Introduction to Infectious Bacterial Disease and Pathology: Human Microbiome & Etiology

Unit 4 Day 3: Intro to Infectious Bacterial Disease - Pathology

Overview

This unit introduces foundational concepts in infectious bacterial diseases, focusing on pathology, the human microbiome, and the etiology of disease. Key definitions, mechanisms, and classifications are provided to support understanding of how diseases develop and spread.

Pathology and Disease Terminology

Key Definitions

• Pathology: The study of disease, including its causes, development, and effects on the body.

• Etiology: The cause or origin of a disease.

• Pathogenesis: The manner in which a disease develops and progresses in the host.

• Infection: The invasion or colonization of the body by pathogenic microorganisms.

• Infectious Disease: Occurs when an infection leads to any change in the state of health. Not all infections result in detectable disease.

Example: Streptococcus pyogenes infection can cause strep throat, an infectious disease.

Human Microbiome

Establishment and Composition

The human microbiome consists of all the microorganisms living in and on the human body. It begins to establish before birth and continues to develop throughout life.

• Begins to establish in utero (before birth).

• Placental microbiome includes members of the Enterobacteriaceae and Propionibacterium.

• Lactobacilli colonize the neonate's intestine, especially during vaginal birth.

• Additional microorganisms are acquired from food, people, and pets.

• Microbiome composition changes throughout life, influencing health and disease.

Estimated Size: $4 \times 10^{13}$ bacterial cells in the human microbiome.

Human Microbiome Project: Analyzes relationships between microbial communities and human health.

Types of Microbiota

• Normal microbiota (normal flora): Permanently colonize the host and do not cause disease under normal conditions.

• Transient microbiota: Present for days, weeks, or months, but do not permanently colonize the host.

Factors Influencing the Normal Microbiota

• Nutrients

• Physical and chemical factors

• Mechanical factors

• Body's defenses against microbes

• Age, diet, nutritional status, geography, personal hygiene, living conditions, occupation, lifestyle

Normal microbiota play a crucial role in the development of the immune system.

Symbiosis: Relationships Between Host and Microbiota

Microorganisms interact with the host in various ways, classified as symbiotic relationships:

Type	Description	Example	Effect on Host
Commensalism	One organism benefits, the other is unaffected	Staphylococcus epidermidis on skin	Host is unaffected; bacteria get nutrients
Mutualism	Both organisms benefit	Escherichia coli in large intestine	E. coli aids digestion and produces vitamin K for host
Parasitism	One organism benefits at the expense of the other	Influenza virus in respiratory tract	Virus harms host to survive

Etiology: Determining the Cause of Disease

Koch's Postulates

Koch's postulates are a set of criteria used to establish a causative relationship between a microbe and a disease.

1. The same pathogen must be present in every case of the disease.

2. The pathogen must be isolated from the diseased host and grown in pure culture.

3. The pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal.

4. The pathogen must be isolated from the inoculated animal and shown to be the original organism.

Application: Koch's postulates are fundamental in identifying the microbial causes of emerging diseases.

Exceptions to Koch's Postulates

• Some pathogens can cause several disease conditions.

• Some pathogens cause disease only in humans.

• Some microbes have never been cultured in laboratory conditions.

• Several different pathogens may cause the same signs and symptoms.

Classifying Infectious Diseases

Symptoms, Signs, and Syndromes

• Symptoms: Subjective changes in body function felt by the patient (e.g., pain, fatigue). Not apparent to an observer.

• Signs: Objective changes that can be measured or observed (e.g., fever, rash).

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

Types of Infectious Diseases

• Communicable disease: A disease that is spread from one host to another. Examples: COVID-19, chicken pox, measles, influenza, genital herpes, tuberculosis.

• Contagious disease: Diseases that are easily and rapidly spread from one host to another.

• Non-communicable disease: A disease that is not spread from one host to another. Example: Tetanus.

Summary Table: Disease Classification

Disease Type	Definition	Example
Communicable	Spread from one host to another	Influenza, COVID-19
Contagious	Easily and rapidly spread	Measles
Non-communicable	Not spread between hosts	Tetanus

Additional info:

• Koch's postulates are foundational in microbiology for linking specific microbes to specific diseases, but modern molecular techniques (e.g., PCR, sequencing) are often used when culturing is not possible.

• The human microbiome is increasingly recognized for its role in immunity, metabolism, and even mental health.