ch 16 host-microbe interactions

a glimpse of history

ancients thought diseases were divine punishment

Leeuwenhoek’s discovery of microorganisms in 17th century led people to suspect they might cause disease

1876: Robert Koch offered evidence for what is now known as Germ Theory of Disease

• showed Bacillus anthracis causes anthrax; later work on tuberculosis

• formalized criteria for establishing the cause of a specific disease, now known as Kock’s postulates

bacteria are ubiquitous

we contact microorganisms daily

• breathing, ingestion with food/drink, pick up on skin

• most so not harm us, some can cause disease

  • most vulnerable are those with weak or defective defenses

    • immunocompromised

      • due to malnutrition, disease (cancer, ADS), surgery, genetic defects, alcohol or drug abuse, immunosuppressive therapy

• remove by

  • washing

  • shed with dead epithelial cells

  • swallowed microorganisms: killed by stomach acid or eliminated in feces

the anatomical barriers as ecosystems

barriers: skin, mucous membranes

• ecosystem for microorganisms

symbiotic relationships between host and microbes

• mutualism: both partner’s benefit

  • E. coli in human colon

    • host: receives vitamin K and B vitamins from intestinal bacteria

    • bacteria: receive food, shelter from host

• commensalism: one partner benefits, other is unharmed

  • microbes on skin: obtain food from host

  • host: neither harmed nor helped

• parasitism: one organism benefits at expense of other

  • all pathogens

the human microbiome (normal microbiota)

microbes found on the body of healthy individuals

• normally do not cause disease

• resident microbiota - permanent

• transient microbiota - temporarily

acquisition

composition changes due to:

• physiological changes within host

• changes in activity of host (weight loss, change in diet)

• presence and condition of other microbes

the human microbiome (normal microbiota) - has protective role

1. protect against pathogens by competitive exclusion

• covers binding sites to prevent attachment

• consumes available nutrients

• produces compounds toxic to other bacteria

when killed or suppressed, pathogens may infect and cause disease

• examples

  • Lactobacillus vs. Candida albicans (vulvovaginal candidiasis)

  • Intestinal microbiota vs. Clostridium difficile

2. stimulate adaptive immune system

• antibodies against normal microbiota also bind to pathogens

3. important in development or oral tolerance

• lessens immune response to microbes in gut and food

  • basis of hygiene hypothesis, insufficient exposure to microbes leads to allergies

4. aid in digestion: break down of fibers

5. produces vitamin K and B vitamins

can become opportunistic pathogens

• do not cause disease in their normal habitat, but do so when in different environments

• cause disease when:

  • immune system is weakened

  • microbe number increases

  • microbe enters other sites

principles of infectious disease

infectious disease: diseases that can spread

infection: microbe colonizes and multiplies in host

• subclinical: no or mild symptoms

• symptomatic: show characteristic signs/symptoms

  • symptoms: changes felt by patient

    • pain, nausea, malaise

  • signs: changes that can be measured or observed

    • rash, fever, pus formation, and swelling

primary infection: the initial infection in a healthy person

• caused by primary pathogen: a microbe that causes disease in healthy individuals

• causes damage that makes the host vulnerable to developing a secondary infection

secondary infections

• due to opportunistic pathogens: only cause disease when the host’s immune defenses are weakened or when introduced into an unusual location

examples…

stages of an infectious disease

1. incubation period:

• time between contact with microbe to onset of symptoms

• may vary with pathogen (few days for cold, years for leprosy)

2. illness: patient experiences signs/symptoms of disease

• immune system trying to fight off infection

3. convalescence period: patient starts to recover

• prone to secondary infections

• are carriers – have no signs or symptoms, more likely to spread infectious agent

• incubation period → illness → convalescence

Characteristics of Infectious Disease

1. Spread:

• Communicable disease – can spread from one host to another

  • Contagious disease - easily spread

• Non-communicable disease – cannot spread from one host to another

2. Duration of symptoms

• Acute infections: symptoms develop quickly and more severe, last a short time (strep throat)

• Chronic infections: develop slowly, last for months or years (TB)

• Latent infections: microbe never eliminated

  • Symptoms may reoccur when immunity is weakened (reactivation)

  • Chicken pox → shingles, cold sores, genital herpes

3. Distribution of pathogen in host

• Localized infection: microbe limited to small area (boil caused by Staphylococcus aureus)

• Systemic infection: agent spread throughout the body via blood (measles)

  • Suffix -emia means “in the blood”

    • Bacteremia: bacteria circulating in blood stream

    • Septicemia: bacteria multiplying in bloodstream

    • Toxemia: toxins circulating in bloodstream

    • Viremia: viruses circulating in bloodstream

  • Sepsis: acute, life-threatening inflammation caused by infectious agents/products in bloodstream

Establishing the Cause of Infectious Diseases, pic

Koch’s Postulates

• Developed by Robert Koch to establish that Bacillus anthracis causes anthrax

• Today, provide the criteria for establishing that a given microbe causes a specific infectious disease

1. Microorganism is present in every case of disease

2. Organism is grown in pure culture from diseased

host

3. When pure culture is introduced into susceptible

hosts → produce same disease

4. Organism is recovered from experimentally infected hosts

Mechanisms of Pathogenesis

Successful pathogen overcome host defenses only long enough to multiply and exit

• If pathogen overwhelms and kills host, it loses the chance for multiplying and opportunities for transmission

Thus, pathogens and hosts generally evolve toward balanced pathogenicity

• Pathogen becomes less virulent while host becomes less susceptible

Pathogens cause disease by:

1. Producing toxins

• Clostridium botulinum, Staphylococcus aureus

2. Colonizing mucous membranes, producing toxins

• Vibrio cholerae, E. coli O157:H7, Corynebacterium diphtheriae

3. Invading host tissues by evading host defenses

• Mycobacterium tuberculosis, Yersinia pestis, Salmonella enterica