Infection, Infectious Diseases, and Epidemiology: Microbiology Study Notes

Symbiotic Relationships Between Microbes and Their Hosts

Types of Symbiosis

Symbiosis refers to the close association between two different biological species. In microbiology, humans and microbes engage in various symbiotic relationships, which can be classified as mutualism, commensalism, or parasitism.

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

• Commensalism: One organism benefits, while the other is neither helped nor harmed. Example: Staphylococcus on human skin.

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

Type	Organism 1	Organism 2	Example
Mutualism	Benefits	Benefits	Bacteria in human colon
Commensalism	Benefits	Neither benefits nor is harmed	Staphylococcus on skin
Parasitism	Benefits	Is harmed	Tuberculosis bacteria in human lung

Normal Microbiota in Hosts

Normal microbiota, also known as normal flora or indigenous microbiota, are microorganisms that colonize the body's surfaces without causing disease under normal conditions. They are classified as resident or transient microbiota.

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

• Transient microbiota: Present for short periods; eliminated by competition, body defenses, or changes in the body.

Some Resident Microbiota

Resident microbiota inhabit various body regions, including the upper respiratory tract, digestive tract, urinary and reproductive systems, and skin. Their composition varies by location and individual.

Acquisition and Opportunism of Normal Microbiota

Humans are axenic (microbe-free) in the womb. Microbiota are acquired during birth and early life. Normal microbiota can become opportunistic pathogens if introduced to unusual sites, if the immune system is suppressed, or if the balance of microbiota changes.

• Opportunistic pathogens: Normal microbiota that cause disease under certain conditions.

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

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

• Animal reservoirs (zoonoses): Diseases naturally spread from animals to humans via direct contact, consumption, or vectors.

• Human carriers: Infected individuals who may be asymptomatic but can transmit pathogens.

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

The Movement of Microbes into Hosts: Infection

Contamination vs. Infection

Contamination is the presence of microbes on the body, while infection occurs when microbes evade defenses, multiply, and establish themselves.

Portals of Entry

Pathogens enter the body through specific portals:

• Skin: Entry via cuts, openings, or by burrowing/digesting outer layers.

• Mucous membranes: Line body cavities; respiratory tract is the most common entry site.

• Placenta: Usually a barrier, but some pathogens can cross and infect the fetus.

• Parenteral route: Pathogens deposited directly into tissues beneath skin/mucosa (not a true portal).

The Role of Adhesion in Infection

Adhesion is the process by which microorganisms attach to host cells, a critical step for colonization and infection. Adhesion factors include specialized structures and attachment proteins (ligands) that bind to host cell receptors. Disruption of these interactions can prevent infection.

• Some bacteria form biofilms for collective attachment (e.g., dental plaque).

The Nature of Infectious Disease

Manifestations of Disease

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

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

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

• Asymptomatic (subclinical) infections: No symptoms, but signs may be present.

Causation of Disease: Etiology

Etiology is the study of disease causation. The germ theory states that diseases are caused by infections of pathogenic microorganisms. Robert Koch developed postulates to prove causation:

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 disease in a healthy host.

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

Exceptions exist, such as pathogens that cannot be cultured or diseases with multiple causes.

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 enzymes that help pathogens invade tissues, evade defenses, and maintain infection by dissolving structural chemicals in the body.

Toxins

Chemicals that damage host tissues or trigger damaging immune responses. Toxemia is the presence of toxins in the bloodstream.

• Exotoxins: Secreted proteins that kill host cells or disrupt function.

• Endotoxins: Lipid A component of Gram-negative bacteria released upon cell death, causing fever, inflammation, and shock.

Antiphagocytic Factors

Mechanisms that prevent phagocytosis by host immune cells, such as capsules (not recognized as foreign, slippery) and chemicals that inhibit phagocyte function.

The 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: Recovery; no signs or symptoms.

The Movement of Pathogens Out of Hosts: Portals of Exit

Pathogens exit the host through portals often identical to entry sites, typically in secretions or excretions (e.g., saliva, feces, blood).

Modes of Infectious Disease Transmission

Transmission occurs from a reservoir or portal of exit to a new host's portal of entry. Main modes include:

• Contact transmission: Direct, indirect, or droplet spread.

• Vehicle transmission: Airborne, waterborne, or foodborne.

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

Classification of Infectious Diseases

Diseases can be classified by the body system affected, taxonomic category, longevity/severity, transmission mode, or population effects. Key terms include:

• Acute: Rapid onset, short duration.

• Chronic: Slow development, long duration.

• Subacute: Intermediate characteristics.

• Latent: Pathogen inactive for a time.

• Communicable: Spread from host to host.

• Contagious: Easily spread between hosts.

Epidemiology of Infectious Diseases

Frequency of Disease

Epidemiologists track disease occurrence using:

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

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

Occurrence is also evaluated by frequency and geographic distribution.

Epidemiological Studies

• Descriptive epidemiology: Tabulation of data, location, time, and patient information to identify the index case.

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

• Experimental epidemiology: Tests hypotheses about disease causation, often using Koch's postulates.

Hospital Epidemiology: Nosocomial Infections

Nosocomial (hospital-acquired) infections are classified as:

• Exogenous: Acquired from the healthcare environment.

• Endogenous: Arise from normal microbiota due to healthcare factors.

• Iatrogenic: Result from medical procedures.

Hand washing is the most effective prevention method.

Epidemiology and Public Health

Public health agencies (local, national, global) monitor and control disease transmission, ensure water and food safety, and educate the public to limit disease spread.