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

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Normal Flora and Symbiosis

Introduction to Symbiosis and the Human Microbiome

Symbiosis refers to the close association between different species living together, often with significant impacts on their biology. In humans, the microbiome—the collection of all microorganisms living in and on the body—plays a crucial role in health and disease. The human body is a complex ecosystem, with bacterial cells outnumbering human cells.

Normal microbiota (or normal flora): Microorganisms that colonize the body without causing disease under normal conditions.
Two main types: Resident microbiota (permanently colonize) and transient microbiota (temporarily present).
Microbiota are found on the skin, oral cavity, intestines, upper respiratory tract, urinary tract, and reproductive tract.

Human cheek cell with resident bacteria Distribution of cell number and mass for human and bacterial cells Sites of normal microbiota on the human body

Types of Symbiotic Relationships

Symbiotic relationships between humans and microbes can be classified into four main types:

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 in the Environment: The Rhizosphere

The rhizosphere is the narrow region of soil influenced by root secretions and associated microorganisms. Nitrogen-fixing bacteria such as Rhizobium form mutualistic relationships with plants, converting atmospheric nitrogen into forms usable by plants, which is essential for plant and ecosystem health.

Root microbiome: Includes bacteria, fungi, and other microbes living in association with plant roots.
Nitrogen fixation: Conversion of atmospheric nitrogen (N2) to ammonia (NH3), a process vital for plant nutrition.

Diagram of rhizosphere and root microbiome Plant roots with nodules Close-up of root nodules

Human Microbiome: Acquisition and Development

Development of the Microbiome

Humans are born with a sterile internal environment, but colonization by microbes begins during birth and continues through early life. The mode of delivery (vaginal vs. Caesarian) and feeding (breastfeeding vs. formula) significantly influence the composition of the infant microbiome.

Vertical transmission: Transfer of microbiota from mother to child during birth.
Breast milk contains complex carbohydrates that nourish beneficial bacteria such as Bifidobacterium longum infantis.
Early microbiota establishment is critical for immune and metabolic development.

SEM of bacteria on intestinal surface

Sites and Examples of Normal Microbiota

Different body sites harbor distinct microbial communities, each adapted to their local environment. Common genera include Staphylococcus (skin), Streptococcus (oral cavity), Bacteroides (intestines), and Lactobacillus (female reproductive tract).

Site	Common Microbes
Skin	Staphylococcus, Micrococcus, Propionibacterium
Oral cavity	Streptococcus, Neisseria, Haemophilus
Intestines	Bacteroides, Lactobacillus, Escherichia
Female reproductive tract	Lactobacillus, Streptococcus, Candida

Table of resident microbiota by body site

Microbiota and Human Health

Benefits of Normal Microbiota

Normal microbiota provide numerous benefits to the host, including:

Competing with pathogens for nutrients and attachment sites (competitive exclusion).
Producing vitamins (e.g., vitamin K, B vitamins) and other beneficial metabolites.
Stimulating immune system development and function.
Maintaining proper pH in certain body regions (e.g., vagina).

Disruption of the normal microbiota (e.g., by antibiotics or illness) can lead to opportunistic infections, where normally harmless microbes cause disease.

Probiotics and Microbiome Restoration

Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. They are used to restore or maintain a healthy microbiome, especially after antibiotic use or gastrointestinal disturbances.

Probiotic supplement bottle

Infectious Diseases: Reservoirs, Transmission, and Epidemiology

Reservoirs of Infection

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

Animal reservoirs: Zoonoses are diseases naturally spread from animals to humans (e.g., rabies, plague).
Human carriers: Infected individuals who may or may not show symptoms but can transmit pathogens.
Nonliving reservoirs: Soil, water, and food contaminated with infectious agents.

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

Table of common zoonoses

Transmission of Infectious Diseases

Diseases can be transmitted by direct or indirect means:

Contact transmission: Direct (person-to-person) or indirect (via fomites such as doorknobs).
Vehicle transmission: Through air, water, food, or bodily fluids.
Vector transmission: Biological (e.g., mosquitoes) or mechanical (e.g., flies carrying pathogens on their bodies).

Stages of Infectious Disease

The progression of infectious disease typically follows five stages:

Incubation period: Time between exposure and appearance of symptoms.
Prodromal period: Onset of mild, general symptoms.
Illness: Most severe signs and symptoms.
Decline: Symptoms subside as immune response controls infection.
Convalescence: Recovery and return to normal health.

Graph of stages of infectious disease Incubation periods of various diseases

Manifestations and Classification of Disease

Diseases are characterized by symptoms (subjective, felt by patient) and signs (objective, observed by others). A syndrome is a group of symptoms and signs that characterize a disease.

Symptoms (Patient)	Signs (Observer)
Pain, nausea, headache, fatigue	Fever, rash, vomiting, diarrhea
Itching, malaise, abdominal cramps	Swelling, pus, anemia, tachycardia

Table of typical manifestations of disease

Epidemiology: Occurrence and Spread of Disease

Epidemiology is the study of when and where diseases occur and how they are transmitted in populations. Key terms include:

Incidence: Number of new cases in a given time period.
Prevalence: Total number of cases at a given time.
Endemic: Constantly present in a population.
Epidemic: Sudden increase in cases above expected levels.
Pandemic: Worldwide epidemic.
Sporadic: Occasional cases.
Herd immunity: Resistance of a population due to immunity of most members.

Graph of incidence and prevalence

Summary Table: Terms Used to Classify Infectious Diseases

Term	Definition
Acute disease	Symptoms develop rapidly, run course quickly
Chronic disease	Symptoms develop slowly, last a long time
Latent disease	Pathogen remains inactive for a time
Communicable disease	Easily spread from host to host
Noncommunicable disease	Not spread from host to host

Table of terms used to classify infectious diseases

Conclusion

The human microbiome and its symbiotic relationships are fundamental to health, influencing immunity, metabolism, and disease susceptibility. Understanding reservoirs, transmission, and epidemiology is essential for controlling infectious diseases and promoting public health.