Normal Flora, Symbiosis, and Infectious Disease: Microbiome and Epidemiology

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Normal Flora and Symbiosis

Introduction to the Human Microbiome

The human body is a complex ecosystem inhabited by trillions of microorganisms, collectively known as the microbiota or normal flora. These microbes play essential roles in health and disease, forming intricate relationships with their human host. The study of these interactions is central to understanding infectious diseases and the maintenance of health.

Human cells vs. Bacterial cells: The human body contains approximately as many bacterial cells as human cells, with bacteria making up a significant portion of the total cell count but a much smaller fraction of body mass.
Microbiome: The collective genomes of the microbiota, studied using metagenomics, which allows for the identification and cataloging of microbes without the need for culturing.

Human cheek cell with resident bacteria Distribution of cell number and mass for different cell types in the human body

Sites of Normal Microbiota in the Human Body

Normal microbiota colonize various body sites, each with distinct microbial communities. These sites include the skin, oral cavity, upper respiratory tract, intestines, urinary tract, and female reproductive tract.

Skin: Dominated by Gram-positive bacteria such as Staphylococcus and Micrococcus.
Oral cavity: Contains diverse genera including Streptococcus and Neisseria.
Intestines: Populated by Bacteroides, Lactobacillus, and others.
Female reproductive tract: Rich in Lactobacillus and Streptococcus.

Major sites of normal microbiota on the human body

Types of Symbiotic Relationships

Symbiosis refers to the close association between two different species. In the context of microbiology, it describes the interactions between humans and their microbiota. There are four main types of symbiotic relationships:

Type	Organism 1	Organism 2	Example
Mutualism	Benefits	Benefits	Bacteria in human colon
Commensalism	Benefits	Neither benefits nor is harmed	Mites in human hair follicles
Amensalism	Is harmed	Neither benefits nor is harmed	Fungus secreting antibiotic, inhibiting bacteria
Parasitism	Benefits	Is harmed	Tuberculosis bacteria in human lung

Table of types of symbiotic relationships

Microbiota Beyond Humans: The Rhizosphere

Plant-Microbe Interactions

The rhizosphere is the narrow region of soil influenced by root secretions and associated microbial activity. This 'root microbiome' is crucial for nutrient cycling, especially nitrogen fixation, and plant health.

Rhizobium: Nitrogen-fixing bacteria that form symbiotic nodules on plant roots, converting atmospheric nitrogen into forms usable by plants.
Soil Microbiome: Includes mycorrhizal fungi, bacterial endophytes, and other organisms that interact with plant roots.

Diagram of rhizosphere and root microbiome Roots with nodules formed by nitrogen-fixing bacteria Close-up of root nodules

Acquisition and Development of the Human Microbiome

Establishment of Microbiota

Humans are born with a sterile internal environment. Colonization by microbiota begins during birth and continues through early life, influenced by delivery mode (vaginal vs. Caesarian), feeding (breast milk vs. formula), and environment.

Vaginal birth: Infants acquire microbiota similar to their mother's vaginal and gut flora.
Caesarian birth: Infants are colonized by environmental microbes, which may increase risk for certain diseases.
Breastfeeding: Human milk contains complex carbohydrates that nourish beneficial bacteria such as Bifidobacterium longum infantis.

Probiotic supplement bottle

Microbiota and Human Health

Benefits of Normal Microbiota

Normal microbiota provide numerous benefits to the host, including:

Competing with pathogens for resources and attachment sites
Producing vitamins (e.g., vitamin K, B vitamins)
Maintaining proper pH (e.g., in the vagina)
Stimulating immune system development
Protecting against chronic diseases

Scanning electron micrograph of bacteria on intestinal lining

Opportunistic Pathogens

Normal microbiota can become opportunistic pathogens under certain conditions, such as:

Introduction into unusual body sites
Immune suppression
Disruption of normal microbiota (e.g., antibiotic use)

Infectious Diseases: Concepts and Epidemiology

Reservoirs of Infection

Pathogens are maintained in reservoirs, which can be:

Animal reservoirs: Zoonoses are diseases transmitted from animals to humans.
Human carriers: Infected individuals who may or may not show symptoms.
Nonliving reservoirs: Soil, water, and food contaminated with pathogens.

Disease	Causative Agent	Animal Reservoir	Mode of Transmission
Malaria	Plasmodium spp.	Monkeys	Bite of Anopheles mosquito
Rabies	Rabies virus	Bats, skunks, foxes, dogs	Bite of infected animal

Table of common zoonoses

Transmission of Infectious Diseases

Diseases can be transmitted by direct contact, indirect contact (fomites), vehicles (air, water, food), and vectors (biological or mechanical).

Person sneezing, illustrating droplet transmission

Stages of Infectious Disease

The progression of infectious disease typically follows these stages:

Incubation period: No signs or symptoms
Prodromal period: Mild, general symptoms
Illness: Most severe signs and symptoms
Decline: Declining signs and symptoms
Convalescence: Recovery; no signs or symptoms

Graph of stages of infectious disease

Classification of Infectious Diseases

Term	Definition
Acute disease	Symptoms develop rapidly and run a short course
Chronic disease	Symptoms develop slowly and last a long time
Latent disease	Pathogen remains inactive for a time but can reactivate
Communicable disease	Transmitted from one host to another
Noncommunicable disease	Not transmitted from host to host

Table of terms used to classify infectious diseases

Manifestations of Disease

Symptoms (Sensed by Patient)	Signs (Detected by Observer)
Pain, nausea, headache, chills, fatigue, malaise, discomfort, itching, abdominal cramps	Swelling, rash, vomiting, diarrhea, fever, pus, anemia, leukocytosis/leukopenia, bubo, tachycardia/bradycardia

Table of typical manifestations of disease

Epidemiology: Occurrence and Spread of Disease

Key Epidemiological Terms

Incidence: Number of new cases in a given area during a specific period.
Prevalence: Total number of cases in a given area during a specific period.
Endemic: Disease constantly present in a population.
Epidemic: Disease acquired by many hosts in a short time.
Pandemic: Worldwide epidemic.
Sporadic: Disease that occurs occasionally.
Herd immunity: Immunity in most of a population, reducing disease spread.

Graph of incidence and prevalence

Models of Disease Spread

The SIR model (Susceptible, Infected, Removed) is used to understand and predict the spread of infectious diseases in populations.

SIR model diagram

Index Case

The index case (or "patient zero") is the first documented patient in an outbreak, crucial for tracing the source and understanding the spread of disease.

Summary Table: Types of Symbiotic Relationships

Type	Definition	Example
Mutualism	Both organisms benefit	Bacteria in human colon
Commensalism	One benefits, other unaffected	Mites in hair follicles
Amensalism	One harmed, other unaffected	Fungus secreting antibiotic
Parasitism	One benefits, other harmed	Tuberculosis in lungs

Additional info: These notes integrate foundational concepts from microbiology, including the human microbiome, symbiosis, infectious disease transmission, and epidemiology, as outlined in standard college-level microbiology curricula.