Symbiotic Relationships Between Microbes and Their Hosts

Types of Symbiosis

Symbiosis refers to the close association between two different biological species. In microbiology, humans have various symbiotic relationships with microorganisms, which can be classified as follows:

• Mutualism: Both organisms benefit. Example: Bacteria in the human colon synthesize vitamins for the host and receive nutrients.

• Commensalism: One organism benefits, the other is neither helped nor harmed. Example: Mites in human hair follicles.

• Amensalism: One organism is harmed, the other is unaffected. Example: Fungus secreting antibiotics that inhibit nearby bacteria.

• Parasitism: One organism benefits at the expense of the other. Example: Mycobacterium tuberculosis in human lungs.

Microbiome of Humans

The microbiome consists of all microorganisms that colonize the body's surfaces without causing disease under normal conditions. These are also called normal microbiota or normal flora.

• Resident microbiota: Remain a part of the normal microbiota throughout life; mostly commensal.

• Transient microbiota: Remain in the body for only a short period due to competition, elimination, or chemical/physical changes in the host.

Acquisition and Opportunism of Microbiota

Humans are free of microorganisms in the womb. The microbiome begins to develop during birth and is established in the first months of life. Under certain conditions, normal microbiota can become opportunistic pathogens and cause disease, such as:

• Introduction into unusual body sites

• Immune suppression

• Changes in the normal microbiota (e.g., antibiotics)

• Stressful conditions

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as a source of infection. Most pathogens cannot survive long outside their host. There are three main types:

• Animal reservoirs (Zoonoses): Diseases naturally spread from animal hosts to humans (e.g., rabies, plague, malaria).

• Human carriers: Infected individuals who are asymptomatic but can transmit pathogens to others.

• Nonliving reservoirs: Soil, water, and food contaminated with pathogens (often by feces or urine).

The Invasion and Establishment of Microbes in Hosts: Infection

Contamination vs. Infection

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

• Infection: When microbes evade external defenses, multiply, and become established in the body.

Portals of Entry

Pathogens enter the body through specific sites called portals of entry:

• Skin

• Mucous membranes (respiratory, gastrointestinal, urogenital tracts)

• Placenta (rare, but can transmit pathogens to fetus)

• Parenteral route (circumvents usual portals, e.g., via punctures, cuts)

The Role of Adhesion in Infection

Adhesion is the process by which microorganisms attach to host cells, a critical step for colonization and infection. Microbes use adhesion factors such as specialized structures (fimbriae, pili) or attachment molecules (ligands) to bind to host cell receptors.

The Nature of Infectious Disease

Infection and Disease

• Infection: Invasion of the host by a pathogen.

• Disease (Morbidity): Results when infection alters normal body functions.

Manifestations of Disease

• Symptoms: Subjective characteristics felt only by the patient (e.g., pain, nausea).

• Signs: Objective manifestations observed or measured by others (e.g., fever, rash).

• Syndrome: A group of symptoms and signs that characterize a disease.

• Asymptomatic (subclinical) infections: Lack symptoms but may have signs.

Causation of Disease: Etiology

Etiology is the study of the cause of disease. The germ theory of disease states that infections by pathogenic microorganisms cause disease. Koch’s postulates are a set of criteria used to prove that a specific pathogen causes a specific disease:

1. The suspected agent must be present in every case of the disease.

2. The agent must be isolated and grown in pure culture.

3. The cultured agent must cause the disease when inoculated into a healthy host.

4. The same agent must be reisolated from the diseased experimental host.

Exceptions: Some pathogens cannot be cultured, diseases may have multiple causes, and ethical considerations may prevent testing in humans.

Virulence Factors of Infectious Agents

Pathogenicity is the ability to cause disease; virulence is the degree of pathogenicity. Virulence factors include:

• Adhesion factors

• Biofilms

• Extracellular enzymes

• Toxins (exotoxins and endotoxins)

• Antiphagocytic factors

Extracellular Enzymes

Secreted by pathogens to dissolve structural chemicals in the body, aiding invasion and evasion of host defenses. Mutants lacking these enzymes are often avirulent.

Toxins

Chemicals that harm tissues or trigger damaging immune responses. Toxemia is the presence of toxins in the bloodstream. Two main types:

• Exotoxins: Secreted by bacteria; can kill host cells.

• Endotoxins: Released from dead Gram-negative bacteria; can cause fever, inflammation, shock.

Antiphagocytic Factors

These factors help pathogens evade phagocytosis by host immune cells:

• Bacterial capsules: Composed of chemicals not recognized as foreign; slippery and hard to engulf.

• Antiphagocytic chemicals: Prevent fusion of lysosome and phagocytic vesicle; leukocidins destroy phagocytes.

Stages of Infectious Disease

Infectious diseases typically progress through five stages:

1. Incubation period: No signs or symptoms.

2. Prodromal period: Mild, general symptoms.

3. Illness: Most severe signs and symptoms.

4. Decline: Declining signs and symptoms.

5. Convalescence: No signs or symptoms; recovery.

The Movement of Pathogens Out of Hosts: Portals of Exit

Pathogens leave the host through specific portals of exit, often the same as portals of entry. They are commonly found in materials the body secretes or excretes (e.g., saliva, feces, urine, blood).

Modes of Infectious Disease Transmission

Transmission Pathways

Transmission occurs from a reservoir or portal of exit to another host’s portal of entry. There are three main groups:

• Contact transmission: Direct (person-to-person), indirect (via fomites), or droplet (via mucus droplets).

• Vehicle transmission: Airborne (aerosols), waterborne (fecal-oral), foodborne, or via bodily fluids.

• Vector transmission: Biological vectors (e.g., mosquitoes) or mechanical vectors (e.g., flies).

Classification of Infectious Diseases

Epidemiology of Infectious Diseases

Frequency of Disease

• Incidence: Number of new cases in a given area during a given period.

• Prevalence: Total number of cases in a given area during a given period.

Epidemiological Studies

• Descriptive epidemiology: Tabulation of data, location, time, and patient information; identifies index case.

• Analytical epidemiology: Determines probable cause, mode of transmission, and prevention; often retrospective.

Healthcare-Associated (Nosocomial) Infections

Nosocomial infections are acquired in healthcare settings. Types include:

• Exogenous: Pathogen acquired from the healthcare environment.

• Endogenous: Pathogen arises from normal microbiota within the patient.

• Iatrogenic: Result from modern medical procedures.

• Superinfections: Use of antimicrobials inhibits resident microbiota, allowing others to thrive.

Prevention: Aggressive control measures are required; handwashing is the most effective method.

Epidemiology and Public Health

Public health agencies (local, national, global) play a key role in interrupting disease transmission by:

• Enforcing cleanliness of water and food supplies

• Reducing disease vectors and reservoirs

• Establishing immunization schedules

• Locating and treating exposed individuals

• Implementing isolation and quarantine

• Providing public health education

Examples of agencies: U.S. Public Health Service (CDC), World Health Organization (WHO).