13. Principles of Disease and Epidemiology

• Pathology


  The scientific study of disease
• Etiology


  The cause of the disease
• Pathogenesis


  The manner in which a disease develops
• Infection


  The invasion or colonization of the body by pathogenic microorganisms
• Infection may exist in the absence of detectable disease
• Infection is the presence of a particular type of microorganism in a part of the body where it is not normally found
• Disease


  Occurs when an infection results in any change from a state of health
• An abnormal state in which part or all of the body is incapable of performing its normal functions
• Examples of disease


  • AIDS
  • UTI
  • Athlete's foot
  • Pneumonia
• Development of the Human Microbiome

  1. Normal characteristic microbial populations begin to establish themselves in utero

  2. The placental microbiome consists of only few different bacteria, mostly Enterobacteriaceae and Propionibacterium, found in the newborn's intestine

  3. More microorganisms are introduced to the newborn's body from the environment when breathing and feeding start

  4. An individual's microbiome changes rapidly during the first three years as the personal microbiome becomes established

  5. These microorganisms remain there throughout life and, in response to altered environmental conditions, may increase or decrease in number and contribute to health and disease
• A typical human body contains 3 x 10^13 body cells, and harbors as many bacteria cells—an estimated 4 x 10^13 bacterial cells
• The Human Microbiome Project

  Began in 2007 to analyze microbial communities called microbiomes that live in and on the human body<|>The goal is to determine the relationship between changes in the human microbiome and human health and disease<|>More diverse than previously thought
• Normal microbiota


  Microorganisms that establish more or less permanent residence but do not produce disease under normal conditions
• Transient microbiota


  Microbes that may be present for several days, weeks, or months and then disappear
• Representative Normal Microbiota for Different Regions of the Body

  • Bacteria on the surface of the nasal epithelium
  • Bacteria in the small intestine
  • Bacteria on the lining of the stomach
• Factors Affecting the Normal Microbiota

  • Nutrients
  • Physical and chemical factors
  • The host's defenses
  • Mechanical factors
• Nutrients


  Microbes vary with respect to the types of nutrients they can use as an energy source<|>Microbes colonize only those body sites that can supply the appropriate nutrients
• Physical and Chemical Factors

  • Temperature
  • pH (acidity and basicity)
  • Available oxygen and carbon dioxide
  • Salinity
  • Sunlight
• The host's defenses


  Molecules and activated cells that kill microbes, inhibit their growth, prevent their adhesion to host cell surfaces, and neutralize toxins that microbes produce<|>Childhood exposure to microorganisms helps the immune system develop
• Mechanical Forces


  Chewing actions of the teeth and tongue movements can dislodge microbes<|>Flow of saliva and digestive secretions and the various muscular movements of the throat, esophagus, stomach, and intestines can remove unattached microbes<|>Flushing action of urine removes unattached microbes<|>Mucus traps microbes in the respiratory system, which cilia then propel toward the throat for elimination
• Other Factors Affecting the Normal Microbiota

  • Age
  • Nutritional status
  • Diet
  • Health status
  • Disability
  • Hospitalization
  • Stress
  • Climate
  • Geography
  • Personal hygiene
  • Living conditions
  • Occupation
  • Lifestyle
• Germ-Free Animals


  Animals with no microbiota whatsoever can be reared in the laboratory<|>Research with germ-free animals has shown that microbes are not absolutely essential to animal life<|>Germ-free animals have undeveloped immune systems and are unusually susceptible to infection and serious disease<|>Germ-free animals also require more calories and vitamins than do normal animals
• Microbial Antagonism


  Also called competitive exclusion<|>Happens when an established normal microbiota prevents the overgrowth of harmful microorganisms, benefitting the host<|>Involves competition among microbes<|>The normal microbiota protect the host against colonization through different mechanisms
• How the normal microbiota protect the host against colonization by potentially pathogenic microbes

  • Competing for nutrients
  • Producing substances harmful to the invading microbes
  • Affecting conditions such as pH and available oxygen
• Examples of microbial antagonism

  • The normal microbiota of the large intestine effectively inhibit Clostridium difficile
  • E. coli cells produce bacteriocins that inhibit the growth of other bacteria
  • The normal bacterial microbiota of the adult human vagina maintains a local pH of about 4, inhibiting the overgrowth of Candida albicans
• Commensalism


  One organism benefits and the other is unaffected
• Examples of commensalism


  • Staphylococcus epidermis on the skin surface
  • Corynebacterium spp. on the surface of the eye
  • Saprophytic Mycobacterium spp. in the ear and external genitals
• Mutualism


  Benefits both organisms
• Example of mutualism


  • Bacteria in the large intestine such as E. coli synthesize vitamin K and some B vitamins that are absorbed in the bloodstream and distributed for use by body cells
• Parasitism


  One organism benefits by deriving nutrients at the expense of the other
• Many disease-causing bacteria are parasites
• Opportunistic pathogens


  Microbes that do not cause disease in their normal habitat in a healthy person but may do so in a different environment<|>Usually cause secondary infections to the host<|>Possess other features that contribute to their ability to cause disease
• Some opportunistic pathogens may be found in locations in or on the body that are somewhat protected from the body's defenses, and some are resistant to antibiotics
• Many people carry other microorganisms that are generally regarded as pathogenic but that may not cause disease in those people
• Cooperation among microorganisms


  Can be a factor in causing disease
• Example of cooperation among microorganisms

  • Pathogens that cause periodontal disease and gingivitis have been found to have receptors, not for the teeth, but for the oral streptococci that colonize the teeth
• The Etiology of Infectious Diseases

  Some diseases have a well-known etiology<|>Some have an etiology that is not completely understood<|>Other diseases have unknown etiology<|>Not all diseases are caused by microbes
• Koch's Postulates


  Developed by Robert Koch to establish the cause of infectious diseases<|>Includes 4 criteria that must be met to prove a microbe causes a disease
• Exceptions to Koch's Postulates
• Symptoms


  Changes in body function, such as pain and malaise, that are subjective and not apparent to an observer