BACTERIA AND DISEASES

• Koch's postulates


  General guidelines developed in the 19th century to identify pathogens that could be isolated with the techniques of the day

  
• Koch's postulates


  • The suspected organism must be absent in healthy individuals but present in those with the 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 a susceptible animal
  • The same organism must be isolated from pure culture from the experimentally infected host

  
• Viruses were not yet able to be cultured during the 1800's, so Koch's postulates could not be fully met for viral diseases
• The third postulate stipulates that the experimental host "should" exhibit disease, not "must", because asymptomatic carriers, immunity, and genetic resistance are possible
• Koch's Postulates do not account for prion diseases and other agents that cannot be grown in culture
• Most of the human bacterial pathogens satisfy Koch's postulates except for Mycobacterium leprae and Treponema pallidum, the causative agents of leprosy and syphilis, which have not yet been grown in cell-free culture media
• How organisms produce disease

  • Mechanical: Invasiveness
  • Chemical: Toxin Production
• Exotoxins


  Soluble in body fluids and easily diffused into the blood and rapidly transported throughout the body
• Endotoxins


  Exert their effects when the gram negative bacteria die and their cell walls undergo lysis, thereby releasing the endotoxin
• Classification of infectious diseases based on how they behave within a host and within a given population


  • Communicable disease
  • Contagious disease
  • Fulminant infection
  • Non communicable disease
• Classification of infectious diseases based on the source of the microorganism


  • Exogenous
  • Endogenous
• Classification of infectious diseases based on the occurrence of a disease


  • Sporadic disease
  • Endemic disease
  • Epidemic
  • Pandemic
• Classification of infectious diseases based on the severity or duration of a disease


  • Acute disease
  • Chronic disease
  • Latent disease
• Stages of infection


  • Incubation period
  • Prodromal period
  • Period of illness
  • Period of decline
  • Period of convalescence
• Antimicrobial susceptibility testing


  Used to determine which antimicrobials will inhibit the growth of the bacteria or fungi causing a specific infection
• Importance of susceptibility testing


  • Helps determine which drugs are likely to be most effective in treating a person's infection
  • Aids in the evaluation of treatment services provided by hospitals, clinics and national programs for control and prevention of infectious diseases
  • Monitors for resistance patterns due to the mutations in bacterial DNA

  
• Kirby-Bauer disk diffusion susceptibility test
• Communicable disease


  a disease that is spread from one host to another, either directly or indirectly
• Contagious disease


  is easily and rapidly spread from one person to another
• Fulminant infection


  if the infection results in the death of the patient over a short period of time
• Non communicable disease


  is one that is not spread from one person to another
• Exogenous disease

  • if the source of the infectious agent is from outside the body
  • example: hospital acquired infection or nosocomial infections
• Endogenous disease


  is one where the source of the causative organism is from inside the body
• Sporadic disease


  a disease that occurs occasionally
• Epidemic disease

  • if a great number of people in a given locality develop an infectious disease in a relatively short period of time
  • example: influenza
• Pandemic


  if a disease has a worldwide occurrence or involves at least three regions in the world
• Acute disease

  • is one that develops rapidly but lasts for only a short period of time
  • example: common cold
• Chronic disease

  • if the disease develops more slowly and occurs for long periods of time
  • example: tuberculosis
• Latent disease


  is one in which the causative organism remains inactive for a time but can become active again and produce symptoms of the disease.
• Localized infection

  • is one in which the invading organisms are limited to a relatively small area of the body
  • examples: Boils and abscesses
• Systemic or generalized infection

  • is one where the causative organisms or their products are spread throughout the body through blood or lymph
  • example: Sepsis or blood poisoning
• Primary infection


  is an acute infection that causes the initial illness
• Secondary infection


  is one which is caused by opportunistic pathogens after the primary infection has weakened the body’s defenses
• Incubation period


  It is the period starting from the entry of the pathogen until the appearance of the first sign.
• Prodromal period


  starts from the onset of non-specific signs and symptoms such as fever, headache, and fatigue
• Factors affecting the length of incubation period:

  • virulence of the organisms
  • resistance of the host
  • number of infecting microorganisms
• Period of illness

  • corresponds to the period of maximal invasion.
  • the patient manifests signs and symptoms distinctive of the disease
  • when the patient does not successfully overcome the disease producing agents, he or she may develop severe disease
  • infection may also progress from an acute form into a chronic form
• Period of decline

  • the signs and symptoms of the patient start to subside
  • Body temperature may return to normal and the feeling of weakness may diminish
  • it is also during this period that the patient becomes vulnerable to secondary infections
• Period of convalescence

  • is marked by recovery of the patient from the disease
  • patient regains strength and the body returns to its pre diseased, normal condition
• Classification of infectious diseases based on the extent of host involvement

  • Localized infection
  • Systemic or generalized infection
  • Primary infection
  • Secondary infection