Microbiology: Symbiotic Relationships

Symbiotic Relationships

• 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

• 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.

Acquisition of Normal 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

• 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 versus Infection

• 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

• 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.

Skin

• 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

• 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.

Parenteral Route

• 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

• 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 versus Disease

• 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.

Manifestations of Disease

• 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.

Etiology

• Etiology is the study of the cause of disease.
• The germ theory of disease states that disease is caused by infections of pathogenic microorganisms.