study of bacteria (Microscopy) Virulence factors/ determinants Determinants of bacterial pathogenesis Virulence of virus and fungi  Definitions Pathogen: An organism capable of causing disease. Pathogenesis: includes initiation of the infectious process and the mechanism that lead to the development of signs and symptoms of the disease. Pathogenicity: The ability of a microorganism to cause a disease ( depends on virulence factors).  Virulence: The quantitative ability of a microorganism to cause disease. Toxigenicity: The ability of a microorganism to produce a toxin. Incubation period: starts with the time of infection and ends when the individual develops symptoms Carrier: a person who harbors the pathogen without suffering from any disease from it. Morphology of bacteria  Study of bacteria (Microscopy) A) Wet preparations: Examine motility, molds, ova/cysts B) Stained preparations: ü Simple stain ü Negative stain ü Differential stain Wet preparations o Gram stain o Ziehl Neelsen stain o Albert stain ü Special stain Stained preparations  Study of bacteria (Microscopy) A) Wet preparations/ wet mount: Stained preparations Gram stain Principle: Based on the cell wall composition (peptidoglycan) of the bacteria. Gram positive bacteria: thick peptidoglycan – retain the primary stain complex (purple). Gram negative bacteria: thin peptidoglycan – do not retain the primary stain complex – stained by counter stain.  IMPORTANCE OF GRAMS STAIN 1. Guide the clinician on initial choice of antibiotic when the results of culture and sensitivity is pending. 2. Contribute to selection of culture media especially when it’s a mixed flora. 3. Judge the specimen quality. Ex: sputum Negative stain: The background is stained leaving the bacteria contrastingly colourless. Ex: India ink stain 4- Special stains: - These techniques are used to stain certain bacterial species. e.g. Endospore stain, capsule stain. Endospore stain Structure of Bacteria Cell wall It is a complex, semi-rigid structure - According to the cell wall structure (peptidoglycan thickness), by GRAMS STAIN the bacterial cells are divided into two classes: a- Gram positive bacteria e.g. Staphylococcus aureus b- Gram negative bacteria e.g. Escherichia coli, Shigella flexneri, Salmonella species  Gram positive cell wall 1. Peptidoglycan 2. Teichoic acid 3. Lipoteichoic acid v Peptidoglycan (50-90% of the wall) Function - Gives rigid support, protects against osmotic pressure v Teichoic acid : polysaccharide containing glycerophosphate residue. Medical importance: Mediates attachment to host cells. Antigenic , induce septic shock. v Lipoteichoic acid: Teichoic acid associated with lipid.They cause release of TNF, IL-1 from macrophages. Function – antigenic, cytotoxic and adhesin  Gram negative cell wall 1. Peptidoglycan 2. Lipoprotein layer 3. Outer membrane  Gram negative cell wall 1. Peptidoglycan layer (PG) : thin (2 nm, 5-10%). 2. Lipoprotein layer : covalently joined to PG. - Stabilize the outer membrane 3. Outer membrane : contain outer membrane proteins (OMPs) and Lipopolysaccharides.  COMPARISON OF CELL WALL COMPONENT Gram positive Gram negative bacteria bacteria Peptidoglycan Thicker; Thinner; single multilayer layer Teichoic acids Present Absent Lipopolysaccharide Absent Present  Structure Function Peptidoglycan (Cell wall) Gives rigid support, protects against osmotic pressure Outer membrane of Gram-negative bacteria (Cell wall) Toxic component of endotoxin Capsule Protects against phagocytosis Two types: (1) mediates attachment to Pilus cell surfaces; (2) sex pilus mediates attachment of two bacteria during conjugation Flagellum Motility Plasmid Contains a variety of genes for antibiotic resistance and toxins  Virulence ?? Virulence factors help bacteria to Invade the host Cause disease Escape from host defenses The following are some of the types of virulence factors/ determinants: Adherence Factors/ adhesins Capsule Endotoxin Exotoxin Flagella Enzymes Adhesins: adherence to host cell 2 types of adhesins: u Fimbrial adhesins – Example : P-pili u Non-Fimbrial adhesins: Examples § Protein A Pili: hairlike filaments that extend from cell surface. Importance: 1. Mediate attachment (adherence) to host cell 2. Special kind of pilus called sex pilus forms attachment between the male (donor) and female bacteria during sex pilus conjugation (exchange of genetic material).  Capsule: 1. Negative charges on capsular polysaccharide repel the negatively charged cell membrane of neutrophil and prevent it from bacterial ingestion (phagocytosis). 2. Play a role in bacterial adherence to human tissues 3. Used in certain vaccines 4. Specific identification of organism can be made by using antiserum.  