microniology exam2

Introduction to Microbe–Human Interactions

  • Learning changes everything.
  • Chapter 13: Microbe–Human Interactions: Health and Disease
  • Authors: Marjorie Kelly Cowan, Heidi Smith
  • Source: McGraw Hill LLC

Section 13.1: The Human Host and Its Microbiome

  • Learning Outcomes:
    • Differentiate among colonization, infection, and disease.
    • Identify sites where normal biota is found in humans.
    • Discuss the impact of the Human Microbiome Project on understanding normal biota.

The Human Microbiome and Host

  • Each human body houses trillions of microbes, with some necessary and beneficial, all impactful.
  • The resident microbiota, known as the human microbiome, colonizes us long-term and typically does not cause disease.
  • Holobiont: a human plus all of its resident microbiota.

Colonization, Infection, Disease

  • Definitions:

    1. Infection:
      • Condition in which pathogenic microorganisms penetrate host defenses, enter tissues, and multiply.
    2. Disease:
      • Pathological state resulting from cumulative infection effects; disrupts tissues and organs.
      • Any deviation from health.

  • Factors that Cause Disease:

    • Infections
    • Genetics
    • Aging
    • Malfunctions of systems or organs
    • Infectious Disease:
      • Disruption caused by microbes or their products.

Resident Biota

  • Normal Biota:
    • Large, diverse microbes inhabiting the human body.
    • Known also as resident or indigenous biota or normal flora.
    • Includes bacteria, fungi, protozoa, and viruses.
    • Profoundly influences human biology.

Relationships Among Microbes and the Human Host

  • The human body is colonized by resident microbes, transient microbes, and disease-causing microbes.
  • Resident microbes are crucial for maintaining health; transient microbes do not typically remain in the body long.

The Human Microbiome Project

Preliminary Results

  • Human cells have 21,000 protein-coding genes; microbes have 8-20 million.
  • Microbes are found in previously sterile areas of the body.
  • Healthy individuals harbor various viruses, including pathogens, in low numbers.
  • The microbiome can influence the success of viral infections (e.g., influenza, HIV) and the efficacy of treatments.

Current Understanding of Sites with Normal Microbiota

Definitive Sites
  • Skin and adjacent mucous membranes
  • Upper respiratory tract
  • Gastrointestinal tract (mouth included)
  • Outer urethra, bladder, urine
  • External genitalia, vagina
  • External ear canal, external eye (lids and conjunctiva)
  • Breast milk
Additional Sites with Microbiota Identity
  • Normal Biota Presence Considered:
    • Lungs (lower respiratory tract)
    • Placenta, amniotic fluid, fetus
    • DNA from microbiota: found in the brain and bloodstream.

Acquiring the Microbiota

Benefits of Normal Biota

  • Influence organ development and growth.
  • Prevent overgrowth of harmful microorganisms

Microbial Antagonism

  • The antagonistic effect that beneficial microbes have against intruder microorganisms.
  • Established relationships with microbes discourage incoming species.

Factors Weakening Host Defenses

  • Increased Susceptibility:
    • Age (very young or very old)
    • Genetic defects in immunity or acquired defects (e.g. AIDS)
    • Pregnancy
    • Surgery and organ transplants
    • Underlying diseases (cancer, diabetes)
    • Chemotherapy/immunosuppressive drugs
    • Physical and mental stress
    • Other infections

Endogenous Infections

  • Caused by biota present in the body.
  • Occurs when normal biota enters a site that was previously sterile (e.g., Streptococcus pneumoniae can cause pneumonia in AIDS patients).

The Start of Colonization

  • Fetuses may be seeded with microbiota in utero.
  • Exposure occurs during birth (mother’s vaginal biota).
  • Breast milk contains ~600 species of bacteria and sugars for gut bacteria.

