Microbiology: Symbiotic Relationships

Study Notes

Symbiosis means "to live together" and describes the relationship between microorganisms and their host.
There are three types of symbiotic relationships: mutualism, commensalism, and parasitism.

Normal microbiota, also termed normal flora and indigenous microbiota, refers to the organisms that colonize the body's surfaces without normally causing disease.
There are two types of normal microbiota: resident microbiota and transient microbiota.
Resident microbiota are a part of the normal microbiota throughout life, and most are commensal.
Transient microbiota remain in the body for only hours to months before disappearing, and are found in the same regions as resident microbiota.

Development in the womb is generally free of microorganisms (axenic).
Microbiota begins to develop during the birthing process.
Much of one's resident microbiota is established during the first months of life.

Opportunistic pathogens are normal microbiota that can cause disease under certain circumstances.
Conditions that provide opportunities for pathogens include immune suppression, changes in the normal microbiota, and introduction of normal microbiota into unusual sites in the body.

Contamination is the mere presence of microbes in or on the body.
Infection results when the organism has evaded the body's external defenses, multiplied, and become established in the body.

Portals of entry are sites through which pathogens enter the body.
The four major types of portals of entry are skin, mucous membranes, placenta, and parenteral route.

The outer layer of packed, dead, skin cells usually acts as a barrier to pathogens.
Some pathogens can enter through openings or cuts, while others enter by burrowing into or digesting the outer layers of skin.

Mucous membranes line the body cavities that are open to the environment.
They provide a moist, warm environment that is hospitable to pathogens.
The respiratory tract is the most commonly used site of entry, and pathogens able to survive the acidic pH of the stomach may use the gastrointestinal tract as a route of entry.

The parenteral route is not a true portal of entry but a means by which pathogens can be circumvented.
Pathogens are deposited directly into tissues beneath the skin or mucous membranes.

Adhesion is the process by which microorganisms attach themselves to cells.
Adhesion is required to successfully establish colonies within the host.
Microorganisms use adhesion factors, such as specialized structures, attachment proteins, and surface lipoproteins or glycoproteins, called ligands, that bind host cell receptors.

Infection is the invasion of the host by a pathogen.
Disease results only if the invading pathogen alters the normal functions of the body.
Disease is also referred to as morbidity.

Symptoms are subjective characteristics of disease felt only by the patient.
Signs are objective manifestations of disease that can be observed or measured by others.
A syndrome is a group of symptoms and signs that characterize a disease or abnormal condition.
Asymptomatic, or subclinical, infections lack symptoms but may still have signs of infection.