Infection, Infectious Diseases, and Epidemiology: Symbiosis, Microbiota, and Pathogen Entry

Symbiotic Relationships Between Microbes and Their Hosts

Types of Symbiosis

Symbiosis refers to the close association between two different biological species, often involving microorganisms and their hosts. These relationships can be classified based on the effects on each participant:

• Mutualism: Both organisms benefit from the relationship.

• Commensalism: One organism benefits, while the other is neither helped nor harmed.

• Amensalism: One organism is harmed, while the other is unaffected.

• Parasitism: One organism (the parasite) benefits at the expense of the other (the host).

Example: Termites and their gut protozoa exhibit mutualism, where protozoa digest cellulose for the termite, and the termite provides a habitat and nutrients for the protozoa.

Table: Types of Symbiotic Relationships

Purpose: To compare and classify the different types of symbiotic relationships based on the benefit or harm to each organism.

Type	Effect on Host	Effect on Microbe	Example
Mutualism	Benefit	Benefit	Gut bacteria synthesizing vitamins
Commensalism	No effect	Benefit	Staphylococcus epidermidis on skin
Amensalism	Harmed	No effect	Antibiotic-producing fungi inhibiting bacteria
Parasitism	Harmed	Benefit	Pathogenic bacteria causing disease

Microbiome of Humans

Normal Microbiota

The microbiome consists of all microorganisms that colonize the body's surfaces without normally causing disease. These are also called normal microbiota, normal flora, or indigenous microbiota. There are two main types:

• Resident microbiota: Remain throughout life and are mostly commensal.

• Transient microbiota: Present for short periods; cannot persist due to competition, immune responses, or environmental changes.

Example: The image shows normal microbiota (pink) colonizing the surface of human tissue.

Table: Some Resident Microbiota

Purpose: To classify the main genera of resident microbiota in different regions of the human body and their significance.

Body Region	Genera	Notes
Upper Respiratory Tract	Fusobacterium, Haemophilus, Lactobacillus, Moraxella, Staphylococcus, Streptococcus, Candida (fungus)	Similar to mouth; includes bacteria and fungi.
Upper Digestive Tract	Actinomyces, Bacteroides, Corynebacterium, Haemophilus, Lactobacillus, Moraxella, Neisseria, Streptococcus, Treponema, Entamoeba (protozoan), Candida (fungus)	Microbes colonize surfaces of teeth, gums, and mucosa.
Lower Digestive Tract	Bacteroides, Bifidobacterium, Clostridium, Enterococcus, Escherichia, Lactobacillus, Proteus, Klebsiella, Candida (fungus), Entamoeba (protozoan)	Most diverse; strict anaerobes predominate.
Female Urinary & Reproductive Systems	Lactobacillus, Candida (fungus), Streptococcus, Staphylococcus	Microbiota change with hormonal cycles.
Male Urinary & Reproductive Systems	Lactobacillus, Streptococcus, Staphylococcus	Lower urethra colonized; upper sterile.
Eyes & Skin	Corynebacterium, Micrococcus, Propionibacterium, Staphylococcus, Candida (fungus)	Microbes on outer skin and conjunctiva; deeper tissues sterile.

Acquisition of the Microbiome

Humans develop their microbiome after birth. The womb is typically sterile, but colonization begins during the birthing process and continues in the first months of life.

Opportunistic Pathogens

Some normal microbiota can become opportunistic pathogens and cause disease under certain conditions, such as:

• Introduction into unusual body sites

• Immune suppression

• Changes in the normal microbiota (e.g., after antibiotics)

• Stressful conditions

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

Pathogens require reservoirs to survive outside a host. There are three main types:

• Animal reservoirs: Zoonoses are diseases that spread from animals to humans via direct contact, consumption, or vectors (e.g., arthropods).

• Human carriers: Asymptomatic individuals can transmit pathogens to others.

• Nonliving reservoirs: Soil, water, and food can harbor infectious agents, often due to contamination.

Table: Some Common Zoonoses

Purpose: To classify zoonotic diseases by their causative agents.

Type	Examples
Helminthic	Schistosomiasis
Protozoan	Malaria
Fungal	Ringworm
Bacterial	Anthrax, Lyme disease
Viral	Rabies, Influenza

The Invasion and Establishment of Microbes in Hosts: Infection

Contamination vs. Infection

Contamination is the presence of microbes on body surfaces, while infection occurs when microbes evade defenses, multiply, and establish themselves in the body.

Portals of Entry

Pathogens enter the body through specific sites called portals of entry:

• Skin: Pathogens enter through cuts, openings, or by burrowing/digesting skin layers.

• Mucous membranes: Line body cavities open to the environment; respiratory tract is the most common entry site.

• Placenta: Normally a barrier, but some pathogens can cross and infect the fetus.

• Parenteral route: Not a true portal; involves direct deposition into tissues (e.g., via punctures).

Example: The image shows common portals of entry, including the conjunctiva, mouth, skin, and placenta.

Skin as a Portal of Entry

The skin's outer layer acts as a barrier, but some pathogens can enter through cuts or by penetrating the skin.

Mucous Membranes as Portals of Entry

Mucous membranes provide a moist environment for pathogens. The respiratory tract is the most common entry point, but the gastrointestinal tract is also significant, requiring pathogens to survive stomach acid.

Placenta as a Portal of Entry

The placenta usually protects the fetus, but some pathogens can cross, causing miscarriage, birth defects, or premature birth.

Parenteral Route

This route bypasses normal portals of entry, allowing pathogens to be deposited directly into tissues beneath the skin or mucous membranes (e.g., via needles or insect bites).

The Role of Adhesion in Infection

Adhesion is the process by which microorganisms attach to host cells, a critical step for colonization and infection. Microbes use specialized structures or attachment molecules (adhesion factors) to bind to host tissues.