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Published byMarjorie Richards Modified over 8 years ago
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Reservoirs and vectors Reservoirs Animal, soil, water etc - source of infection. Vectors Arthropods, especially fleas, ticks, and mosquitoes Mechanical vectors Insects carry pathogens on feet Biological vectors Pathogen replicates in vector
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Microbes: Portals of entry 1)Skin: Most microbes are not able to penetrate intact skin Some fungi infect skin 2) Mucous membranes Respiratory tract: eg. Influenza virus Gastrointerstinal tract: eg. Cholera Urogenital tract: HIV Eye (conjunctiva): eg. adenovirus
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Microbes: Portals of entry 3) Parenteral (Blood-borne) route: Microbe is deposited below the skin into the tissue Eg. Blood transfusion, insect bite, tatooing (eg. HIV) 4) Vertical transmission: Transmission from mother to child (eg. HIV)
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What are bacteria ?
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Bacterial Structure Chromosome: DNA material of the cell Cell wall: Composed of peptidoglycan the cell wall maintains the overall shape of a bacterial cell Cell membrane: Lining the inside of the cell wall it provides a boundary for the contents of the cell and a barrier to substances entering and leaving. Cytoplasm: Describes the inside of the cell and the contents
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Bacterial Shapes There are 3 different shapes of bacteria Spirals (Campylobacter) Rods (Lactobacillus) Balls or cocci (Staphylococcus) Bacteria with flagella are motile
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Gram stain Diff. in cell wall 1884: Hans Christian Gram; he developed this method to better visualize bacteria All bacteria may be classified as Gram positive and Gram negative
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Classification of bacteria Shape Motility Gram stain Other property (eg.aerobic /ananerobic)
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Mechanisms of bacterial pathogenicity
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Portals of entry Penetration or evasion of host defenses Damage to host cells Portal of exit Number of invading microbes Adherence Virulence factors Microbial mechanism of pathogenicity
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(1) Adherence Adherence (attachment) is often an essential step in bacterial pathogenesis or infection, required for colonizing a new host – Adhesion - Process by which microorganisms attach themselves to cells. Requires the participation of two factors: a receptor and an ligand – Microbial adherence to a eukaryotic cell or tissue surface involves complementary chemical interactions between the host cell or tissue surface and the bacterial surface.
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(1) Adherence Adhesions/ligands bind to receptors on host cells – Fimbriae/ pili Escherichia coli Neisseria gonorrhoeae – M protein Streptococcuspyogenes – Glycocalyx Streptococcus mutans
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Adherence examples M protein: Group A Streptococcus Pili /fimbriae: E.Coli / N. gonorrhoeae
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Why is adherence important ? Ability to colonize and cause disease Ability to adhere determines the host specificity Potential drug target
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(2)Formation of bacterial biofilms Quorum sensing: ability to sense population density and alter gene expression
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Examples of biofilms Dental plaques
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Biofilms outside the body Biofilm mats on stagnant water Biofilm mats on rocks – Yellowstone national park
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What triggers biofilm formation ? Bacterial attachment to surfaces Nutritional depletion Sub-inhibitory concentrations of antibiotics A critical population density of bacteria
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How do biofilms help bacteria ? Protection from immune repsonse Protection from antibiotics Help tide over periods of low nutrition Microbial chatter (communication)
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Biofilms: Why do they matter ? Key mechanism in bacterial pathogenesis Important part of food chain Major cause of corrosion of metal pipes
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(3) Bacterial capsule Some bacteria have a polysaccharide layer outside the cellwall called capsule Helps in attachment Helps evade host defences Streptococcus pneumoniae Haemophilus influenzae Bacillus anthracis
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How does the capsule help bacteria evade host defences ? Prevent complement binding and phagocytosis Escape TLR recognitionAllows survival inside phagocytosis Capsulated bacteria
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Capsule inhibits phagocytosis
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Capsule allows survival inside phagocytes Capsule may help resist digestion by lysosomal emzymes
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Capsules allow escape of TLR recognition (TLR) No immune response
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Capsules contribute to pathogenesis
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(4) Bacterial cell wall Outer layer Peptidoglycan (sugars and aminoacids) The cell wall may carry some surface proteins
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Mycolic acid in cellwall helps resists lysosomal digestion Mycolic acid in the cell wall resists digestion by lysosomal enzymes Mycolic acid in the cell wall Eg. Mycobacterium tuberculosis, Mycobacterium leprae
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Proteins on the cell wall help evade phagocytosis Phagocytosis Immune response X Disease X Phagocytosis X Immune response Disease
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(5) Bacterial enzymes (a)Coagulase and kinase (b)Hyaluronidase and collagenase (c)IgA protease
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(a) Coagulase and kinase Inhibits phagocytosis Immune response – no access Antibiotics not effective Eg. Staphylococcus aureus – skin infections Strains that do not produce coagulase are not pathogenic
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(b) Hyaluronidase and collagenase Digestion of connective tissues and Invasion of tissues Hyaluronidase: is present in Staphylococcus aureus (Skin infections) and Streptococcus pyogenes (Sore throat) Collagenase: is present in Clostridium perfringens (gas gangrene)
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(c) IgA protease An enzyme that can degrade IgA antibodies Eg. Haemophilus influenza – causes respiratory tract infection / meningitis Help negotiate mucosal defenses.
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(6) Membrane ruffling Invasins : proteins expressed on the cell surface of various pathogens that alter actin filaments of host cell cytoskeleton, allowing microbes to enter cells.
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