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Microbiology lec2

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  • 1. Medical Microbiology Lecture 2 Dr. Saleh M Y OTH PhD Medical Molecular Biotechnology and Infectious Diseases 02/10/2010 IMS - MSU
  • 2. Infection: The growth of microorganisms in the host without causing damages to the host. Definaations Bacteria may be divided into three types based on their potential to cause diseases; - pathogens , - commensals , and - nonpathogens . Must to know: Morbidity, mortality, infections, parasite, host, vector, fomite, contagious (infectious) disease, epidemic, endemic, pandemic, Zoonosis, Epizootic, Attack rate.
  • 3.
    • Normal flora; (commensals, indigenous organisms): Bacteria which do not normally cause diseases and can be found commonly in healthy individuals.
    • They may cause diseases
    • (1) in an immunocompromised host, or
    • (2) after being introduced into different body sites, or (3) overgrow in favorable conditions.
    • They are considered as opportunistic pathogens .
    • Nonpathogens: Certain bacteria, such as
    • Lactobacillus acidophilus , almost never cause disease.
  • 4.
      • Normal flora;
    • Frequently found on or within the body of healthy persons, protect the host by competing with pathogens.
      • * 10 6 streptomycin-resistant Salmonella were needed to infect an experimental animal.
    • * Fewer than 10 cells were needed to infect an animal pre-treated with streptomycin to suppress the normal flora in the GI tract.
  • 5.
    • The composition of the normal flora varies in different body sites.
    • Ultimately, the flora composition is determined by ecological factors including the presence of receptors of host cell surface for bacterial adherence, pH, oxygen, availability of nutrients, water, host defense and personal hygiene.
  • 6. Some tissues, organs are dense with normal flora, others are normally sterile a. colonized sites i. alimentary/intestinal tract ii. upper respiratory tract iii. distal genitourinary tract iv. skin b. normally sterile sites i. blood ii. CSF iii. interstitial fluid and spaces iv. lymph
  • 7. Disease ; damage caused by presence of microorganisms or their products. Colonization - presence of microorganisms without disease at that point. This term applies to surfaces only , i.e., the blood cannot be colonized and host cells with intracellular infection are not colonized.
  • 8. Carrier state; colonization with a pathogen Pathogen ; any organism that has the potential to cause disease
  • 9.
    • Pathogenicity: The ability of a bacterium to inflict damages on the host.
    • Virulence: The degree of pathogenicity associated with a bacterium.
    • Virulence factors : Bacterial products or strategies that contribute to virulence.
  • 10.
      • Adherence and colonization:
      • Bacteria need to avoid being washed away by non-specific host defense mechanisms (e.g., saliva).
      • Most bacteria possess specific adherence factors which bind receptors on the surface of the host cells or tissue.
      • The specific interaction between bacterial adhesins and host receptors explains the observed tissue specificity of bacterial infections.
      • For example, N. gonorrhoeae adheres better to human urogenital epithelia than to other tissue.
  • 11. Pili and Afimbrial Adhesins Pili - fimbrial Adhesins Afimbrial Adhesins (No pili) Adherence and colonization ……..
  • 12.
      • Pili (fimbriae): E. coli as an example..
      • Most the strains of E. coli do not adhere to the small intestine epithelia.
      • They are normally found in large intestine.
      • Pathogenic strains can colonize small intestine and cause diarrhea.
    Adherence and colonization …….. Assignment (1): write in 6 pages about the pathogenic strains of E. coli.
  • 13.
      • The ability of the pathogenic strains of to cause diseases is intimately associated with the expression of various pili.
      • E. coli colonization factor antigen I (CFA) pili found in enterotoxigenic E. coli ( ETEC ) strains.
      • ETEC strains adhere to small intestinal mucosa and produce symptoms by elaborating toxins that act on mucosal cells to cause diarrhea.
    Adherence and colonization …….. Assignment (1): write in 6 pages about the pathogenic strains of E. coli.
  • 14.
      • Nonfimbrial Adhesins (Afimbrial Adhesins)
      • - S. aureus : Lipoteichoid acid
      • - Staphylococcus : Slime
      • - Group A streptococci : F protein (non-fibrillar, mediates attachment to fibronectin).
      • - Streptococcus pneumonia : Capsule
    Adherence and colonization ……..
  • 15.
      • Growth:
      • Bacteria have two distinct phases in their life cycle: active or vegetative stage and inactive or spore-forming stage.
      • Bacteria need nutrients to grow.
      • Iron is an essential growth factor for many bacteria.
      • At neutral pH, iron usually exists in a highly insoluble ferric state (Fe3+).
  • 16.
    • Within human body;
    • transferrin, lactoferrin, ferritin and hemin bind most of the available iron.
    • Many bacteria produce iron-chelating compounds call siderophores to help getting iron.
    Growth …..
  • 17.
    • Within human body;
    • Siderophores are low molecular weight compounds that chelate iron with very high affinity.
    • The siderophores are excreted into the medium and then the iron-siderophore complex is taken up by the cells through receptors.
    Growth …..
  • 18.
    • Within human body;
    • Many bacteria also can use transferrin, lactoferrin, ferritin or hemin as a source of iron.
    Growth …..
  • 19. Mechanisms of Bacterial Pathogenesis Virulence factors are bacterial strategies or products which contribute to each of these steps leading ultimately
  • 20. Functions and stages of pathogens and disease 1. encounter 2. entry 3. spread (+/-) 4. multiplication 5. evasion of host defenses 6. damage 7. outcome a. transmission to new host (+/-) b. recovery or not
  • 21.
      • Growth and Toxins produced by pathogens
      • Toxins:
      • Toxicity
      • Exotoxins: Toxic proteins, many are excreted into the medium; may damage host tissue away from where bacteria infections occur.
      • Endotoxins: LPS, embedded in the G- organism outer membrane, released during bacterial lysis.
  • 22. P P