chapter 14:


Introduction to Microbes

  • Microbes are the focus of study in the context of infectious diseases.

Normal Flora

  • Definition: Normal flora refers to the collection of microbes that inhabit the surfaces (inside and outside) of a healthy person.

  • Population: There is a staggering ratio of microbes to human cells; approximately 10 times more microbes are present than actual human cells.

  • Weight of Microbes: Collectively, the microbes residing in and on the body weigh about half a pound.

Acquisition of Normal Flora

  • Sterile Environment: During fetal development, the uterus provides a sterile environment; consequently, the fetus has no microbes.

  • Birth Process: The process of birth is crucial for the acquisition of normal flora:

    • As the baby passes through the birth canal, it acquires microbes from the mother.

    • Upon exposure to the external environment, the baby inhales non-sterile air, further introducing microbes.

  • Diet: Additionally, the food consumed post-birth is not sterile, contributing to the microbiome.

  • Physical Contact: Any physical interaction, including touching by caregivers, exposes the baby to microbes.

Locations of Flora in Healthy Individuals

  • Skin: The skin is heavily populated by a variety of microbes, forming a key component of the body's normal flora.

  • (Additional details or slides regarding other body areas may follow but are not provided in the current excerpt.)

Microbial Distribution in the Human Body

  • Skin Flora

    • Microbes are not on the surface but deeply embedded in:

      • Hair follicles

      • Glands and ducts

    • Normal flora is crucial for skin health.

  • Digestive System

    • High density of microbes exists in:

      • The upper part of the digestive system, especially the mouth (highest density)

      • Large intestines

    • Flora is predominantly found at the two ends of the digestive tract.

  • Respiratory System

    • Flora present in upper regions:

      • Nasal cavities

      • Trachea

    • Not found in the lungs under normal conditions.

  • Urogenital System

    • Flora present in:

      • Distal end of the urethra in both males and females

      • Vagina in females

    • Normally absent in the uterus, which is a sterile environment for the fetus.

Symbiotic Relationships

  • Definition

    • Symbiosis: A close relationship between two organisms.

  • Types of Symbiosis

    • Commensalism: One organism benefits, the other is unaffected.

      • Common among body flora.

    • Mutualism: Both organisms benefit.

    • Parasitism: One organism benefits at the other's expense, potentially leading to disease.

Benefits of Normal Flora

  • Immune Development

    • Aids in the normal development of the immune and digestive systems.

  • Vitamin Production

    • Example: E. Coli produces Vitamin K in the large intestines.

  • Microbial Antagonism

    • Normal flora competes with potential pathogens, helping to ward off diseases.

Pathogens

  • Definition

    • Pathogens are microbes that can cause diseases.

  • Types of Pathogens

    • Opportunistic Pathogens: Generally harmless but can cause disease if they overgrow or invade other body areas.

    • Obligate Pathogens: Require a host for survival and must infect to replicate and obtain nutrients.

Flora Types

  • Transient Flora

    • Picked up during daily activities (e.g., touch) and often responsible for diseases.

  • Permanent Normal Flora

    • Less likely to cause disease compared to transient flora.

Disease Prevention

  • Handwashing

    • Essential to protect against diseases caused by transient flora.

Germ Theory of Disease

  • Introduction

    • Associated with Dr. Cook and the concept that microbes can cause diseases.

  • Cook's Postulates

    • Process to identify disease-causing microbes.

    1. Isolate and identify the microbe from individuals with the disease.

    2. The same microbe must be present in all individuals with the disease.

    3. Expose a healthy test organism to the microbe and it must develop the same disease.

    4. Isolate and identify the same microbe from the test organism, confirming it as a causative agent of the disease.

  • Key Takeaway

    • Consistency of the microbe in both the original and test organisms

Classifying Diseases

  • Classification of diseases involves using specific terms to categorize different types of diseases.

    • Important to memorize a list of microbes as they will help in classifying diseases during exams.

Signs vs. Symptoms

  • Signs:

    • Objective measures of disease, observed/measured by health care providers.

    • No input from the patient required to identify.

    • Examples include:

      • Fever (measured with a thermometer)

      • Septicemia (microbes in the blood)

      • Chest sounds, skin eruptions

      • Leukocytosis (high white blood cell count) and leukopenia (low white blood cell count)

      • Swollen lymph nodes (can be palpated)

      • Abscesses, tachycardia (increased heart rate), antibodies in serum.

  • Symptoms:

    • Subjective experiences/feelings described by the patient.

    • Important for diagnosis but cannot be directly observed by health care providers.

    • Examples include:

      • Chills, pain, nausea, malaise, chest tightness, itching, headache, weakness, abdominal cramps, anorexia (loss of appetite), sore throat.

Disease Transmission

  • Types of Transmission:

    • Non-communicable diseases:

      • Not transmitted from person to person.

