Infection, Infectious Diseases, and Epidemiology: Symbiosis, Microbiota, and Reservoirs

Symbiotic Relationships Between Microbes and Their Hosts

Definition and Types of Symbiosis

Symbiosis refers to the close and long-term biological interaction between two different biological organisms. In the context of microbiology, it describes the various ways microbes interact with their hosts, including humans. These relationships can be classified into several types 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 benefits at the expense of the other, often causing harm.

Example: Termites and their gut protozoa/bacteria exhibit mutualism, as the microbes help digest cellulose, benefiting both parties.

Table: Types of Symbiotic Relationships

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

Definition and Types of Normal Microbiota

The microbiome refers to the collection of microorganisms that colonize the surfaces of the human body without normally causing disease. These are also known as normal microbiota, normal flora, or indigenous microbiota. There are two main types:

• Resident microbiota: Microbes that are a permanent part of the normal microbiota throughout life, typically without causing harm. Most are commensal.

• Transient microbiota: Microbes that remain in the body for a short period and are found in the same regions as resident microbiota but do not persist due to competition, immune responses, or environmental changes.

Example: The skin and mucous membranes are colonized by a variety of bacteria that form part of the normal microbiota.

Table: Some Resident Microbiota

Body Site	Genera	Notes
Upper Respiratory Tract	Fusobacterium, Haemophilus, Lactobacillus, Moraxella, Staphylococcus, Streptococcus, Candida (fungus)	The nose is cooler than the rest of the respiratory system and has distinct microbiota. The microbiota of the trachea and bronchi are similar to those of the nose and mouth. The alveoli of the lungs are usually axenic (free of microbes), but can be colonized in disease.
Upper Digestive Tract	Actinomyces, Bacteroides, Corynebacterium, Haemophilus, Lactobacillus, Moraxella, Neisseria, Streptococcus, Treponema, Entamoeba (protozoan), Trichomonas (protozoan)	Microbes colonize surfaces of teeth, gingiva, lining of cheeks, and pharynx. They are found in saliva in large numbers. Dozens of species have never been cultured.
Lower Digestive Tract	Bacteroides, Bifidobacterium, Clostridium, Enterococcus, Escherichia, Lactobacillus, Proteus, Klebsiella, Candida (fungus), Trichomonas (protozoan)	The lower bowel is mostly anaerobic, though some facultative anaerobes are present.
Female Urinary and Reproductive Systems	Lactobacillus, Candida (fungus), Trichomonas (protozoan)	Microbiota change as activity in the vagina changes during menstrual cycles. The flow of urine prevents extensive colonization of the urinary bladder or urethra.
Male Urinary and Reproductive Systems	Lactobacillus, Mycobacterium, Peptostreptococcus, Staphylococcus, Streptococcus	The flow of urine prevents extensive colonization of the urinary bladder or urethra.
Eyes and Skin	Corynebacterium, Micrococcus, Propionibacterium, Staphylococcus, Candida (fungus), Malassezia (fungus)	Microbes live on the outer, dead layers of the skin and in hair follicles and pores of glands. The deeper layers (dermis and hypodermis) are axenic, except when broken by cuts or wounds.

Acquisition of the Microbiome

Humans develop their microbiome beginning at birth. The womb is typically free of microorganisms, but colonization begins during the birthing process and continues in the first months of life as the infant is exposed to the environment.

How Normal Microbiota Become Opportunistic Pathogens

Some normal microbiota can become opportunistic pathogens under certain conditions, causing disease. These conditions include:

• Introduction of normal microbiota into an unusual site in the body

• Immune suppression

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

• Stressful conditions

Example: Escherichia coli is normally harmless in the gut but can cause urinary tract infections if introduced into the urinary tract.

Reservoirs of Infectious Diseases of Humans

Definition and Types of Reservoirs

Most pathogens cannot survive for long outside their host. Reservoirs of infection are sites where pathogens are maintained as a source of infection. There are three main types:

• Animal reservoirs: Animals harbor pathogens that can be transmitted to humans (zoonoses).

• Human carriers: Infected individuals who may not show symptoms but can transmit pathogens to others.

• Nonliving reservoirs: Soil, water, and food can serve as reservoirs, often contaminated by feces or urine.

Animal Reservoirs and Zoonoses

Zoonoses are diseases that naturally spread from animal hosts to humans. Transmission can occur through direct contact, consumption of animal products, or via bloodsucking arthropods. Humans are often dead-end hosts, making eradication difficult.

Table: Some Common Zoonoses

Type	Examples
Helminthic	Schistosomiasis, Trichinosis
Protozoan	Malaria, Toxoplasmosis
Fungal	Ringworm
Bacterial	Lyme disease, Plague
Viral	Rabies, Influenza

Example: Rabies is transmitted from animals (such as bats or dogs) to humans via bites.

Human Carriers

Some humans can harbor pathogens without showing symptoms (asymptomatic carriers) and can transmit infections to others. Some carriers eventually develop illness, while others remain healthy due to effective immune responses.

Nonliving Reservoirs

Soil, water, and food can act as nonliving reservoirs for pathogens. Contamination is often due to fecal or urinary matter, and these reservoirs can be important sources of infection for humans.