Symbiotic Relationships Between Microbes and Their Hosts

Types of Symbiosis

Symbiosis refers to the interaction between two different biological organisms living in close physical association. Humans have symbiotic relationships with numerous microorganisms, which can be classified as follows:

• Mutualism: Both organisms benefit from the relationship.

• Commensalism: One organism benefits, while the other is neither helped nor harmed.

• Amensalism: One organism is harmed, and the other is unaffected.

• Parasitism: One organism benefits at the expense of the other.

Normal Microbiota in Hosts

Definition and Types

Normal microbiota, also known as normal flora or indigenous microbiota, are organisms that colonize the body's surfaces without causing disease under normal circumstances. They are divided into:

• Resident microbiota: Remain throughout life and are mostly commensal.

• Transient microbiota: Present for a short period; cannot persist due to competition, elimination by host defenses, or changes in the body.

Distribution of Resident Microbiota

Resident microbiota are found in various regions of the body, including the digestive tract, respiratory tract, urinary and reproductive systems, and eyes. Their composition varies by location and environmental conditions.

Acquisition of Normal Microbiota

Humans develop microbiota during the birthing process, as the womb is typically free of microorganisms. The resident microbiota is established in the first months of life.

Opportunistic Pathogens

How Normal Microbiota Become Opportunistic Pathogens

Opportunistic pathogens are normal microbiota that cause disease under certain conditions, such as:

• Introduction into unusual sites

• Immune suppression

• Changes in abundance

• Stressful conditions

Reservoirs of Infectious Diseases

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as sources of infection. They include:

• Animal reservoirs: Zoonoses are diseases naturally spread from animals to humans.

• Human carriers: Asymptomatic individuals can transmit pathogens.

• Nonliving reservoirs: Soil, water, and food contaminated by feces or urine.

Common Zoonoses

Invasion and Establishment of Microbes in Hosts: Infection

Exposure to Microbes: Contamination and Infection

• Contamination: Presence of microbes in or on the body.

• Infection: Microbes evade defenses, multiply, and establish themselves.

Portals of Entry

Pathogens enter the body through:

• Skin: Through cuts, openings, or by burrowing.

• Mucous membranes: Respiratory tract (most common), gastrointestinal tract.

• Placenta: Can be crossed by some pathogens, causing fetal harm.

• Parenteral route: Circumvents usual portals; pathogens deposited directly into tissues.

The Role of Adhesion in Infection

Adhesion Factors

Adhesion is the process by which microorganisms attach to host cells, essential for colonization. Adhesion factors include specialized structures and attachment molecules (ligands or adhesins). Blocking these can prevent infection.

The Nature of Infectious Disease

Manifestations: Symptoms, Signs, and Syndromes

• Symptoms: Subjective, felt only by the patient.

• Signs: Objective, observed or measured by others.

• Syndrome: Group of symptoms and signs characterizing a disease.

• Asymptomatic: No symptoms, but signs may be present.

Etiology and Koch’s Postulates

Determining the Cause of Disease

Etiology is the study of disease causation. Koch's postulates are criteria used to prove a specific pathogen causes a specific disease, forming the basis of the germ theory of disease.

Exceptions to Koch’s Postulates

• Some pathogens cannot be cultured.

• Diseases may be caused by multiple pathogens or cofactors.

• Ethical constraints prevent testing in humans.

Virulence Factors of Infectious Agents

Pathogenicity and Virulence

• Pathogenicity: Ability to cause disease.

• Virulence: Degree of pathogenicity.

• Virulence factors include adhesion, biofilms, extracellular enzymes, toxins, and antiphagocytic factors.

Extracellular Enzymes

Secreted enzymes dissolve structural chemicals, aiding invasion and evasion of host defenses. Mutants lacking these enzymes are often avirulent.

Toxins

Toxins are chemicals that harm tissues or trigger damaging immune responses. Toxemia is the presence of toxins in the bloodstream. There are two main types:

• Exotoxins: Secreted by bacteria; highly toxic.

• Endotoxins: Part of Gram-negative bacterial cell walls; released upon cell death.

Antiphagocytic Factors

• Bacterial capsules: Not recognized as foreign, slippery, difficult to engulf.

• Antiphagocytic chemicals: Prevent fusion of lysosome and phagocytic vesicles.

• Leukocidins: Destroy phagocytic white blood cells.

The Stages of Infectious Disease

Stages Following Infection

Many infectious diseases progress through five stages:

1. Incubation period

2. Prodromal period

3. Illness

4. Decline

5. Convalescence

Movement of Pathogens Out of Hosts: Portals of Exit

Portals of Exit

Pathogens exit the host through portals often identical to entry sites, typically in materials secreted or excreted by the body.

Transmission of Infectious Diseases

Contact Transmission

Transmission occurs from a reservoir or portal of exit to another host's portal of entry. Contact transmission includes direct, indirect, and droplet transmission.

Vehicle and Vector Transmission

Vehicle transmission involves water, food, or air. Vector transmission involves arthropods such as mosquitoes and ticks.

Classification of Infectious Diseases

Methods of Classification

Diseases can be classified by body system affected, taxonomic category, longevity and severity, mode of transmission, and population effects.

Epidemiology of Infectious Diseases

Frequency of Disease

Epidemiology is the study of disease occurrence and transmission. Frequency is measured by:

• Incidence: Number of new cases in a given area and time.

• Prevalence: Total number of cases in a given area and time.

Reporting and Distribution

Data are reported in various ways, including maps, curves, and bar charts to illustrate incidence and prevalence.

Occurrence Terms

• Endemic: Disease normally present in a population.

• Sporadic: Occasional cases.

• Epidemic: Sudden increase in cases.

• Pandemic: Epidemic on multiple continents.

Nationally Notifiable Infectious Diseases

Certain diseases must be reported to public health authorities for monitoring and control.

Epidemiological Studies

Descriptive Epidemiology

Involves tabulation of data, recording location and time, collecting patient information, and identifying the index case.

Analytical Epidemiology

Determines probable cause, mode of transmission, and prevention methods; often retrospective.

Hospital Epidemiology: Healthcare-Associated (Nosocomial) Infections

Types and Control

• Exogenous: Acquired from healthcare environment.

• Endogenous: Arises from patient's own microbiota.

• Iatrogenic: Result of medical procedures.

• Superinfections: Antimicrobial drugs allow other microbes to thrive.

Handwashing is the most effective control measure.

Epidemiology and Public Health

Public Health Organizations

Data sharing among local, state, national, and global agencies is essential. The CDC and WHO coordinate public health efforts worldwide.