Bacterial Pathogenesis

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Bacterial Pathogenesis. The term infection describes the process that pathogenic microorganisms multiply,release toxin within the body and produce a change in the normal physiology of the body. Section 1 Normal flora and opportunistic pathogens. - PowerPoint PPT Presentation


  • Bacterial Pathogenesis

  • The term infection describes the process that pathogenic microorganisms multiply,release toxin within the body and produce a change in the normal physiology of the body.

  • Section 1 Normal flora and opportunistic pathogensDefinition: microorganisms that live on or in human bodies, and ordinarily do not cause human diseases but under certain condition can cause disease

  • pathogen

  • The distribution of normal flora

  • Normal floraSkinStaphylococcus epidermidisDiphtheroids StreptococciPeptococcus

  • The significance of normal floraconstitute a protective host defense mechanism: Competition of nutrients and receptors Metabolic substances by normal flora: e.g., bacteriocins, antibiotics, etc. serve a nutritional function:several B vitamins and vitamin Kkeep our immune systems in tune normal flora share many antigenic determinants with pathogenic organisms

  • Opportunistic pathogens Definition: normally nonpathogenic microorganisms capable of causing infection disease in an immunosuppressed host. Conditions of causing diseases by opportunistic pathogens:Alteration of colonization sitesDeclination of host immune system function

  • DysbacteriosisDefinition: the state in which the proportion of bacterial species and the number of the normal flora colonizing in certain site of a host present large-scale alteration.

  • Nosocomial infectionsInfectious diseases acquired as a result of a hospital stay are known as nosocomial infections.Surgical procedures and lower defenses permit resident floraIndwelling devicesFomites ,medical equipment,other patients

  • Section 2 Bacterial pathogenesisbacteriaImmune status of the hostinbodyouterbody

  • Why do Animal get infectious diseases?From the organisms perspectives The number of organisms The virulence of these organismsFrom the hosts perspective Innate immunity acquired immunity


  • Pathogenicity of bacteria Pathogenicity and virulence: refer to an organism's ability to cause disease. LD50 (median lethal dose) or ID50 (median infectious dose): refers to the number of bacteria or amount of bacterial products, such as toxins, that cause death or bacterial disease in 50% of animals in a defined period after the bacteria are administrated by a designated route.

  • Pathogenicity of bacteriapathogenicity determined by):virulence factors of the bacteriumthe number of infecting bacteriaroute of entry into the body

  • Virulence factors Invasiveness Definition: the ability of a microorganism to invade human cells or tissues,and to multiply on or within them.

  • Capsules and slime layers: e.g., pneumococci Interfere with the ability of phagocytic blood cells to engulf and destroy bacteria and protect bacteria against some antimicrobial substance

  • Bacterial infections are usually initiated by adherence of the microbe to a specific epithelial surface of the host,otherwise the organism is removedPeristalsis and defecation ciliary action,coughing and sneezing or urinationA specific stickiness2.Adhesins

  • (1)Finbrial adhesins involved in mediating attachment of some bacteria to mammalian cell surfaces

  • (2)Non-fimbrial adhesinIncluding the filamentous haemagglutinin of Bordetella pertussia,a mannose-resistant haemagglutinin from Salmonella serotype Typhimurium and a fibrillar haemagglutinin from helecobacte pylori

    Pili: e.g., Neisseria gonorrhoeae Other surface structures of bacteria: LTA

  • Other extracelluar aggressinsInvasive enzymes: e.g.Coagulase:working in conjunction with serum factors to coagulate plasma.contributes to the formation of fibrin walls around staphylococcal lesions.

  • Toxins Exotoxin Definition: a soluble protein toxin usually secreted from a living bacterium.

  • Origin and release: produced by Gram-positive bacteria as well as Gram-negative cells Physical and chemical properties: proteins and usually heat-labile.

  • Virulence factorsToxinsExotoxinImmunity: excellent antigens that elicit specific antibodies called antitoxins.

  • Antitoxin: Definition: a specific antibody capable of neutralizing the exotoxin that stimulates its production. Toxoid:Definition: a modified exotoxin that has been treated to destroy its toxicity and remains immunogenicity.

  • Virulence factorsExotoxinComponent characteristics: most exotoxins consist of two parts, an A (active) component and a B (binding) component.

