Microbe–Human Interactions: Infection and Disease

Chapter 13 – Microbe–Human Interactions: Infection, Disease, and Epidemiology

A & P Word Roots

  • Understanding A & P (Anatomy and Physiology) words using root meanings can aid in deciphering terms encountered during the course.

  • Important roots to know:

    • cyto- = cell

    • hepat- = liver

    • hemo- = blood

    • leuko- = white (as in white blood cells)

    • entero- = intestine

    • neuro- = nerve/nervous system

    • kerat- = skin

    • derma- = skin

    • gastro- = stomach

    • encepha- = brain

    • meningo- = membrane (referring to the meninges covering the brain)

    • osteo- = bone

    • cardio- = heart

    • endocardio- = lining of heart

    • nephro- = kidney (nephron)

    • pneumo- or pulmo- = lung

    • myo- = muscle

    • necro- = death

    • -itis = inflammation

    • -emia = blood condition

    • -osis = abnormal condition/disease

    • -oma = growth/tumor

    • -pathy = disease/disorder

    • -lysis = burst/destruction

Resident Microbes and Humans

  • Microbial Interaction Types:

    • Some microbes are commensal (do not cause harm).

    • Some are beneficial (mutualistic), while others cause disease.

    • Transient microbes inhabit the body temporarily and can be removed by body defenses or hygiene before colonization.

  • Resident Microbes (Normal Flora):

    • Microbes that colonize the body and are normally found in specific regions (known as human microbiota or microbiome).

    • The Human Microbiome Project identified 10,000 different species (bacteria, fungi, archaea, protozoa, viruses).

    • Balanced normal flora is beneficial for:

    • Digestion of carbohydrates.

    • Formation of protective mucous layers.

    • Generation of vitamins.

    • Development of immune system.

    • Dysfunctional normal flora is associated with conditions such as obesity, allergies, autoimmune diseases, and diabetes.

  • Colonization Areas:

    • Epidermis and skin follicles.

    • Mucous membranes (mouth, colon, respiratory tract, urogenital tract, eyes, and ears).

    • 3 to 10 times more microbes than human cells.

    • Example: 30% of fecal volume is composed of bacteria.

  • Sterile Areas:

    • Internal organs (heart, liver, kidneys, lungs, brain, spinal cord, etc.) are typically sterile.

    • Presence of microbes in these areas indicates infection.

  • Infection Potential:

    • Resident microbes generally do not cause infections unless:

    • The immune system is compromised.

    • They invade areas inappropriately (e.g., Escherichia coli in urinary tract infections).

  • Microbial Presence in Pregnancy:

    • Minimal species present in the uterus and amniotic fluid.

    • Normal flora established post-birth, expanding through maternal contact.

  • Microbiota Development Influences:

    • Age, diet, disease state, and antibiotic usage.

    • Probiotics: microorganisms taken to adjust resident microbes.

Microbes and Disease

  • Definition of Infection:

    • Invasion and colonization of tissues by microbes.

    • When infection results in tissue damage, it’s termed infectious disease and the microbe is a pathogen.

  • Noninfectious Diseases:

    • Caused by factors such as genetics, diet, aging, etc. (Not covered in this course).

  • Host Definition:

    • An individual that contracts a pathogen.

  • Host Immune System Role:

    • Immunocompetent hosts: individuals with normal immune function.

    • Immunocompromised hosts: individuals with weakened immune systems (e.g., children, elderly, those with certain diseases).

  • Pathogen Types:

    • True (Primary) Pathogens: can cause disease in healthy individuals (e.g., flu virus).

    • Opportunistic Pathogens: cause disease predominantly in immunocompromised hosts; may arise from overgrowth of normal flora.

  • Pathogenicity and Virulence:

    • Pathogenicity: the ability of a microbe to cause disease.

    • Microbes are classified on a scale of pathogenicity (Level 1 to Level 4).

    • Virulence: the likelihood of a pathogen causing infection; determined by virulence factors.

Infection Phases from the Microbe’s Perspective

  1. Entry:

    • Pathogen enters through specific portals (e.g., mouth, nose, wounds).

    • Some pathogens can enter through multiple portals; others require specific ones.

    • Parenteral route: bypassing skin/mucous membranes.

    • Sexually transmitted infections (STIs): enter via urogenital tract.

    • Transplacental transmission: possible during pregnancy, exposing fetus to pathogens.

    • Infectious Dose (ID): minimum number of microbes required for infection.

  2. Attachment:

    • Pathogen adheres to host tissues using structures specific to their type.

    • *Attachment Structures:

      • Bacteria: fimbriae, slime layer, or capsule.

