Ch 14 Infection, Infectious Diseases, and Epidemiology: Core Concepts and Mechanisms

Ch 14 - Symbiotic Relationships Between Microbes and Their Hosts

Types of Symbiosis

Symbiosis refers to the close and long-term interaction between different biological species. In microbiology, symbiotic relationships between microbes and their hosts are fundamental to understanding health and disease.

• Mutualism: Both organisms benefit from the relationship. Example: Bacteria in the human colon synthesize vitamins and aid digestion.

• Commensalism: One organism benefits, while the other is neither helped nor harmed. Example: Mites in human hair follicles.

• Amensalism: One organism is harmed, while the other is unaffected. Example: Fungus secreting an antibiotic that inhibits nearby bacteria.

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

Microbiome of Humans

Resident and Transient Microbiota

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

• Resident microbiota: Remain throughout life, mostly commensal, and are found in specific body regions.

• Transient microbiota: Present for short periods; cannot persist due to competition, immune defenses, or chemical/physical changes in the body.

Distribution of Resident Microbiota

Resident microbiota are found in various body sites, each with characteristic genera and ecological notes.

Body Site	Genera	Notes
Eyes and Skin	Corynebacterium, Micrococcus, Propionibacterium, Staphylococcus, Candida (fungus), Malassezia (fungus)	Microbiota are on the outer, dead layers of skin and hair follicles; deeper layers are mostly axenic.
Female Urinary and Reproductive Systems	Acinetobacter, Clostridium, Lactobacillus, Staphylococcus, Streptococcus, Candida (fungus), Trichomonas (protozoan)	Microbiota change as vaginal pH changes during menstrual cycle; flow of urine prevents extensive colonization.
Male Urinary and Reproductive Systems	Bacteroides, Pseudomonas, Lactobacillus, Mycobacterium, Peptostreptococcus, Staphylococcus	Flow of urine prevents extensive colonization.
Upper Digestive Tract	Actinomyces, Bacteroides, Corynebacterium, Haemophilus, Neisseria, Streptococcus, Treponema, Candida (fungus), Entamoeba (protozoan)	Microbes colonize surfaces of teeth, gums, cheeks, and pharynx; saliva contains hundreds of species.
Lower Digestive Tract	Bacteroides, Clostridium, Enterococcus, Escherichia, Lactobacillus, Proteus, Shigella, Candida (fungus), Entamoeba (protozoan), Trichomonas (protozoan)	Bacteria are mostly strict anaerobes; some facultative anaerobes are also present.
Upper Respiratory Tract	Prevotella, Haemophilus, Lactobacillus, Moraxella, Staphylococcus, Streptococcus, Candida (fungus)	Nose is cooler than rest of respiratory system; microbiota of trachea and bronchi are sparse; alveoli are axenic.

Acquisition of the Microbiome

Humans develop their microbiome during and after birth. The womb is typically sterile, and colonization begins during the birthing process and continues in the first months of life.

How Normal Microbiota Become Opportunistic Pathogens

Normal microbiota can cause disease under certain conditions, becoming opportunistic pathogens. These conditions include:

• Introduction into unusual sites in the body

• Immune suppression

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

• Stressful conditions

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

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

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

• Human carriers: Asymptomatic individuals can transmit pathogens; some may never develop illness.

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

Disease	Causative Agent	Animal Reservoir	Mode of Transmission
Tapeworm infestation	Dipylidium caninum	Dogs	Ingestion of larvae in dog saliva
Malaria	Plasmodium spp.	Monkeys	Bite of Anopheles mosquito
Anthrax	Bacillus anthracis	Domestic livestock	Direct contact with infected animals
Rabies	Lyssavirus sp.	Bats, foxes, dogs	Bite of infected animal
Yellow fever	Flavivirus sp.	Monkeys	Bite of Aedes mosquito

The Invasion and Establishment of Microbes in Hosts: Infection

Exposure to Microbes: Contamination and Infection

Contamination is the mere presence of microbes in or on the body. 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:

• Skin: Entry through cuts, openings, or by burrowing/digesting outer layers.

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

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

• Parenteral route: Not a true portal; pathogens are deposited directly into tissues beneath the skin or mucous membranes (e.g., via punctures).

