Bacterial Pathogenesis: Introduction, Concepts, and Relevance in Microbiology

Bacterial Pathogenesis: Course Overview and Foundations

Course Structure and Logistics

This course introduces students to the principles of bacterial pathogenesis, focusing on the mechanisms by which bacteria cause disease, their interactions with hosts, and the broader impact on human health and society.

• Schedule: Tuesday and Thursday, 2:00-3:30 pm

• Location: 100 Genetics & Plant Biology Building (GPBB)

• Instructors: Dan Portnoy (course director), Russell Vance, Pia Andrade (GSI)

• Supplementary Texts: Brock's "Biology of Microorganisms" and Janeway's "Immunobiology"

• Grading: Three midterms (each 33%), with the final exam held in-person

Course Content and Relevance

The course covers foundational and advanced topics in bacterial pathogenesis, including:

• Basic bacteriology and bacterial genetics

• Immunology relevant to host-pathogen interactions

• Mechanisms of pathogenesis, toxins, effectors, and invasion strategies

• Extracellular and intracellular pathogens

Foundational Concepts in Microbial Biology

Evolutionary Context and Diversity

Microbiology is grounded in evolutionary biology, as highlighted by the quote: "Nothing in biology makes sense except in the light of evolution." (Theodosius Dobzhansky).

• Three Domains of Life: Bacteria, Archaea, and Eukarya

• Bacteria: Exhibit extensive biochemical diversity

• Eukaryotes: Exhibit extensive morphological diversity

Microbial Abundance and Ecological Impact

• There are more bacterial cells (microbiota) than human cells in the body

• Microbes outnumber all other species and constitute ~20% of Earth's biomass

• Microbial cycling of elements (carbon, nitrogen) is essential for life

• Less than 1% of microbial species have been identified; many cannot be cultured

Habitats and Extremophiles

Microbes thrive in diverse and extreme environments:

• Underground: Basalt deposits 1500m underground

• Atmosphere: Some bacteria live and reproduce in clouds

• Polar Ice: Bacteria found in glaciers and polar snows at -17 to -85°C

• Hot Springs: Some species grow at 75-95°C; Archaea at 106°C in hydrothermal vents

• Deep Sea: Extreme barophiles survive at >10,000m depth, >1000 atmospheres pressure

Global Burden of Infectious Disease

Major Pathogens and Public Health

• The Big Three: AIDS (virus), Tuberculosis (Mycobacterium tuberculosis), Malaria (Plasmodium falciparum)

• The Big Four: Adds SARS-CoV-2 (Covid-19)

• No effective vaccines for AIDS, TB, or Malaria

• Emergence of drug-resistant pathogens (e.g., MDR-TB, XDR-TB)

Impact of Medical Microbiology

• Vaccines and antibiotics have dramatically reduced morbidity and mortality from infectious diseases

• Example: Diphtheria cases dropped sharply after vaccine introduction

Key Terminology and Concepts in Pathogenesis

Definitions

• Pathogen: A microorganism capable of producing disease in a percentage of normal (non-immune) individuals

• Expanded Definition: Causes pathology by manipulating host cell processes, evading innate immunity, and promoting dissemination

• Opportunistic Pathogen: Rarely causes disease in healthy hosts, but does so in immunocompromised individuals (e.g., organ transplant recipients, AIDS patients, elderly)

• Accidental Pathogen: Not evolved to cause human disease (e.g., Legionella pneumophila)

Types of Pathogens

• Obligate Pathogen: Only found in association with its host

• Facultative Pathogen: Can survive in the environment and the host

• Obligate Intracellular Pathogen: Only grows inside host cells (all viruses)

• Facultative Intracellular Pathogen: Can grow inside and outside cells; culturable in laboratory

Stages in Bacterial Pathogenesis

Overview of Stages

• Encounter: Initial contact with host (e.g., microbiota, breathing, sexual contact, vector-borne, zoonoses)

• Entry: Penetration of mucosal membranes

• Colonization: Attachment to host cells via pili or fimbriae

• Signal Transduction: Bacteria detect/respond to environmental changes (oxygen, pH, nutrients)

• Multiplication: Compete for nutrients (e.g., iron); exponential growth ($N_t = N_0 imes 2^{t/d}$, where $d$ is doubling time)

• Manipulation of Host Cell Biology: Use secretion systems to inject proteins into host cells

• Invasion: Penetration and spread within host tissues

• Evasion of Host Immunity: Stealth (capsules), offensive (toxins), infiltration (intracellular lifestyle)

• Transmission: Spread to new hosts

Quantitative Measures of Pathogenesis

• Pathogenesis: Ability to cause infection

• Virulence: Quantitative measure of pathogenesis

• LD50 (Lethal Dose 50): Number of microorganisms/toxin required to kill 50% of test animals

• ID50 (Infectious Dose 50): Number of organisms/toxin required to infect 50% of test animals

Determinants of Pathogenesis (Virulence Factors)

• Virulence Factors: Components responsible for a pathogen's ability to cause infection

• Often identified by mutations in specific genes (virulence genes) that reduce virulence

• Pathogenesis is multifactorial; not all gene mutations result in loss of virulence

Table: Types of Pathogens and Their Characteristics

Type	Definition	Example
Obligate Pathogen	Only found in association with host	Mycobacterium tuberculosis
Facultative Pathogen	Can survive in environment and host	Legionella pneumophila
Opportunistic Pathogen	Causes disease in immunocompromised hosts	Pseudomonas aeruginosa
Accidental Pathogen	Not evolved to cause human disease	Legionella pneumophila

Summary and Importance

Bacterial pathogenesis is a central topic in microbiology, integrating evolutionary biology, genetics, immunology, and ecology. Understanding the mechanisms by which bacteria cause disease, evade host defenses, and spread is essential for developing new therapies, vaccines, and public health strategies.