Bacteria and Disease

• Disease


  Result of an undesirable relationship between host and pathogen, marked by interruption in the normal functioning of a body part or parts

  
• Infection


  Invasion of the body by pathogenic microorganisms. The term is not synonymous with disease

  
• Symbiosis


  Prolonged and close interaction between organisms of different species
• Mutualism


  Both organisms benefit
• Commensalism


  One organism benefits, while the other is unharmed
• Parasitism


  One organism benefits from another and causes harm to the other
• Pathogenicity


  Ability of an organism to produce disease

  
• Virulence


  Describes the degree of pathogenicity of an organism
• Contamination


  Presence of unwanted materials (biological, chemical, radiological)
• Pollution


  Presence of contaminants that can cause adverse biological effects. All pollutants are contaminants but not all contaminants are pollutants
• Bacteremia


  Presence of bacteria in the blood
• Septicemia


  Presence of actively multiplying bacteria in the blood usually from a source infection. The condition is called sepsis
• Pyemia


  Presence of pus-producing bacteria in the bloodstream
• Viremia


  Presence of viruses in the blood
• Toxemia


  Presence of toxins in the blood
• Koch's Postulate


  • The suspected organism must be absent in healthy individuals but present in those with disease
  • The suspected organism must be isolated from the infected host and grown in pure culture
  • The organisms grown from pure culture must produce the same disease as that of the infected source when inoculated to susceptible animal
  • The same organism must be isolated from pure culture from the experimentally-infected host

  
• Robert Koch


  A German physician who made significant contributions to the field of microbiology
• Koch's Postulates were published in 1884
• Limitations of Koch's Postulates

  • The ability of Koch's postulates lies in the ability of the pathogen to grow in the laboratory
  • Viruses are obligate intracellular parasites that need to be grown in living cells
  • Not all people who acquire an infection develop overt disease
  • The reaction of humans to specific pathogens may differ given a specific microorganism
  • Some microorganisms are species-specific
  • There are organisms that produce disease only in animals in the same manner that there are infectious agents that produce disease only in humans

  
• Factors Influencing the Occurrence of Infection

  • Etiologic agent
  • Host
  • Environment
• Reservoirs


  Source of disease-causing microorganisms. Site where an infectious agent normally reside, and multiply. Reservoirs could be an animal, a human, or the environment
• Infections that can be transmitted from animal to human (zoonotic infection)


  • anthrax
  • plague
  • rabies
• Environmental Reservoirs


  Water, soil, and plants could all harbor organisms that could cause disease
• Human Reservoirs


  The human reservoir may not necessarily manifest with the disease. There are certain infected humans who may harbor the organism and only develop sub-clinical disease
• Types of Human Reservoirs

  • Carriers - people who developed the disease, got well but still harbor the organism thereby transmitting them to others
  • Asymptomatic (healthy)
  • Incubatory
  • Chronic
  • Convalescent
• Portal of Exit


  Where microorganisms exit their host
• Mode of Transmission


  • Direct or Indirect
  • Direct Contact - Contact with environmental sources harboring infectious agents, Person-to-person contact, Droplet spread
  • Indirect Contact - Mechanical Transmission, Biological Transmission
• Portal of Entry


  How the infectious agent enters a susceptible host. Provides access to tissue where the agent can multiply
• Host


  Final link of infection. The host's susceptibility is affected by factors such as constitutional or genetic factors and immune status of the host

  
• How Organisms Produce Disease

  • Mechanical: Invasiveness
  • Chemical: Toxins
  • Immunologic

  
• Mechanical: Invasiveness


  Organisms can produce disease by directly damaging tissues or body surface. Encompasses colonization, ability to evade host immune defenses, and production of extracellular substances

  
• Adhesins


  Substances produced by organisms that facilitate adhesion
• Ability to Evade the Immune Response


  Bacteria have various mechanisms to evade the immune system. For example: S. aureus is coagulase-positive which creates coagulum. M. tuberculosis can survive and multiply inside macrophages. Some bacteria can produce biofilm
• Production of Substances


  • Neiserria gonorrheae can enter and multiply inside cell
  • Clostridium perfringens produce collagenase
• Chemical: Toxins


  • Endotoxin and Exotoxin
  • Endotoxins are LPS
  • Exotoxins are products released into the surroundings. Three types: Cytotoxin, Neurotoxin, Enterotoxin
• Immunologic


  Consequence of the immune response. For example, hepatitis caused by hepatitis viruses
• Classification of Infectious Disease

  • Communicable Disease - May spread from one person to another, Contagious, Fulminant infection
  • Non-Communicable Disease - May not be communicated from one person to another
  • Exogenous - Infectious agent from outside
  • Endogenous - Infection from the inside of the body
  • Nosocomial - Hospital-acquired infection

  
• Occurrence of Disease


  • Sporadic - Occurs occasionally
  • Endemic - Present in a low population
  • Epidemic - High number of people in a given locality develop an infectious disease in a short period of time
  • Pandemic - Worldwide occurrence

  
• Severity or Duration of Disease

  • Acute - Develops rapidly but lasts for a short period of time
  • Chronic - Develops more slowly and occurs for a long period of time
  • Latent - Causative agent remains inactive for a time but may become active again
• Host Involvement


  • Localized infection - Invading organisms are limited to a relatively small area of the body
  • Systemic or generalized infection - Causative organisms or their products (toxins) throughout the body
  • Focal Infection - Pathogens' toxins may enter the blood vessel or lymphatic vessel spread to a specific parts of the body and become confined to specific areas