Chapter 14

Symbiotic Relationships Between Microbes and Their Hosts

Types of Symbiosis

Symbiosis refers to the close association between two different biological species. In microbiology, humans and microbes engage in various forms of symbiotic relationships, which can be classified as mutualism, commensalism, or parasitism.

• Mutualism: Both organisms benefit from the relationship. Example: Bacteria in the human colon synthesize vitamins for the host and receive nutrients in return.

• Commensalism: One organism benefits, while the other is neither helped nor harmed. Example: Staphylococcus on human skin.

• Parasitism: One organism benefits at the expense of the other. Example: Mycobacterium tuberculosis in the human lung.

Type	Organism 1	Organism 2	Example
Mutualism	Benefits	Benefits	Bacteria in human colon
Commensalism	Benefits	Neither benefits nor is harmed	Staphylococcus on skin
Parasitism	Benefits	Is harmed	Tuberculosis bacteria in human lung

Normal Microbiota in Hosts

Normal microbiota, also known as normal flora or indigenous microbiota, are microorganisms that colonize the body's surfaces without causing disease under normal conditions. They are classified as resident or transient microbiota.

• Resident microbiota: Remain a part of the normal microbiota throughout life and are mostly commensal.

• Transient microbiota: Remain in the body for a short period and cannot persist due to competition, elimination by body defenses, or chemical/physical changes.

Some Resident Microbiota

Resident microbiota inhabit various regions of the human body, including the upper respiratory tract, digestive tract, urinary and reproductive systems, and eyes and skin. Their composition varies by location and environmental conditions.

Body Site	Representative Genera	Notes
Upper Respiratory Tract	Staphylococcus, Streptococcus, Haemophilus	Microbiota similar to those of the mouth; nasal cavity is cooler and drier.
Upper Digestive Tract	Actinomyces, Bacteroides, Streptococcus	Microbiota colonize surface of teeth, gingiva, lining of cheeks, and pharynx.
Lower Digestive Tract	Bacteroides, Clostridium, Escherichia	Most are strict anaerobes; microbiota are important for digestion and vitamin production.
Female Urinary and Reproductive Systems	Lactobacillus, Streptococcus	Microbiota change with hormonal fluctuations.
Male Urinary and Reproductive Systems	Staphylococcus, Streptococcus	Microbiota exist primarily in the anterior urethra.
Eyes and Skin	Staphylococcus, Corynebacterium	Microbiota live on the outer, dead layers of skin and in the conjunctiva of the eye.

Acquisition and Opportunism of Normal Microbiota

Humans are born axenic (free of microorganisms), but microbiota begin to develop during the birthing process and are established in the first months of life. Normal microbiota can become opportunistic pathogens if introduced to unusual sites, if the immune system is suppressed, or if changes in the microbiota occur.

• Opportunistic pathogens: Normal microbiota that cause disease under certain circumstances.

• Conditions for opportunism: Introduction to unusual sites, immune suppression, or changes in abundance.

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

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

• Animal reservoirs: Diseases naturally spread from animal hosts to humans (zoonoses) via direct contact, consumption, or arthropod vectors.

• Human carriers: Infected individuals who are asymptomatic but can transmit pathogens to others.

• Nonliving reservoirs: Soil, water, and food contaminated by feces or urine can harbor infectious agents.

The Movement of Microbes into Hosts: Infection

Contamination and Infection

Contamination is the mere presence of microbes in or on the body, while infection occurs when an organism evades external defenses, multiplies, and becomes established in the body.

Portals of Entry

Pathogens enter the body through specific portals of entry, including the skin, mucous membranes, placenta, and parenteral route.

• Skin: Acts as a barrier, but pathogens can enter through cuts or by burrowing/digesting outer layers.

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

• Placenta: Usually an effective barrier, but some pathogens can cross and infect the fetus.

• Parenteral route: Pathogens are deposited directly into tissues beneath the skin or mucous membranes, bypassing normal barriers.

