Bacterial Pathogenesis (PDF)

Assoc. Prof. Emrah Ruh Near East University Faculty of Medicine Department of Medical Microbiology and Clinical Microbiology ’ Pathogenesis of bacterial infection: ’ Initiation of the infectious process ’ Mechanisms that lead to the development of...

Assoc. Prof. Emrah Ruh Near East University Faculty of Medicine Department of Medical Microbiology and Clinical Microbiology ’ Pathogenesis of bacterial infection: ’ Initiation of the infectious process ’ Mechanisms that lead to the development of signs & symptoms of disease Characteristics of pathogenic bacteria: ’ Transmissibility ’ Adherence to host cells ’ Invasion of host cells and tissues ’ Toxigenicity ’ Ability to evade the host’s immune system ’ Many bacterial infections are asymptomatic! ’ Disease occurs if the bacteria or immune reactions cause sufficient harm to the person  Glossary Adherence (adhesion, attachment): ’ The process by which bacteria stick to the surfaces of host cells ’ Once bacteria enter the body, adherence is the initial step in the infection process  Glossary Infection: ’ Multiplication of an infectious agent within the body ’ Even the person is asymptomatic! Infectious disease: ’ Clinically evident illness (characteristic medical signs and/or symptoms of disease) resulting from infection  Glossary Invasion: ’ The process whereby microorganisms enter host cells or tissues and spread in the body  Glossary Nonpathogen: ’ A microorganism that does not cause disease ’ May be part of the normal flora Opportunistic pathogen: ’ A microorganism which is capable of causing disease only when the host’s immune system is impaired Pathogen: ’ A microorganism which is capable of causing disease  Glossary Pathogenicity: ’ The ability of an infectious agent to cause disease Toxigenicity: ’ The ability of a microorganism to produce a toxin Virulence: ’ The quantitative ability of an agent to cause disease ’ Involves adherence, invasion and toxigenicity ’ Differentiates the pathogenic strain of a species The transmission Portals of entry of pathogenic bacteria into the body ’ Respiratory tract (upper and lower airways) ’ Gastrointestinal tract (primarily mouth) ’ Genital and urinary tracts ’ Capillary ’ Skin ’ Eyes ’ Ears The transmission Routes of entry of pathogenic bacteria into the body ’ Ingestion ’ Inhalation ’ Trauma ’ Needlestick ’ Arthropod bite ’ Sexual transmission  Some of the natural defense mechanisms and barriers: ’ Mucus and ciliated epithelium (upper respiratory tract) ’ Antibacterial secretions (lysozyme,...) (tears and mucus) ’ Acid and bile (gastrointestinal tract) Bacteria can pass through the barriers: ’ The barriers are sometimes broken ’ Cuts, burns and other injuries in the skin; a tumour or ulcer in the bowel,… ’ Bacteria may have the means to compromise the barrier and invade the body ’ Outer membrane of Gram-negative bacteria: ’ Resistance to lysozyme, acid and bile ’ Enterobacteriaceae can colonize the gastrointestinal tract  The infectious process ’ In the body, bacteria attach or adhere to host cells, usually epithelial cells ’ After bacteria establish a primary site of infection, they multiply and spread; ’ Directly through tissues ’ Or via the lymphatic system to the bloodstream  The infectious process ’ Bacteremia allows bacteria; ’ To spread widely in the body ’ To reach tissues for multiplication Bacterial pathogenicity and genetics: ’ Exchange of genetic information between bacteria: ’ Natural transformation ’ Plasmids, transposons, and bacteriophages ’ Exchange of genetic information between bacteria can result in transfer of virulence factors Bacterial pathogenicity and genetics: ’ Plasmids: ’ Extrachromosomal pieces of DNA ’ Capable of replicating ’ Transposons: ’ Highly mobile segments of DNA ’ Recombination between extrachromosomal DNA and the chromosome ’ Bacteriophages (phages): ’ DNA is transferred from phage to the bacterium Bacterial pathogenicity and genetics: Bacterial pathogenicity and genetics: Bacterial pathogenicity and genetics: Pathogenicity Islands ’ Large group of genes that are associated with pathogenicity ’ Located on bacterial chromosome Bacterial virulence factors Colonization Escape from host immune response Producing