Flagella: whip like appendages that move the bacteria (locomotion) towards nutrients and other attractants. Importance: Chemotaxis : movement of an organism in response to a chemical stimulus Adhesion Invasion of host TOXINS: produced by bacteria to damage the host cell. 2 types: Exotoxins Endotoxins u Exotoxins are enzymes which are produced by the live bacterial cell and excreted into the environment. u Endotoxins are lipopolysaccharides “Cell wall” components. Endotoxin: u Lipopolysaccharides in nature u Integral part of Gram-negative cell wall u Released during bacterial lysis or disintegration of bacterial cell wall u Toxicity depends on lipid A u Heat stable  Mode of action of Bacterial Endotoxin Mode of action of endotoxin. Endotoxin is the most important cause of septic shock, which is characterized primarily by fever, hypotension, and disseminated intravascular coagulation (DIC). Endotoxin causes these effects by activating three critical processes: (1) activating macrophages to produce interleukin-1 (IL-1), tumor necrosis factor (TNF), and nitric oxide; (2) activating the alternative pathway of complement; and (3) activating tissue factor, an early component of the coagulation cascade. (DIC: Disseminated intravascular coagulation) Citation: Chapter 7 Pathogenesis, Chin-Hong P, Joyce EA, Karandikar M, Matloubian M, Rubio L, Schwartz BS, Levinson W. Levinson's Review of Medical Microbiology & Immunology, A Guide to Clinical Infectious Diseases, 18th Edition; 2024. Available at: https://accessmedicine.mhmedical.com/ViewLarge.aspx?figid=284618117&gbosContainerID=0&gbosid=0&groupID=0&sectionId=0&multimediaId=undefined Accessed: October 16, 2024 Copyright © 2024 McGraw-Hill Education. All rights reserved  Exotoxin : Proteins Heat labile Can be converted into toxoids* The mode of action are: Pore formation, Protein synthesis inhibition, Effects on nerve muscle transmission, etc Major categories of exotoxins include cytotoxins, neurotoxins, and enterotoxins *A toxoid is an inactivated toxin whose toxicity has been suppressed while immunogenicity is maintained. BOTULISM caused by Clostridium botulinum (botulinum toxin) Flaccid paralysis/ Muscle paralysis EXOTOXIN: botulinum toxin (neurotoxin) Property Exotoxin Endotoxin Source Certain species of Gram positive Cell wall of Gram-negative and Gram-negative bacteria bacteria Secreted from cell Yes No Chemistry Protein Lipopolysaccharide Location of genes Plasmid or bacteriophage Bacterial chromosome Antigenicity Induces high titer antibodies Poorly antigenic Vaccines Toxoids used as vaccines No toxoids formed and no vaccines available Typical diseases Tetanus, botulism, diphtheria Meningococcemia, sepsis A toxoid is an inactivated toxin whose toxicity has been suppressed while immunogenicity is maintained. ØEnzymes: Bacterial enzymes: promote the spread of the pathogen. Example: u Collagenase: degrade collagen, thus allowing spread through subcutaneous tissue.  Horizontal gene transfer Primary mechanisms for exchange of genetic information between bacteria include Determinants of bacterial pathogenesis: 1) Exposure - Transmission 2) Adhesion 3) Invasion, Inflammation and intracellular survival 4) Toxin production 5) Immunopathogenesis 1. Transmission: important because interrupting the chain of transmission can prevent infectious diseases. Mode of transmission – a. Human to human – direct contact, blood, transplacental.. b. Nonhuman to human – soil, water, animals, fomites.. Four important portals of entry: u Respiratory tract u Gastrointestinal tract u Genital tract u Skin (breached skin) 2. Adherence to host cell surfaces: by adhesins. Ex: pili, capsules etc.. 3. Invasion, inflammation and intracellular survival: Invasion by u Enzymes Invasion of host cells by bacteria is dependent on interaction of specific bacterial proteins and specific host cellular receptors. Intracellular survival : enhances ability to cause disease. These bacteria are called intracellular pathogens. (Mycobacterium, legionella, listeria) 4. Toxin production: Endotoxins Exotoxins 5. Immunopathogenesis : symptoms of disease is not caused by the organism but due to the immune response to the presence of the organism. Ex: Rheumatic fever A biofilm is an aggregate of interactive bacteria attached to a solid surface or to each other and encased in an exopolysaccharide matrix. Biofilms form a slimy coat on solid surfaces (prosthetic joints, intravenous catheters, prosthetic heart valves) Biofilm protects bacteria from antibiotics and host immune defenses.  Shigella species u Causes bacillary dysentery (Shigellosis) u Low infectious dose (10 -100 organisms) u Transmission: fecal – oral route u Principal factors in transmission – fingers, flies, food, feces. u Virulence factors: endotoxin, shiga toxin, intestinal adherence factor u Critical factor in pathogenesis – invasion (bloody diarrhea invading cells of ileum and colon)  Laboratory diagnosis: Specimen : stool u Stool routine/ microscopy – WBCs, RBCs u Stool culture - Mac Conkeys agar – non lactose fermenting colonies u Gram stain from colonies – Gram negative bacilli u Biochemical tests u Agglutination test – confirmation of the organism Gram negative bacilli Mac Conkey Agar Lactose non fermenter Gram stain reaction Virus Virulence Virulence of Virus Adhesion to host cell: Surface proteins Cell entry mechanisms: Endocytosis/ fusion of envelope with host cell membrane Immune evasion: inhibit presentation of viral antigens Modulation of host cellular machinery: manipulate host cellular processes to favor their own replication Immunomodulatory proteins: Inhibit interferon production, disrupt antiviral signaling pathways Fungus Virulence of Fungi Virulence factors Functions Cell wall glycoproteins Adhesion to host cell Capsule Antiphagocytic Exoenzymes Tissue invasion Toxins Tissue necrosis Thank you