      • Examples include:

        • Systemic mycosis (fungal disease from inhaled spores)

        • Tetanus (caused by bacteria entering through wounds)

    • Communicable diseases:

      • Transmitted from person to person.

      • Example diseases include those on the list of microbes.

    • Contagious diseases:

      • A subset of communicable diseases that are very easily transmitted.

      • Illustrated by R0 (basic reproduction number): how many people one infected person will likely infect.

Examples of R0 Values

  • Measles: Airborne, R0 of 12 to 18 (very contagious).

  • Smallpox: Airborne, R0 of 5 to 7.

  • Influenza: R0 of 2 to 3.

  • Ebola: Bodily fluids, R0 of 1.5 to 2.5.

  • COVID-19: R0 of 1.4 to 2.5, similar to the seasonal flu (1 to 2).

Frequency of Occurrence

  • Four terms used to describe how common a disease is in a population:

    • Endemic:

      • Constantly present within a population.

      • Example: The common cold is always present.

    • Sporadic:

      • Occasional increases in disease occurrence; small outbreaks.

      • Example: Measles outbreaks due to vaccination drops.

    • Epidemic:

      • Large numbers affected over a short time; risk of sporadic could escalate to epidemic.

    • Pandemic:

      • Many affected globally—an epidemic across multiple countries or continents.

Severity of Disease

  • Terms relating to the duration and severity of signs and symptoms:

    • Acute:

      • Short duration (days to weeks) with strong symptoms.

      • Examples include: Common cold, flu, COVID-19.

    • Chronic:

      • Long duration (months to years) with mild symptoms.

      • Example: Hepatitis.

    • Latent:

      • Inactive microbe period with no symptoms, can reactivate later.

      • Examples: HIV/AIDS, chickenpox leading to shingles.

Host Involvement

  • Terms regarding the involvement of the host in disease:

    • Localized Infection:

      • Infection restricted to one area of the body; example: Staph infection.

    • Systemic Infection:

      • Microbe spreads throughout the body; examples include measles.

      • Related terms:

        • Toxemia: Toxins in the blood.

        • Viremia: Viruses in the blood.

        • Bacteremia: Bacteria in the blood.

        • Septicemia: Actively replicating bacteria in the blood (very dangerous).

    • Focal Infection:

      • Infection starts in one place and travels to a specific second location; example: Streptococcus pyogenes causing strep throat, leading to heart valve issues if untreated.

Primary and Secondary Infections

  • Primary Infection:

    • Initial infection caused by the first microbe that infects the host.

    • Example: Chickenpox causing skin lesions.

  • Secondary Infection:

    • Occurs due to damage from a primary infection, allowing another microbe to invade.

Introduction to Epidemiology

  • Epidemiology: Study of how disease moves in a population.

  • Focus: Story of disease from the microbe's perspective.

The Life Cycle of a Microbe

  • Source of Microbes: Originates from a reservoir, which is the continual source where microbes can replicate and obtain nutrients.

  • Transmission Pathway:

    • Microbe must be transmitted to an individual who will become ill.

    • Once entered, the microbe causes disease.

    • Finally, the microbe exits the body to infect new hosts.

Reservoirs of Microbes

Types of Reservoirs

  • Nonliving Reservoirs: Natural habitats for various pathogens.

    • Soil:

      • Home to fungi and Clostridium species (e.g., Clostridium tetani, Clostridium botulinum).

    • Water:

      • Pathogens like Entamoeba histolytica which causes traveler's diarrhea.

  • Living Reservoirs: Organisms (animals or humans) that harbor microbes.

    • Common Living Reservoirs:

      • Pigs and poultry: Known for harboring influenza viruses.

      • Bats: Reservoirs for rabies, Ebola, and potentially COVID-19.

      • Children:

        • Particularly concerning in the U.S. due to undeveloped immune systems and lack of hygiene.

    • Ill Individuals and Carriers:

      • Ill individuals actively replicate pathogens, increasing transmission risk.

      • Carriers (e.g., Typhoid Mary): Do not show symptoms but can transmit the pathogen.

Modes of Transmission

Types of Transmission

  • Direct Contact: Involves touching someone harboring the microbe (e.g., handshake, hugs, sexual contact, bites).

  • Indirect Contact: Microbe travels from an infected person to an object (fomite) and then to a new person.

    • Examples of fomites: Glasses, tissues, doorknobs.

  • Droplet Transmission:

    • Involves the release of microscopic droplets through coughing, sneezing, talking.

    • Requires close proximity (within 1 meter) to transmit.

  • Vehicle Transmission: Medium that transmits microbes from reservoirs to individuals.

    • Water: Contaminated water sources transmit pathogens like cholera and E. coli.