Life on Humans: Sites with Established Biota

Anatomical Sites and Common Genera

  1. Skin:
    • Common Genera:
      • Gram-positive: Staphylococcus, Propionibacterium, Streptococcus, Corynebacterium, Lactobacillus
      • Gram-negative: Bacteroides, Prevotella, Haemophilus
      • Fungi: Candida
  2. GI Tract:
    • Oral cavity:
      • Gram-positive: Streptococcus, Actinomyces
      • Gram-negative: Haemophilus, Prevotella
      • Fungi: Candida
    • Intestinal tract:
      • Gram-negative: Bacteroides, Prevotella
      • Fungi: Candida
  3. Respiratory Tract:
    • Nose:
      • Gram-positive: Propionibacterium, Corynebacterium
      • Gram-negative: Moraxella
    • Throat:
      • Similar bacteria as in oral cavity.
    • Lungs:
      • Previously thought to be sterile.
  4. Vagina:
    • Gram-positive: Lactobacillus, Streptococcus
  5. Urinary Tract:
    • Gram-positive: Lactobacillus, Enterococcus

Concept Check—Section 13.1

  1. Infection: A condition of pathogen penetration and multiplication.
  2. The Human Microbiome Project.
  3. False: The lungs are not sterile.
  4. True: Gut biota impacts mood and mental health.
  5. Factors: Age, immunodeficiencies, health conditions.
  6. False: C-section babies have different biota than vaginally born babies.

Section 13.2: When Colonization Leads to Disease

Learning Outcomes

  • Variables influencing microbe disease causation.
  • Differentiate pathogenicity and virulence.
  • Steps for microbes to cause disease.
  • Sources and targets of microbial entry and exit.
  • Define infectious dose (ID) and its role.

Pathogen Definitions

  • Pathogen: Microbe in a parasitic relationship causing disease.
  • True Pathogens: Cause disease in healthy individuals.
  • Opportunistic Pathogens: Cause disease when host defenses are compromised or in incorrect body location.

The Biosafety Levels

  • Biosafety Levels (BSLs):
    • System adopted by CDC based on pathogenicity and handling danger.
Biosafety Levels Overview:
  1. Level 1:
    • Low risk; non-pathogenic microbes.
  2. Level 2:
    • Moderate risk; can infect healthy individuals.
  3. Level 3:
    • Severe disease risk via inhalation; stringent controls.
  4. Level 4:
    • High virulence; extreme risk; isolated facilities required.

Virulence Factors

  • Virulence: Degree of pathogenicity based on ability to establish in host and cause damage.

  • Virulence Factors: Characteristics that enable a pathogen to cause damage.

Infectious Dose (ID)

  • Infectious Dose: Minimum microbes required for infection.
    • Smaller ID indicates higher virulence.

Host Variability

  • **Host-Related Variables: **
    • Genetics, health, and previous exposure affecting susceptibility.

Polymicrobial Infections

  • Many infections are caused by several microbes (e.g., influenza leading to bacterial pneumonia).

Steps to Disease

Portal of Entry

  • A characteristic route taken by a microbe to initiate infection via skin or mucous membranes.
    • Exogenous vs. Endogenous Sources:
    • Exogenous: outside the body.
    • Endogenous: already existing in the body.

Methods of Entry

  • Skin:
    • Entry via nicks and abrasions.
  • Gastrointestinal Tract:
    • Entry through ingested substances.
  • Respiratory Tract:
    • Entry via inhalation through oral and nasal passages.
  • Urogenital Tract:
    • STIs via skin or mucosa.
  • Transplacental Infections:
    • Some pathogens can cross the placenta during pregnancy.

Step 2: Attachment to the Host

  • Adhesion: Process allowing stable foothold on host tissues.
    • Pathogens can only attach to specific cells.
    • Quorum Sensing: Bacterial communication that aids infection establishment.

Step 3: Surviving Host Defenses

  • Phagocytes: White blood cells that destroy pathogens.
    • Antiphagocytic Factors: Mechanisms preventing phagocytosis.

Step 4: Causing Disease

  • Virulence Factors: Mechanisms that enable pathogens to inflict damage.
    • Damage can occur via secretion of enzymes/toxins or excessive host response.

Microbial Damage Methods

  • Secreting Proteins:
    • Breakdown and inflict damage on host tissues.
  • Excessive Immune Response:
    • Damage caused by host immunity, making disease severity higher.
  • Altering Host Cell Processes:
    • Changing gene expression through epigenetic modifications.

Types of Damage

  • Direct Damage via Exoenzymes:
    • Secreted by pathogens to dissolve barriers and promote their spread.
    • Examples include mucinase, keratinase, and hyaluronidase.