      • Viruses: spikes.

      • Helminths: hooks and suckers.

  3. Invasion:

    • Pathogen evades immune response or destroys tissue to advance into deeper tissues.

    • Virulence Factors: Types:

      • Antiphagocytic Factors: evade immune cells (e.g., capsules and leukocidins).

      • Exoenzymes: destroy host tissues (e.g., collagenase, hyaluronidase, coagulase).

      • Exotoxins: proteins secreted that target specific cells (e.g., hemolysins, neurotoxins).

      • Endotoxins: released as bacteria lyse, can cause systemic inflammation.

  4. Exiting Host:

    • Pathogens exit through portals specific to them (e.g., skin, secretions, excretions, blood).

Course of Clinical Infection from the Host’s Perspective

  • Stages:

    1. Incubation Period: time from exposure to first symptoms (typically 2-30+ days).

    2. Prodromal Stage: general symptoms (headache, fatigue). Duration: 1-2 days.

    3. Period of Invasion: worst symptoms, high pathogen replication.

    4. Convalescent Period: immune system clears the pathogen; recovery begins.

Patterns of Infection

  • Types:

    • Acute Infection: rapid development, short-lived symptoms.

    • Chronic Infection: develops slowly, long-lasting.

    • Latent Infection: dormant, reactivation possible.

    • Local Infection: confined to a specific area.

    • Focal Infection: spreads, toxins affect other areas.

    • Systemic Infection: spreads throughout the body.

    • Mixed Infection: multiple pathogens at the same site.

    • Primary Infection: initial infection.

    • Secondary Infection: follows a primary due to immune suppression.

    • Sequelae: long-term effects post-infection.

Diagnosis and Symptoms

  • Signs: observable indicators (e.g., temperature, swollen lymph nodes).

  • Symptoms: subjective experiences (e.g., pain, nausea).

  • Inflammation Signs:

    • Edema, granulomas, swollen lymph nodes.

  • **Blood Indicators:

    • Leukocytosis: increased white blood cells.

    • Bacteremia: small bacteria presence in blood.

    • Septicemia: large bacterial numbers leading to sepsis.

    • Antibodies: indicators of specific infections.

  • Asymptomatic/Infections: some individuals show no symptoms yet can still be infected.

Epidemiology

  • Definition: study of disease frequency, distribution, and factors affecting health in populations.

  • Key Aspects: investigate the who, when, where, how, and why of disease spread.

Epidemiological Focus

Where are Pathogens Found?

  • Reservoirs: living or non-living spaces where pathogens survive.

  • Living Reservoirs: humans (carriers/asymptomatic) and animals (vectors).

  • Carrier Types:

    • Asymptomatic Carrier: infected, shows no symptoms.

    • Passive Carrier: contaminated, transfers without infection.

    • Zoonotic Diseases: pathogens from animals to humans (e.g., through bites).

Patterns of Transmission

  • Methods of pathogen transmission:

    • Direct Contact Transmission: direct interaction between infected carriers and susceptible hosts.

    • Indirect Contact Transmission: pathogen transfer via vehicles (inanimate objects).

    • Vehicles: food, water, air (including aerosols, droplet nuclei).

    • Mechanical Vectors: animals that carry pathogens on their surface but are not infected.

Epidemiological Investigation and Surveillance

  • Surveillance: collecting and analyzing disease occurrence data.

  • Statistics:

    • Prevalence: total cases at a time.

    • Incidence: new cases over time.

    • Morbidity Rate: data analyzing illness occurrences.

  • Disease Classification Patterns:

    • Endemic: constant presence in a region.

    • Sporadic: irregular, unlinked cases.

    • Epidemic: sudden increase in cases.

    • Pandemic: global epidemic.

Koch’s Postulates

  • Criteria to establish a causative relationship between a microbe and disease:

    1. The pathogen is found in every case of the disease but not in healthy individuals.

    2. It can be isolated and grown in pure culture.

    3. A healthy subject infected develops similar disease symptoms.

    4. The pathogen must be re-isolated from the new host and identical.

Healthcare-Associated Infections (HAIs)

  • Defined as infections acquired in medical facilities.

  • HAIs affected rates: 0.1% to 20% depending on facility.

  • Common Infections: Surgical, respiratory, GI tract, skin, urinary tract, and bloodstream infections.

  • Infection Spread Methods: Patient-to-patient, worker-to-patient, etc.

  • Prevention Methods:

    • Proper sterilization and aseptic insertion of medical devices.

    • Handwashing, disinfection, and patient isolation.

  • Standard Precautions (SPs): Guidelines for handling patients/bodily substances to reduce infectious spread.