The Role of Adhesion in Infection

Adhesion is the process by which microorganisms attach to host cells, a critical step for colonization and infection. Adhesion factors include specialized structures and attachment proteins (ligands) that bind to host cell receptors, determining host specificity. Inability to adhere renders microbes avirulent. Some bacteria form biofilms for collective attachment.

The Nature of Infectious Disease

Manifestations of Disease: Symptoms, Signs, and Syndromes

Infection is the invasion of the host by a pathogen. Disease (morbidity) results if the pathogen alters normal body functions. Symptoms are subjective and felt only by the patient, signs are objective and observable by others, and a syndrome is a group of symptoms and signs that characterize a disease. Asymptomatic infections lack symptoms but may still have signs.

Causation of Disease: Etiology

Etiology is the study of the cause of disease. The germ theory states that diseases are caused by infections of pathogenic microorganisms. Robert Koch developed postulates to prove causation:

1. The suspected agent must be present in every case of the disease.

2. The agent must be isolated and grown in pure culture.

3. The cultured agent must cause the disease when inoculated into a healthy host.

4. The same agent must be reisolated from the diseased experimental host.

Exceptions exist, such as pathogens that cannot be cultured, diseases with multiple causes, or ethical limitations.

Virulence Factors of Infectious Agents

Pathogenicity is the ability to cause disease, while virulence is the degree of pathogenicity. Virulence factors include adhesion factors, biofilms, extracellular enzymes, toxins, and antiphagocytic factors.

• Extracellular enzymes: Secreted by pathogens to dissolve structural chemicals and aid invasion.

• Toxins: Chemicals that harm tissues or trigger damaging immune responses. Exotoxins are secreted proteins, while endotoxins (lipid A) are released from Gram-negative bacteria upon cell death.

• Antiphagocytic factors: Capsules and chemicals that prevent phagocytosis by host cells.

The Stages of Infectious Disease

Infectious diseases typically progress through five stages: incubation, prodromal period, illness, decline, and convalescence. The number of microorganisms and intensity of symptoms vary throughout these stages.

The Movement of Pathogens Out of Hosts: Portals of Exit

Pathogens exit the host through portals of exit, often the same as portals of entry, and are typically found in materials secreted or excreted by the body.

Modes of Infectious Disease Transmission

Transmission Pathways

Transmission occurs from a reservoir or portal of exit to another host's portal of entry. There are three main groups:

• Contact transmission: Direct, indirect, or droplet spread.

• Vehicle transmission: Airborne, waterborne, or foodborne routes.

• Vector transmission: Biological (e.g., mosquitoes) or mechanical (e.g., flies).

Classification of Infectious Diseases

Classification Criteria

Infectious diseases can be classified by the body system affected, taxonomic categories, longevity and severity, mode of transmission, or population effects. Terms include:

• Acute disease: Rapid onset, short duration.

• Chronic disease: Develops slowly, persists over time.

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Pathogen remains inactive for a period.

• Communicable: Can be spread from host to host.

• Contagious: Easily spread from host to host.

Epidemiology of Infectious Diseases

Frequency of Disease

Epidemiologists track disease occurrence using incidence (new cases in a given period) and prevalence (total cases in a given period). Occurrence is also evaluated by frequency and geographic distribution.

Epidemiological Studies

• Descriptive epidemiology: Tabulates data, records location and time, collects patient information, and identifies the index case.

• Analytical epidemiology: Determines probable cause, mode of transmission, and prevention methods, often retrospectively.

• Experimental epidemiology: Tests hypotheses concerning disease causation, often applying Koch's postulates.

Hospital Epidemiology: Nosocomial Infections

Nosocomial infections are acquired in healthcare settings. Types include:

• Exogenous: Pathogen acquired from the healthcare environment.

• Endogenous: Pathogen arises from normal microbiota due to healthcare factors.

• Iatrogenic: Results from modern medical procedures.

Hand washing is the most effective way to reduce nosocomial infections.

Epidemiology and Public Health

Public health agencies at various levels monitor and share information about disease, work to limit transmission, ensure water and food safety, and educate the public to promote healthful choices.