Toxins

  • Toxin: Poisonous chemical product by organisms.
    • Exotoxin:
  • Secreted by living bacterial cells.
  • Can exert potent effects in small amounts.
    • Endotoxin:
  • Not secreted; part of the gram-negative bacterial cell wall.

Inducing Host Damage

  • Host cellular damage can arise from an overstimulated immune response.

Patterns of Infection

Infection Types

  1. Localized Infection:
    • Confined to specific tissue (e.g., boils).
  2. Systemic Infection:
    • Spreads throughout body (e.g., viral infections).
  3. Focal Infection:
    • Breaks away from local infection.
  4. Mixed Infection:
    • Multiple agents at the infection site.
  5. Primary and Secondary Infection:
    • Initial vs. subsequent infections.

Infection Duration

  • Acute Infections: Rapid onset and short duration.
  • Chronic Infections: Long-lasting and progressive.

Signs and Symptoms

  • Sign: Objective evidence of disease.
  • Symptom: Subjective evidence sensed by the patient.
  • Syndrome: A defined disease based on a combination of signs and symptoms.
  • Inflammation: Earliest symptom of infection characterized by edema, granulomas, and lymphadenitis.

Blood Infections

  • Leukocytosis: Increase in white blood cells.
  • Leukopenia: Decrease in white blood cells.
  • Septicemia: High levels of microbes in blood.
  • Bacteremia: Presence of bacteria without replication.
  • Viremia: Presence of viruses.

Asymptomatic Infections

  • Infections without noticeable symptoms leading to unrecognized disease transmission.

Portals of Exit

  • Pathogens exit through various bodily secretions such as mucus, feces, blood, and urine.

Long-term Effects

  • Latency: Dormant state allowing recurrent disease.
  • Sequelae: Long-term damage due to infections, such as hearing loss from meningitis.

Course of Infection

  • Incubation Period: Initial exposure to first symptoms.
  • Prodromal Period: Early identifiable symptoms.
  • Acute Phase: Peak infection virulence.
  • Convalescent Stage: Recovery phase with declining symptoms.

Reservoirs of Pathogens

  • Reservoir: Habitat where the pathogen lives.
  • Source: Any individual/environment from which infection is acquired.

Carrier States

  • Carriers: Individuals harbor pathogens asymptomatically, facilitating transmission.
    • Examples include Asymptomatic, Incubating, Convalescent, Chronic, and Passive carriers.

Living Reservoirs

  • Humans and animals serve as direct or indirect transmission sources, utilizing vectors for movement between hosts.

Zoonosis

  • Zoonosis: Diseases originating in animals but transmissible to humans.
    • Human infections do not contribute to pathogen's persistence.
    • Risks vary with occupations (e.g. animal handling).

Common Zoonotic Infections

Viral Diseases

  • Examples: Rabies, Yellow fever, Hantavirus.

Bacterial Diseases

  • Examples: Anthrax, Leptospirosis, Plague.

Miscellaneous Diseases

  • Examples: Ringworm, Toxoplasmosis, Trypanosomiasis.

Nonliving Reservoirs

  • Microbes adapted to soil, air, and water; mostly non-pathogenic but occasionally opportunistic.

Disease Communication

  • Communicable Disease: Transferrable from host to host.
  • Contagious Epidemiology: Easily transmissible diseases through direct or indirect contact.
  • Noncommunicable Disease: No host-to-host transmission.

Modes of Transmission

  1. Horizontal: Infectious spread within a population.
  2. Vertical: From parent to offspring.

Koch’s Postulates

  • Developed by Robert Koch to establish causation of diseases via proof protocols.
  1. Evidence of microbe in every case of a specific disease.
  2. Isolate and cultivate microbe in pure culture.
  3. Inoculate healthy subject to observe resulting disease.
  4. Reisolate from the subject.

Molecular Koch’s Postulates

  • Modern criteria using molecular techniques to determine gene contribution to pathogenic abilities.

Concept Check—Section 13.2

  1. False: Ingested virus may cause disease.
  2. Infectious Dose (ID).
  3. Exotoxins are toxic even in minute amounts.
  4. Signs indicate disease objectively.
  5. During acute phase, greatest virulence is observed.

Conclusion

  • Understanding interactions between microbes and the human body is essential for addressing health and disease.
  • Continued research on the human microbiome and pathogen behavior is integral to public health.