    • Food: Bacteria contamination during processing (e.g., salmonella in chicken, E. coli in beef).

    • Air: Can carry microbes over distances greater than one meter (e.g., smallpox).

    • Bodily Fluids: Contaminated bodily fluids (e.g., blood, saliva) are critical in transmission (e.g., Ebola).

  • Vector Transmission: Involves living organisms, typically insects, transferring microbes between hosts.

    • Mechanical Transmission: Microbes on the surface of vectors (e.g., flies transferring E. coli from feces to food).

    • Biological Transmission: Vectors are infected and inject pathogens into hosts (e.g., mosquitoes transmitting malaria, West Nile virus).

Entry Points for Microbes

Portals of Entry

  • Mucous Membranes: Most common entry points. Key areas include:

    • Respiratory system (breathing in microbes).

    • Gastrointestinal system (ingesting contaminated food/drink).

    • Urogenital system (sexual contact leading to STDs).

  • Skin: Secondary entry route – breaks or abrasions can lead to infection.

  • Parenteral Route: Injection into the bloodstream, less common but significant in certain infections.

Exit Points for Microbes

Portals of Exit

  • Similar to entry points:

    • Mucous membranes (respiratory system, gastrointestinal tract).

    • Skin.

    • Parenteral route via blood.

Conclusion

  • The cycle of disease: Reservoir → Transmission → Portal of Entry → Portal of Exit.

Predisposing Factors for Infectious Diseases

1. Gender

  • Physical Structure:

    • Females are predisposed to urinary tract infections (UTIs) due to anatomical distance between the urethra and anus.

    • Shorter distance facilitates E. coli migration to the urethra, increasing UTI risk.

  • Hormonal Influence:

    • Males are more prone to meningitis, influenced by male hormones affecting immune system response.

2. Genetics

  • Inherited Genes:

    • Certain genetic traits can predispose individuals to diseases.

    • Example: Sickle cell anemia results from a genetic mutation that alters red blood cell shape, leading to health complications.

    • Individuals with sickle cell traits often have roots in malaria-prone areas, providing some malaria resistance due to heterozygosity.

3. Climate and Location

  • Weather Impact:

    • Seasonal changes affect disease prevalence, such as respiratory diseases in winter due to drier air drying out mucous membranes.

  • Vector-Borne Diseases:

    • Living in regions with dangerous vectors (e.g., mosquitoes carrying malaria or Zika) increases vulnerability to these diseases.

    • Example: Tsetse flies in Africa can transmit trypanosomiasis (African sleeping sickness).

4. Immunocompromised States

  • Age:

    • Infants (immature immune systems) and elderly individuals (declining immune function) show higher susceptibility to infections.

  • Diet:

    • Malnutrition, whether from famine or poor dietary choices, impacts immune effectiveness.

  • Lifestyle Choices:

    • Smoking and excessive alcohol consumption weaken the immune response, increasing infection risk.

Stages of Disease Progression

  • Incubation Period:

    • Begins upon encountering a microbe; no signs or symptoms present; microbes acclimate to the host.

  • Prodromal Stage:

    • Mild symptoms may arise; not all diseases exhibit this stage.

  • Period of Illness:

    • Most severe symptoms occur; change in white blood cell counts indicates immune response strength.

  • Period of Decline:

    • Microbial numbers decrease as the immune system fights infection; signs and symptoms lessen.

  • Convalescence:

    • Recovery phase where the body heals from damage sustained during the illness; individuals can still be infectious.

Healthcare-Associated Infections (Nosocomial Infections)

  • Definition: Infections acquired by patients in healthcare facilities, often due to compromised immunity.

  • Environmental Factors:

    • High microbial load in hospitals due to infected patients.

  • Transmission:

    • Transmitted by healthcare workers through direct, indirect contact, or droplets.

  • Prevention:

    • Emphasis on handwashing and surface disinfection in healthcare settings.

Summary of Disease Concepts

  • Classification: Understanding types of diseases and their transmission and predisposing factors is crucial for prevention.

  • Respiratory Diseases:

    • Contagious; primarily spread through droplet transmission.

    • Common predisposing factors include age, diet, smoking, and seasonal weather.

  • Gastrointestinal Diseases:

    • Communicable but not contagious; require close contact for transmission.

  • Vector-Borne Diseases:

    • Non-communicable; require vectors for transmission.

Types of Epidemiological Studies

1. Descriptive Epidemiology

  • Originated by John Snow; analyzes disease prevalence post-outbreak; focuses on identifying reservoirs and transmission modes.

2. Analytical Epidemiology

  • Pioneered by Florence Nightingale; examines characteristics that predispose individuals to diseases; focuses on patterns of health and illness.

3. Experimental Epidemiology

  • Tests treatments and interventions to prevent or cure diseases; involves controlled experiments with treatment and placebo groups.