damage in the host Bacterial virulence factors Colonization Adherence Invasion Pilus Adhesin Capsule Biofilm  Bacterial virulence factors Escape from host immune response Escape from innate Escape from adaptive immunity immunity Capsule Protection Antigenic shift against Escape from neutrophils Antigenic mimicry phagosome Inhibition of Binding to Fc region Inhibition of MAC of antibodies complement activation Triggering Release of macrophage immunoglobulin apoptosis proteases Bacterial virulence factors Producing damage in the host Endotoxin Exotoxin Type I (pyrogenic) exotoxins: Superantigens Type II toxins: Membrane damaging toxins Type III toxins: A-B toxins Bacterial virulence factors Colonization Adherence Invasion Pilus Adhesin Capsule Biofilm Bacterial virulence factors Colonization > Adherence > Pili ’ Enable attachment to host receptor  Bacterial virulence factors Colonization > Adherence > Pili ’ Adhesive part is compatible with glycoprotein and glycolipid receptors of the host cell ’ Once bacteria adhere to the host cell, pili depolymerize and enable bacteria to attach Bacterial virulence factors Colonization > Adherence > Adhesins ’ Enable attachment to host receptor  Bacterial virulence factors Colonization > Adherence > Capsule ’ Some bacteria possess polysaccharide capsule (glycocalyx) and form biofilms on host cell ’ Example: Bacterium that causes tooth decays ’ Glycosyltransferase converts glucose (found on tooth surface) to dextran (component of glycocalyx) ’ Bacterium adheres tightly to tooth enamel by its glycocalyx ’ Bacterial cells of the same or different species become entrapped in the glycocalyx and form plaque on the tooth surface ’ Acidic products excreted by these bacteria cause dental caries Bacterial virulence factors Colonization > Adherence > Capsule ’ Formation of dental caries  Bacterial virulence factors Colonization > Adherence > Biofilm ’ An aggregate of interactive bacteria attached to a solid surface or to each other and encased in an exopolysaccharide matrix ’ Form a slimy coat on solid surfaces and occur throughout nature ’ May consist of one or more species  Bacterial virulence factors Colonization > Adherence > Biofilm ’ Facilitates colonization of bacteria especially on surgical appliances (artificial valves, catheters…) ’ Enables resistance to host immune system and antibiotics  Bacterial virulence factors Colonization > Invasion ’ Invasion is the process in which bacteria enter the host cell ’ Some bacteria invade tissues through the junctions between epithelial cells ’ Other bacteria directly enter the host cells  Bacterial virulence factors Colonization > Invasion ’ Invasion of intracellular microorganisms: ’ After internalization, the bacterium can either persist in an intracellular vacuole, or escape to the cytosol  Bacterial virulence factors Colonization > Invasion > Nutrition ’ Iron is one of the most essential nutrients for bacteria ’ Iron has a wide oxidation-reduction potential which makes it important in a variety of metabolic functions ’ Most of iron in the host is located intracellularly (in hemoglobin and myoglobin) ’ The concentration of free iron in its ferric form (Fe3+) is very low (due to transferrin and lactoferrin) and higly insoluble (hydroxides, carbonates and phosphates)  Bacterial virulence factors Colonization > Invasion > Nutrition ’ Siderophores are specific for ferric iron and supply iron to the bacterial cell  Bacterial virulence factors Escape from host immune response Escape from innate Escape from adaptive immunity immunity Capsule Protection Antigenic shift against Escape from neutrophils Antigenic mimicry phagosome Inhibition of Binding to Fc region Inhibition of MAC of antibodies complement activation Triggering Release of macrophage immunoglobulin apoptosis proteases  Bacterial virulence factors Escape from innate immunity ’ Phagocytosis leads to cytokine secretion from macrophages ’ Cytokines and bacterium itself activates complement system ’ Escape from innate immunity refers to protection from phagocytosis and lytic activity of complement system Bacterial virulence factors Escape from innate immunity > Capsule Effects of capsule: ’ Inhibition of formation of C3 convertase ’ Binding