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• Ancients thought diseases were divine punishment
• Leeuwenhoek's discovery of microorganisms in 17th century led people to suspect they might cause disease
• Robert Koch (1876) offered evidence of what is now known as germ theory of disease
• Robert Koch


  • Showed Bacillus anthracis causes anthrax; later work on tuberculosis
  • Formalized criteria for establishing cause of disease, now known as Koch's postulates
• We contact numerous microorganisms daily
• Most microbes are harmless; many are beneficial
• Some microbes can cause disease if there is an opportunity
• Immunocompromised


  Weaknesses or defects in immunity leave people vulnerable to invasion
• Symbiosis


  Living together
• Mutualism

  Both partners benefit
• Commensalism

  One partner benefits, other is unharmed
• Parasitism

  One organism benefits at expense of other
• Normal microbiota or microbiome

  Microorganisms routinely growing on a healthy body
• Resident microbiota

  Microbes that inhabit sites for extended periods
• Metagenomics


  Analysis of DNA to study microbiota
• Colonization begins at birth; microbiome is different after vaginal birth than after caesarian birth
• Breastfeeding affects composition of microbiome
• There are 10^13 to 10^14 microbes in the human microbiome
• Composition of microbiome is different among individuals and over time
• Microbiome changes with physiological state and lifestyle of host
• Dysbiosis


  Imbalance in the microbiome
• Beneficial roles of human microbiome

  • Significant contribution is protection against pathogens
  • Covering of binding sites prevents attachment
  • Consumption of available nutrients
  • Production of compounds toxic to other bacteria
• When microbiome is suppressed (for example, during antibiotic treatment), pathogens may colonize, cause disease
• Some antibiotics inhibit Lactobacillus (in vagina of mature females, suppress growth of Candida albicans); results in vulvovaginal candidiasis
• Oral antibiotics can inhibit intestinal microbiota, allow overgrowth of toxin-producing Clostridioides (Clostridium) difficile
• Beneficial roles of human microbiome

  • Stimulation of adaptive immune system
  • Antibodies against normal microbiota may also bind to pathogens
  • Mice in microbe-free environment have underdeveloped mucosal-associated lymphoid tissue (MALT)
  • Some intestinal microbiota appear to protect against infection and cancer by increasing the patrolling of T cell in the intestine
  • Important in development of immune tolerance
  • Immune system lessens response to many microbes in gut as well as food
• Beneficial roles of human microbiome

  • Aid in digestion: break down fiber; increase nutrients
  • Produce important substances, such as vitamin K
• Colonization

  Microbe establishing itself and multiplying
• Infection


  Pathogen causing disease
• Subclinical

  No symptoms or mild symptoms
• Infectious disease

  Prevents normal function
• Symptoms

  Subjective effects experienced by patient (pain, nausea)
• Signs

  Objective evidence (rash, pus formation, swelling)
• Primary infection


  Initial infection
• Secondary infection

  Damage can predispose individual to developing a secondary infection
• Primary pathogen

  Microbe or virus that causes disease in otherwise healthy individual
• Opportunistic pathogen

  Causes disease only when body's immune defenses are compromised or when introduced into unusual location
• Virulence


  Degree of pathogenicity
• Virulence factors


  Allow microorganism to cause disease
• Communicable or contagious diseases easily spread from one host to another