Ch 14 Infection, Infectious Diseases, and Epidemiology: Study Notes

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 types of symbiotic relationships:

• 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.

Example: Protozoans in termite intestines digest cellulose with the help of bacteria, benefiting both the protozoans and termites (mutualism).

Normal Microbiota of Humans

The microbiome consists of all microorganisms that colonize the body's surfaces without causing disease under normal conditions. These are also called normal microbiota, normal flora, or indigenous microbiota.

• Resident microbiota: Remain throughout a person's life, mostly commensal.

• Transient microbiota: Present for short periods (hours to months), often eliminated by competition, body defenses, or changes in the body.

Distribution of Resident Microbiota

Resident microbiota are found in various body regions, each with characteristic genera:

Region	Genera	Notes
Upper Respiratory Tract	Fusobacterium, Haemophilus, Lactobacillus, Moraxella, Staphylococcus, Streptococcus, Malassezia (fungus), Candida (fungus)	The nose is cooler than the rest of the respiratory system and has unique microbiota. The microbiota of the trachea and bronchi are similar to those of the nose and mouth. The alveoli of the lungs have no normal microbiota—they are axenic.

Region	Genera	Notes
Upper Digestive Tract	Actinomyces, Bacteroides, Corynebacterium, Haemophilus, Lactobacillus, Neisseria, Staphylococcus, Treponema, Entamoeba (protozoan), Neisseria (gonococcus)	Microbes colonize surfaces of teeth, gingiva, lining of cheeks, and pharynx, and they are found in saliva in large numbers. Duodenum of small intestine has fewer bacteria.
Lower Digestive Tract	Bacteroides, Bifidobacterium, Clostridium, Enterococcus, Escherichia, Lactobacillus, Proteus, Shigella, Candida (fungus), Entamoeba (protozoan), Trichomonas (protozoan)	The bacteria are mostly strict anaerobes, though some facultative anaerobes are also resident.

Region	Genera	Notes
Female Urinary and Reproductive Systems	Bacteroides, Clostridium, Lactobacillus, Staphylococcus, Streptococcus, Candida (fungus), Trichomonas (protozoan)	Microbiota change as acidity in the vagina changes during menstrual cycle. The flow of urine prevents extensive colonization of the urinary bladder or urethra.
Male Urinary and Reproductive Systems	Bacteroides, Clostridium, Lactobacillus, Mycobacterium, Pseudomonas, Staphylococcus, Streptococcus	The flow of urine prevents extensive colonization of the urinary bladder or urethra.

Region	Genera	Notes
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.
Eyes	Corynebacterium, Staphylococcus	Tears wash most microbiota from the eyes, so fewer are present compared to the skin.

Development and Acquisition of the Microbiome

The human fetus develops in a sterile environment. Microbiome acquisition begins during birth and continues in the first months of life, influenced by birth mode, diet, and environment. Early microbiome composition can affect immune development and susceptibility to atopic disorders.

Opportunistic Pathogens

Normal microbiota can become opportunistic pathogens under certain conditions, such as:

• Introduction into unusual body sites

• Immune suppression

• Changes in the normal microbiota (e.g., antibiotic use)

• Stressful conditions

Reservoirs of Infectious Diseases

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as sources of infection:

• Animal reservoirs: Zoonoses are diseases naturally spread from animal hosts to humans (e.g., rabies, malaria).

• Human carriers: Asymptomatic individuals can transmit pathogens (e.g., tuberculosis, AIDS).

• Nonliving reservoirs: Soil, water, and food contaminated with pathogens (e.g., Clostridium in soil).

The Invasion and Establishment of Microbes in Hosts: Infection

Contamination vs. Infection

• Contamination: Presence of microbes in or on the body.

• Infection: When microbes evade defenses, multiply, and establish themselves in the body.

Portals of Entry

Pathogens enter the body through specific portals:

• Skin: Barrier to most microbes; entry via cuts, hair follicles, or by burrowing organisms.

• 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: Circumvents normal portals by direct deposition into tissues (e.g., punctures, injections).

Pathogens Crossing the Placenta

Pathogen	Condition in Adult	Effect on Embryo/Fetus
Toxoplasma gondii (protozoan)	Toxoplasmosis	Abortion, epilepsy, microcephaly, blindness, etc.
Treponema pallidum (bacterium)	Syphilis	Abortion, birth defects, death
Listeria monocytogenes (bacterium)	Listeriosis	Granulomatosis infantiseptica, death
Cytomegalovirus (DNA virus)	Usually asymptomatic	Deafness, microcephaly, mental retardation
Lentivirus (HIV, RNA virus)	AIDS	Immunosuppression (AIDS)
Rubivirus (RNA virus)	German measles	Severe birth defects or death

The Nature of Infectious Disease

Manifestations of Disease

• Symptoms: Subjective characteristics felt only by the patient (e.g., pain, nausea).

• Signs: Objective manifestations observed or measured by others (e.g., fever, rash).

• Syndrome: Group of symptoms and signs that characterize a disease.

• Asymptomatic (subclinical): Infections without symptoms but with detectable signs.

