Infection, Infectious Diseases, and Epidemiology (Chapter 14) – Microbiology Study Notes

Infection, Infectious Diseases, and Epidemiology

Learning Objectives

• Describe the interactions between microbes and humans.

• Comprehend the mechanisms of microbial pathogenesis.

• Understand the importance of epidemiology.

This chapter covers microbial life in the human body (microbiota), reservoirs of infection, microbial mechanisms of pathogenesis, principles of epidemiology, and nosocomial infections.

Pathology, Infection, and Disease

Key Definitions

• Pathology: The study of disease.

• Etiology: The cause of a disease.

• Pathogenesis: The development of disease.

• Infection: Invasion or colonization of the body by pathogens.

• Disease: An abnormal state in which the body is not performing normal functions.

Relationships between the Normal Microbiota and the Host

Types of Symbiosis

Symbiosis describes the relationship between normal microbiota and the host. There are several types:

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

• Mutualism: Both organisms benefit.

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

• Amensalism: One species causes harm to another without any cost or benefit to itself.

Some normal microbiota are opportunistic pathogens, meaning they can cause disease under certain conditions.

Comparison Table: Types of Symbiosis

Type	Effect on Host	Effect on Microbe
Commensalism	Unaffected	Benefits
Mutualism	Benefits	Benefits
Parasitism	Harmed	Benefits
Amensalism	Harmed	Unaffected

Normal and Transient Microbiota

Resident vs. Transient Microbiota

• Normal microbiota (resident microbiota) permanently colonize the host and do not cause disease under normal conditions.

• Transient microbiota may be present for days, weeks, or months but do not persist due to competition, elimination by host defenses, or chemical/physical changes.

• The Human Microbiome Project analyzes relationships between microbial communities and human health.

Table: Resident Microbiota in Different Body Regions

Region	Microbiota Example	Notes
Upper Respiratory Tract	Staphylococcus, Streptococcus	Present in nose, throat
Skin	Staphylococcus epidermidis	Resistant to dryness
Digestive Tract	Lactobacillus, Bacteroides	Important for digestion
Genitourinary Tract	Lactobacillus	Protects against pathogens

Microbial Antagonism (Competitive Exclusion)

• Normal microbiota protect the host by competing for nutrients, producing substances harmful to invading microbes, and affecting pH and available oxygen.

Factors Affecting Microbiota Distribution

• Nutrients

• Physical and chemical factors

• Host defenses

• Mechanical factors

Reservoirs of Infection

Types of Reservoirs

• Animal reservoirs: Zoonoses are diseases naturally transmitted from animals to humans (e.g., rabies, Lyme disease).

• Human reservoirs: Carriers may have inapparent infections or latent diseases.

• Nonliving reservoirs: Soil, water, and food can be reservoirs due to contamination.

Table: Some Common Zoonoses

Disease Type	Disease	Causative Agent	Animal Reservoir	Mode of Transmission
Helminthic	Tapeworm infestation	Dipylidium caninum	Dogs	Ingestion of larvae in dog saliva
Protozoan	Malaria	Plasmodium spp.	Monkeys	Bite of Anopheles mosquito
Fungal	Ringworm	Trichophyton spp.	Domestic animals	Direct contact
Bacterial	Anthrax	Bacillus anthracis	Livestock	Contact/inhalation
Viral	Rabies	Lyssavirus sp.	Bats, skunks, dogs	Bite of infected animal

Human Carriers and Inanimate Reservoirs

• Asymptomatic infected individuals can transmit disease.

• Inanimate reservoirs include waterborne, foodborne, and airborne sources.

Microbial Mechanisms of Pathogenesis

Pathogenicity and Virulence

• Pathogenicity: Ability to cause disease.

• Virulence: Degree of pathogenicity.

Exposure to Microbes

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

• Infection: Microbe evades defenses, multiplies, and becomes established.

Portals of Entry

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

• Mucous membranes: Respiratory tract (most common), gastrointestinal tract (must survive stomach acid).

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

• Parenteral route: Not a true portal; pathogens deposited directly into tissues (e.g., via needles, bites).

Numbers of Invading Microbes

• ID50: Infectious dose for 50% of a sample population (measures virulence).

• LD50: Lethal dose for 50% of a sample population (measures toxin potency).

Example: Campylobacter jejuni infectious dose is 500 organisms.

For Bacillus anthracis:

Portal of Entry	ID50
Skin	10–50 endospores
Inhalation	10,000–20,000 endospores
Ingestion	250,000–1,000,000 endospores

Toxin	LD50
Botulinum	0.03 ng/kg
Shiga toxin	250 ng/kg
Staphylococcal enterotoxin	1350 ng/kg

Adherence

• Microorganisms attach to host tissues using adhesion factors (specialized structures, attachment molecules).

• Adhesins (ligands) on pathogens bind to receptors on host cells (e.g., glycocalyx, fimbriae).

• Biofilms: Communities of microbes sharing nutrients and aiding colonization.

• Attachment proteins (found on viruses and bacteria) determine host cell specificity.

• Blocking or changing ligands/receptors can prevent infection.

• Microorganisms unable to make attachment proteins/adhesins are avirulent.

Some bacteria form biofilms by attaching to each other.

Diagram: Contact, Colonization, Infection, Disease

• Not all contacts lead to colonization or infection.

• Successful infection requires adherence and evasion of host defenses.

Checkpoint Questions (for Review)

• Why is an absolute commensalism difficult to prove?

• What is the preferred portal of entry for Bacillus anthracis? Why?

Additional info: These notes are based on textbook-style lecture slides for Chapter 14, covering core concepts in infection, infectious diseases, and epidemiology, suitable for college-level microbiology students.