    Toxicity: high and even fatal; highly tissue specificity

  • Categories: Cytotoxins: exotoxins that destroy the target cells directly by various mechanisms. Neurotoxins: exotoxins that affect nerve transmission of the nerve system. Enterotoxins: exotoxins that stimulate hypersecretion of water and electrolytes from the intestinal epithelium and produce watery diarrhea.

  • Exotoxin Neurotoxin Tetanus toxin ,clostridium tetani glycine spastic paralysisBotulinum toxin, clostridium botulinum acetylcholine flaccid paralysis

    Cytotoxin diphtheria toxin inhibits protein synthesis

    Enterotoxins v. cholerae perturb the processes that regulate ion and water exchange across the intestinal epithelium

  • Virulence factorsEndotoxins Origin and release: produced only by Gram-negative bacteria and released only when bacteria lyze. Chemical and physical properties: lipopolysaccharide of the outer membrane of Gram-negative bacteria; heat-stable

  • Endotoxins

    Immunity: weakly immunogenic

    Biologic activity: lipid A is the primary toxic component all endotoxins present similar biologic effects. Pyrogenicity Leukocyte reaction Endotoxemia and endotoxin shock DIC (dissemiated intravascular coagulation)

  • Endotoxin(especially lipid A))ActivatesmacrophagesActivatescomplementActivatesHageman fatcorIL-1FeverTNFFever and hypotensionNitric oxidehypotensionC3a

    Hypotension EdemaC5a Neutrophil chemotaxisCoagulation cascadeDIC

  • Endotoxin toxicity leukocytoreaction

    pyogenetic reaction endotoxemia and endotoxin shockShwartzman phenomenonDIC

  • Endotoxins

    Detection of endotoxin: The Limulus lysate test

  • The different between indotoxin and exotoxin

    kindsexotoxinEndotoxinsourceG+or a few G-G-


    stability Short ofgood160 2-4h destroyedvirulencestrongweakantigenicitystrong


  • Virulence of pathogenic bacterial

  • .Portals of Entry and the size of the inoculumIf certain pathogen enter the wrong portal,they will not be infectious.Occasionally,an infective agent can enter by more than one portal.e.g.mycobacterium tuberculosis.

  • Portals of entry skinrespiratory systemingestion systemgenitourinary system

    C. tetani

  • The size of the inoculumThe quantity of microbes in the inoculating dose.

  • .the originate and progress of infectionA.The source of the infection

    B.routes of pathogen transmission

    C.Patterns of infection

  • A.The source of the infectionLiving reservoirsPersons or animals with frank symptomatic infection are obvious sources of infectionNonliving reservoirs

  • Exogenous infections: Patients Carriers: those in whom pathogens are present and may be multiplying, but who shows no clinical response to their presence. Contaminated animals Endogenous infections Sources of infectious diseases

  • Carrier stateDefinition of carriers: those in whom pathogens are present and may be multiplying, but who shows no clinical response to their presenceDefinition of carrier state: a type of infections causing no signs of symptoms, in which pathogens multiply and may be transmitted to other individuals

  • two major types of carrier: Convalescent carriers: those who recover from infectious disease and in whom the pathogens remain and multiply without causing overt symptoms. Healthy carriers: those who do not have the clinical symptoms but carry pathogens indeed. Typhoid Mary (Mary Mallon)

  • B.routes of pathogen transmission1.respiratory infections the tiny particles of liquid released into the air form aerosols or droplets2.wound infectons:in soil and feces of human and animal3.intestinal infections: contaminate drinking water and food or when used to fertilize crops

  • infection:directly contact between the skin and mucous membranes of the infected person or animal and that of healthy person5.animal bites infections:the majority of animal vectors are arthropods such as fleas,mosquitos,flies,and ticks

  • acute infection chronic infection C.Patterns of infectionApparent infection1.apparent infectionWhen an infection causes pathological changes leading to disease,it is often accompanied by a variety of signs and symptomsInfectons that come on rapidly,with severe but short-lived effects,are called acute infectionsThe infection persists several months to several years called chronic infection

  • Inapparent infection: also called subclinical infection that has no detectable clinical symptoms

  • local infection generalized/systemic infection

    Localized infection stands for the case that the microbe enters the body and remains confined to a specific tissue

  • Generalized infection BacteremiaDefinition: a transitory disease in which bacteria present in the blood are usually cleared from the vasc


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