Categories of Diseases

Category	Description	Examples
Hereditary	Genetic errors from parents	Sickle-cell anemia, Down syndrome
Congenital	Defects present at birth	Fetal alcohol syndrome
Degenerative	Result from aging	Renal failure
Nutritional	Lack of essential nutrients	Rickets
Endocrine	Hormonal imbalances	Addison’s disease
Mental	Emotional or psychosomatic	Skin rash, GI distress
Immunological	Immune system dysfunction	Allergies, autoimmune diseases
Neoplastic	Abnormal cell growth	Cancer
Infectious	Caused by infectious agents	Colds, influenza
Iatrogenic	Result of medical treatment	Yeast vaginitis after antibiotics
Idiopathic	Unknown cause	Alzheimer’s disease
Nosocomial	Acquired in healthcare settings	Pseudomonas infection in burn patient

Etiology and Koch’s Postulates

Etiology is the study of the cause of disease. Robert Koch developed postulates to prove a specific pathogen causes a specific disease:

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 include pathogens that cannot be cultured, diseases caused by multiple agents, and ethical limitations.

Virulence and Virulence Factors

Pathogenicity is the ability to cause disease; virulence is the degree of pathogenicity. Virulence factors include:

• Adhesion factors

• Biofilms

• Extracellular enzymes

• Toxins (exotoxins and endotoxins)

• Antiphagocytic factors

Extracellular Enzymes

Secreted by pathogens to dissolve host chemicals, aiding invasion and evasion of defenses. Mutants lacking these enzymes are often avirulent.

Biofilms

Bacteria can form biofilms, such as dental plaque, which protect them from host defenses and antibiotics.

Toxins

Toxins are chemicals that damage host tissues or trigger damaging immune responses. Toxemia is the presence of toxins in the blood.

• Exotoxins: Secreted proteins, often highly toxic (e.g., botulinum toxin).

• Endotoxins: Lipid A component of Gram-negative bacterial LPS, released upon cell death.

Feature	Exotoxins	Endotoxins
Source	Mainly Gram-positive and Gram-negative bacteria	Gram-negative bacteria
Chemical Nature	Protein or short peptide	Lipid A of LPS
Toxicity	High	Low (may be fatal in high doses)
Heat Stability	Unstable above 60°C	Stable up to 121°C
Fever Producing?	No	Yes
Antigenicity	Strong	Weak
Toxoid Formation	Possible	Not feasible
Representative Diseases	Botulism, tetanus, diphtheria	Typhoid fever, endotoxic shock

Antiphagocytic Factors

These factors help pathogens evade phagocytosis by host immune cells:

• Bacterial capsules (not recognized as foreign, slippery)

• Antiphagocytic chemicals (prevent lysosome fusion, destroy phagocytes)

The Stages of Infectious Disease

Infectious diseases typically progress through five stages:

1. Incubation period: No signs or symptoms.

2. Prodromal period: Mild, general symptoms.

3. Illness: Most severe signs and symptoms.

4. Decline: Declining signs and symptoms.

5. Convalescence: No signs or symptoms.

Transmission of Infectious Diseases

Portals of Exit

Pathogens leave the host through various portals, often the same as entry points, in secretions or excretions.

Modes of Transmission

• Contact transmission: Direct (person-to-person), indirect (fomites), or droplet (short-range aerosols).

• Vehicle transmission: Airborne (long-range aerosols), waterborne, foodborne, or via bodily fluids.

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

Classification of Infectious Diseases

Term	Definition
Acute disease	Symptoms develop rapidly, short course
Chronic disease	Mild symptoms, develop slowly, last long
Subacute disease	Symptoms and course between acute and chronic
Asymptomatic disease	No symptoms
Latent disease	Appears long after infection
Communicable disease	Transmitted from host to host
Contagious disease	Easily spread communicable disease
Noncommunicable disease	Not spread person-to-person
Local infection	Confined to a small region
Systemic infection	Widespread, often via blood or lymph
Focal infection	Site that serves as a source for other sites
Primary infection	Initial infection
Secondary infection	Follows a primary infection

Epidemiology of Infectious Diseases

Frequency of Disease

• Incidence: Number of new cases in a given area and time period.

• Prevalence: Total number of cases in a given area and time period.

Epidemiological Studies

• Descriptive epidemiology: Tabulation of data, identification of index case.

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

• Experimental epidemiology: Tests hypotheses, applies Koch’s postulates.

Nosocomial (Healthcare-Associated) Infections

• Exogenous: Acquired from the healthcare environment.

• Endogenous: Arise from normal microbiota within the patient.

• Iatrogenic: Result from medical procedures.

• Superinfections: Result from antimicrobial use suppressing normal microbiota.

Public Health and Disease Control

Public health agencies (e.g., CDC, WHO) monitor, report, and work to interrupt disease transmission through:

• Enforcing cleanliness of water and food

• Reducing vectors and reservoirs

• Immunization programs

• Isolation and quarantine measures

Additional info: These notes cover the core concepts of infection, infectious diseases, and epidemiology, including the roles of microbiota, reservoirs, portals of entry and exit, disease progression, transmission, classification, and public health interventions.