Pathogen	Condition in Adult	Effect on Embryo or Fetus
Toxoplasma gondii	Toxoplasmosis	Abortion, epilepsy, encephalitis, microcephaly, mental retardation, blindness, anemia, jaundice
Treponema pallidum	Syphilis	Abortion, birth defects, syphilis
Listeria monocytogenes	Listeriosis	Granulomatosis infantiseptica, death
Rubivirus	German measles	Severe birth defects or death

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 molecules (ligands) that bind to host cell receptors. The specificity of these interactions can determine host and tissue tropism.

• Blocking or altering ligands/receptors can prevent infection.

• Microorganisms unable to produce adhesins are often avirulent.

• Some bacteria form biofilms, enhancing colonization and resistance to host defenses.

The Nature of Infectious Disease

Definitions: Infection, Disease, Morbidity, Pathogenicity, and Virulence

Infection is the invasion of the host by a pathogen. Disease (morbidity) results when the pathogen alters normal body functions. Pathogenicity is the ability to cause disease, while virulence is the degree of pathogenicity.

Manifestations of Disease: Symptoms, Signs, and Syndromes

Diseases are characterized by symptoms, signs, and syndromes:

• 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: A group of symptoms and signs that characterize a disease.

• Asymptomatic (subclinical) infections: Lack symptoms but may have signs.

Symptoms (Sensed by Patient)	Signs (Detected by Observer)
Pain, nausea, headache, chills, sore throat, fatigue, malaise, discomfort, itching, abdominal cramps	Swelling, rash, vomiting, diarrhea, fever, pus, anemia, leukocytosis/leukopenia, bubo, tachycardia/bradycardia

Causation of Disease: Etiology and Koch’s Postulates

Etiology is the study of the cause of disease. Robert Koch developed postulates to prove that 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 with multiple causes, and ethical limitations.

Virulence Factors of Infectious Agents

Virulence factors enhance a pathogen's ability to cause disease. These include:

• Adhesion factors (see above)

• Biofilms

• Extracellular enzymes: Secreted to dissolve host chemicals, aiding invasion and evasion of defenses.

• Toxins: Chemicals that harm tissues or trigger damaging immune responses. Two main types:

  • Exotoxins: Secreted proteins, highly toxic, specific effects.

  • Endotoxins: Lipid A from Gram-negative bacteria, released upon cell death, causes systemic effects.

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

Feature	Exotoxins	Endotoxins
Source	Mainly Gram-positive and some Gram-negative bacteria	Gram-negative bacteria
Chemical Nature	Protein	Lipid A of LPS
Toxicity	High	Low, but fatal in high doses
Heat Stability	Unstable above 60°C	Stable up to 121°C
Fever Producing	No	Yes
Antigenicity	Strong	Weak
Representative Diseases	Botulism, tetanus, diphtheria	Typhoid fever, meningococcal meningitis

The Stages of Infectious Disease

Infectious diseases typically progress through five stages:

1. Incubation period: Time between infection and appearance of symptoms.

2. Prodromal period: Short period of mild symptoms.

3. Illness: Most severe stage; characteristic signs and symptoms.

4. Decline: Symptoms subside as immune response/treatment reduces pathogen numbers.

5. Convalescence: Recovery and return to normal function.

The Movement of Pathogens Out of Hosts: Portals of Exit

Pathogens exit the host through portals often similar to entry sites, typically in materials the body secretes or excretes (e.g., respiratory droplets, feces, urine).

Modes of Infectious Disease Transmission

Contact, Vehicle, and Vector Transmission

• Contact transmission: Direct (person-to-person), indirect (via fomites), or droplet (mucus droplets from coughing/sneezing).

• Vehicle transmission: Airborne (aerosols), waterborne (contaminated water), foodborne (contaminated food), or via bodily fluids.

• Vector transmission: Biological vectors (e.g., mosquitoes) are hosts for part of the pathogen's life cycle; mechanical vectors (e.g., flies) passively carry pathogens.

Classification of Infectious Diseases

Diseases can be classified by taxonomic category, affected body system, longevity/severity, mode of transmission, or population effects. Terms include:

• Acute: Rapid onset, short duration

• Chronic: Slow development, long duration

• Latent: Pathogen remains inactive for a time

• Communicable: Spread from host to host

• Contagious: Easily spread

• Noncommunicable: Not spread between hosts

Epidemiology of Infectious Diseases

Frequency of Disease

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

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

Occurrence is also described as endemic, sporadic, epidemic, or pandemic.

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.

Hospital Epidemiology: Healthcare-Associated Infections (HAIs)

• 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.

Handwashing is the most effective way to reduce HAIs.