protein H (the capsules that are reach in sialic acid) ’ Inhibition of opsonization by masking C3b ’ Masking pathogen-associated molecular patterns  Bacterial virulence factors Escape from innate immunity > Capsule ’ The complement system… Bacterial virulence factors Escape from innate immunity > Escape from phagosome ’ Inhibition of phagosome formation ’ Escape from phagosome ’ Inhibition of phagolysosome formation ’ Inhibition of phagosome acidification ’ Inhibition of phagosome maturation ’ Resistance against phagocytic death  Bacterial virulence factors Escape from host immune response Escape from innate Escape from adaptive immunity immunity Capsule Protection Antigenic shift against Escape from neutrophils Antigenic mimicry phagosome Inhibition of Binding to Fc region Inhibition of MAC of antibodies complement activation Triggering Release of macrophage immunoglobulin apoptosis proteases Bacterial virulence factors Escape from adaptive immunity ’ Important defense mechanisms against infectious agents ’ IgG is an opsonin and causes opsonization ’ IgG, IgM and IgA bind to bacteria and inhibit attachment to host cell ’ IgG and IgM activate complement system (classical pathway) ’ IgM and IgA cause bacteria to aggregate and facilitate their phagocytosis Bacterial virulence factors Escape from adaptive immunity ’ Antigenic mimicry: ’ Bacterial antigens mimic the antigens found on host tissues ’ Several bacteria coat their surfaces with host proteins such as fibronectin, lactoferrin and transferrin ’ Binding to Fc region of antibodies: ’ Bacterium is coated with antibody but opsonization does not occur Bacterial virulence factors Producing damage in the host Endotoxin Exotoxin Type I (pyrogenic) exotoxins: Superantigens Type II toxins: Membrane damaging toxins Type III toxins: A-B toxins Bacterial virulence factors Producing damage > Exotoxins and endotoxins Characteristics of exotoxins and endotoxins -1 Exotoxin Endotoxin Produced by both Gram-positive and Found only in Gram-negative bacteria. Gram-negative bacteria. Excreted by living cell. Released on bacterial death and in part during growth. Integral part of the cell wall of Gram-negative bacteria. Polypeptides. Lipopolysaccharide complexes. Relatively unstable; toxicity often Relatively stable; withstand destroyed rapidly at temperatures above temperatures above 60oC for hours 60oC. without loss of toxicity. Bacterial virulence factors Producing damage > Exotoxins and endotoxins Characteristics of exotoxins and endotoxins -2 Exotoxin Endotoxin Highly antigenic; stimulate formation Weakly immunogenic; antibodies are of high-titer antitoxin. Antitoxin antitoxic and protective. neutralizes toxin. Converted to antigenic, non-toxic Not converted to toxoids. toxoids by formalin, acid, heat, etc. Toxoids are used to immunize. Highly toxic to laboratory animals. Moderately toxic to laboratory animals. Bacterial virulence factors Producing damage > Exotoxins and endotoxins Characteristics of exotoxins and endotoxins -3 Exotoxin Endotoxin Usually bind to specific receptors on Specific receptors are not found on cells. cells. Usually do not produce fever in the Usually produce fever in the host by hosts. release of interleukin-1 and other mediators. Frequently controlled by Synthesis directed by chromosomal extrachromosomal genes. genes. Bacterial virulence factors Producing damage > Endotoxins ’ Lipopolysaccharide (LPS)  Endotoxin of Gram- negative bacteria Bacterial virulence factors Producing damage > Endotoxins ’ Lipopolysaccharide (LPS)  Endotoxin of Gram- negative bacteria  Bacterial virulence factors Producing damage > Endotoxins ’ O-specific polysaccharide: ’ Induces specific immunity (O-antigen) ’ Common core polysaccharide: ’ Same in all Gram- negative bacteria ’ Lipid A: ’ Responsible for primary toxicity Bacterial virulence factors Producing damage > Endotoxins ’ LPS in the bloodstream is initially bound to circulating proteins which then interact with receptors (TLR-4) on macrophages, monocytes and other cells of the reticuloendothelial system ’ IL-1, TNF and other cytokines are released, and the complement and coagulation cascades are activated ’ The following findings can be observed: ’ Fever, leukopenia and