Chap 2

• Host/microbe interaction


  Mechanism, pathogenicity, virulence and toxigenicity

  
• Normal flora


  Microorganisms that colonize the body and usually do not cause disease

  
• Opportunistic microorganisms


  Microbes that normally do not cause disease, but may under certain circumstances

  
• Microbe


  Microscopic organism (microorganism) too small to be seen with the naked eye, especially one that causes disease
• Types of microbes


  • Bacteria
  • Fungi
  • Archaea
  • Protist
  • Viruses
  • Parasites (Toxoplasma, Plasmodium, Babesia etc.)

  
• Disease-causing pathogens


  • Bacteria - TB, cholera, typhoid
  • Fungi - Ringworm, histoplasmosis, candidiasis
  • Viruses - cold, dengue, Covid-19
  • Parasites - malaria, Chagas disease, toxoplasmosis, amebiasis
• Pathogen


  An organism causing disease to its host, with the severity of the disease symptoms referred to as virulence

  
• Pathogenicity


  The ability of an organism to cause disease (i.e., harm the host). This ability represents a genetic component of the pathogen and the overt damage done to the host is a property of the host-pathogen interactions.

  
• Pathogenesis


  The mechanisms by which a disease develops, progresses, and either persists or is resolved
• Pathology


  The study and diagnosis of disease. Clinical pathology involves the examination of surgically removed organs, tissues (biopsy samples), bodily fluids, and in some cases the whole body (autopsy). Chemical pathology (also known as clinical biochemistry) involves the biochemical investigation of bodily fluids such as blood, urine and cerebrospinal fluid.
• Pathogenicity is the ability of an organism to infect a host and cause disease

  
• Aggressiveness is the ability of the pathogen to invade and establish within the host

  
• Virulence


  The severity of the disease in infected hosts (or degree of pathogenicity)
• Virulence factors (VF)


  • Physical structures that the bacterium has (pili, flagellum etc.)
  • Chemical substances that the bacterium can produce (toxins)
  • The genes that code for virulence factors are commonly found clustered on the pathogen's chromosome or plasmid DNA, called pathogenicity islands (PI)
  • PI facilitate the sharing of virulence factors between bacteria due to horizontal gene transfer (HGT), leading to the development of new pathogens over time
  • The genes for virulence factors are often controlled by quorum sensing (QS), to ensure gene activation when the pathogen population is at an optimal density

  
• Quorum sensing (QS)


  Where bacteria "talk" to one another using signaling molecules called autoinducers to measure their population. When the autoinducers reach a critical threshold level, they activate bacterial quorum sensing genes that enable the bacteria to behave as a multicellular population rather than as individual single-celled organisms.

  
• Some pathogens are much more virulent than others
• Low dose of Strain A increases mortality than higher dose of Strain B, so Strain A is more virulent than Strain B
• Pathogenicity


  The ability of an organism to infect a host and cause disease

  
• Aggressiveness


  The ability of the pathogen to invade and establish within the host

  
• Normal flora


  Microorganisms that live on another living organism (human or animal) or inanimate object without causing disease

  
• Opportunistic pathogens


  Microbes that normally do not cause disease, but may under certain circumstances, such as when the host's immune system is compromised or there is a breach in the body's natural defenses

  
• The human body is not sterile, we become colonized by bacteria from the moment we are born
• Normal sites of the human body

  • Brain; Central nervous system
  • Blood; Tissues; Organ systems
  • Sinuses; Inner and Middle Ear
  • Lower Respiratory Tract: Larynx; Trachea; Bronchioles (bronchi); Lungs; Alveoli
  • Kidneys; Ureters; Urinary Bladder; Posterior Urethra
  • Uterus; Endometrium (Inner mucous membrane of uterus); Fallopian Tubes; Cervix and Endocervix
• Knowledge of the normal flora of the human body allows prediction of the pathogens causing infection and investigation for underlying abnormalities in specific areas of the body when bacteria are isolated from normally sterile sites

  
• Brushing teeth


  Linked to lower risks of atrial fibrillation and heart failure