hypoglycemia; hypotension and shock resulting in impaired perfusion of essential organs (brain, heart, kidney); intravascular coagulation; and death from massive organ dysfunction Bacterial virulence factors Producing damage > Endotoxins Bacterial virulence factors Producing damage > Endotoxins Disseminated intravascular coagulation (DIC): ’ Frequent complication of Gram-negative bacteremia (can also occur in other infections) ’ LPS activates factor XII ’ Coagulation cascade: Fibrinogen -> fibrin ’ LPS activates plasminogen to plasmin ’ Fibrin -> fibrin split products ’ LPS causes platelets to adhere to vascular endothelium and occlusion of small blood vessels, causing ischemic or hemorrhagic necrosis in various organs Bacterial virulence factors Producing damage in the host Endotoxin Exotoxin Type I (pyrogenic) exotoxins: Superantigens Type II toxins: Membrane damaging toxins Type III toxins: A-B toxins Bacterial virulence factors Producing damage > Exotoxins ’ Exotoxin synthesis is generally enabled by the genetic information transferred by plasmid or phage ’ Toxin synthesis can be controlled by chromosome or plasmid ’ Genes that control toxin synthesis are activated by environmental factors Bacterial virulence factors Producing damage > Exotoxins ’ Gram-positive bacteria directly release their toxins ’ Gram-negative bacteria release their toxins through one of the secretion systems Bacterial virulence factors Producing damage > Exotoxins > Type I (pyrogenic): Superantigens ’ In normal conditions, antigens are presented to MHC II by means of antigen presenting cells (APC) ’ Superantigens directly bind to MHC II Bacterial virulence factors Producing damage > Exotoxins > Type I (pyrogenic): Superantigens ’ Activation of large amounts of T4 lymphocytes stimulate release of large amounts of IL-2 ’ IL-2 causes several symptoms such as fever, vomiting, diarrhea, malaise Bacterial virulence factors Producing damage > Exotoxins > Type I (pyrogenic): Superantigens ’ IL-2 triggers release of several cytokines including TNF-, IL-1, IL-8 and PAF ’ These cytokines cause endothelial damage, acute respiratory distress syndrome, DIC, shock and organ dysfunction Bacterial virulence factors Producing damage > Exotoxins > Type II: Membrane damaging toxins ’ Toxins that hydrolize membrane phospholipid: ’ Toxins that have phospholipase C, phospholipase D and sphingomyelinase activity ’ Toxins that have detergent-like effect ’ Channel or pore forming toxins Bacterial virulence factors Producing damage > Exotoxins > Type II: Membrane damaging toxins ’ Functions of pore forming toxins: ’ Releasing nutrients and death of host cell ’ Functioning in type III secretion system ’ Damaging vacuole membrane and allowing bacterium to escape into cytoplasm ’ Damaging cell membrane and allowing bacterium to leave the cell Bacterial virulence factors Producing damage > Exotoxins > Type III: A-B toxins ’ Compose of two subunits: “A and B” ’ “Part B” binds to the receptor of the host cell and ’ “A+B” enter into the cell by endocytosis. Inside the cell, “part A” transfers into cytoplasm ’ Only “part A” enters into the cell and passes directly into cytoplasm Bacterial virulence factors Producing damage > Exotoxins > Type III: A-B toxins ’ “A subunit” is the active site that exibits various effects ’ “B subunit” is the binding site ’ Targets and effects of A-B toxins: ’ Cyclic adenosine monophosphate (cAMP), ADP- ribosylation, Rho proteins, metalloproteinase action and RNA Bacterial virulence factors Colonization Escape from host immune response Producing damage in the host Bacterial virulence factors Colonization Adherence Invasion Pilus Adhesin Capsule Biofilm  Bacterial virulence factors Escape from host immune response Escape from innate Escape from adaptive immunity immunity Capsule Protection Antigenic shift against Escape from neutrophils Antigenic mimicry phagosome Inhibition of Binding to Fc region Inhibition of MAC of antibodies complement activation Triggering Release of macrophage immunoglobulin apoptosis proteases Bacterial virulence factors Producing damage in the host Endotoxin Exotoxin Type I (pyrogenic) exotoxins: Superantigens Type II toxins: Membrane damaging toxins Type III toxins: A-B toxins