Symbiosis and Normal Flora

• A fetus in utero is normally free of microbes.
• During birth, a newborn is exposed to microbes, initiating the colonization of the infant's intestine.

Symbiosis

• Symbiosis describes the close relationship between different species within a community.
  • Mutualism: Both organisms benefit in this interaction, for example, E. coli in the human GI tract.
  • Commensalism: One organism benefits, while the other is unaffected; for example, some mycobacteria inhabiting the ear, living on secretions and dead cells.
  • Parasitism: One organism benefits, while the other is harmed; for example, tuberculosis, helminths, and protozoa.
  • Amensalism: One organism inhibits the growth/survival of another without being affected itself; for example, penicillium.

Normal Flora (Microbiota)

• Resident flora: Normal flora present throughout a person's life, for example, S. epidermidis and E. coli.
• Transient flora: Microbes that remain for hours, days, or months before disappearing, e.g., Bacillus Laterosporus.
• Normal flora is typically protective and doesn't cause disease in its normal habitat in healthy individuals.
• When balance is disrupted, normal flora can become opportunistic pathogens.

Portal of Entry

• Exogenous: Pathogens enter from the outside environment
• Endogenous: Pathogens arise from within the body
• Most pathogens have a preferred portal of entry.
• If a pathogen enters the "wrong" portal, infection may not occur.
• Some infectious agents can enter through multiple portals, such as Streptococcus and Staphylococcus.

Portals of Entry

• Skin
• Mucous membranes
• Placenta: Some microbes can cross the placenta, potentially causing spontaneous abortions, birth defects, or premature births, for example, HIV, Rubivirus, Cytomegalovirus, Parvovirus B-19, Treponema pallidum, Listeria monocytogenes, Toxoplasma gondii.

Virulence

• Virulence is the degree of pathogenicity or disease-provoking power of a specific microbe.
  • Adhesion: The initial step of infection, facilitated by pili and cell membrane proteins.
  • Colonization: Occurs in tissues in contact with the external environment, such as the urogenital, digestive, and respiratory tracts, and the conjunctiva.
  • Invasion: Disruption of host cell membranes and barriers by invasins.
  • Evasion of Host Defense: Mechanisms to avoid phagocytes, inhibit engulfment, survive inside phagocytes, or produce substances that harm phagocytes.

Toxins

• Toxins are a major virulence factor
• Toxigenesis is the ability of an organism to produce toxins.
• Bacterial toxins can act on sites remote from the original infection site.

Endotoxin

• Lipopolysaccharides found in the gram-negative cell wall.
• Released during cell lysis, whether by host defense or antibiotics.
• Less potent and less specific than exotoxins.
• Heat-stable but can be affected by certain oxidizing agents like peroxide.
• Example: Shiga toxin.

Exotoxins

• Proteins released by bacteria during the exponential growth phase.
• More potent and specific than endotoxins.
• Heat-labile and can be affected by acids and proteolytic enzymes.
• Examples include enterotoxins, neurotoxins, leukocidins, and hemolysins, named according to their target tissue.

Portal of Exit

• The site where a pathogen leaves an infected person.
• Often the same as the portal of entry.
• Pathogens can also exit through defecation, blood, nasal secretions, saliva, sputum, respiratory droplets, tears, and earwax.

Etiology of Infectious Disease

• The study of the cause of disease.

Patterns of Infection

- Local infections: The organism remains confined to a specific tissue.
- Focal infections: The pathogen spreads from a local infection to other tissues.
- Systemic infections: Infection spreads to multiple sites and tissues, usually through the circulatory system.
    - Bacteremia: Presence of bacteria in the blood.
    - Septicemia: Systemic infection caused by the multiplication of pathogens in the blood.
    - Toxemia: Presence of toxins in the blood.
    - Viremia: Presence of viruses in the blood.
- Incubation period: Time from initial contact with the infectious agent to the appearance of first symptoms.
- Prodromal stage: Earliest, nonspecific symptoms like discomfort, weakness, headache, muscle pain, upset stomach.
- Convalescence: Recovery period.
- Rehabilitation: Maximizing function of diseased tissue.
- Mixed infection: Several infectious agents concurrently establish themselves at the same site.
- Acute infection: Rapid onset with severe symptoms, vanishes quickly.
- Chronic infection: Less severe symptoms but persist for a long period.
- Primary infection: The initial infection.
- Secondary infection: Follows a primary infection.
- Subclinical infection: No apparent symptoms, can last a long time.

Epidemiology and Public Health

• Epidemiology: the study of the distribution and causes of disease in populations.
  • Examines the number of people affected, location, and disease outcomes (recovery, death, disability, etc.).
  • CDC: Central source of epidemiological information.
    • Releases data on morbidity, mortality, and the incidence of specific notifiable diseases.
    • Morbidity: Number of patients with a disease within a group.
    • Mortality: Number of deaths resulting from a particular disease.
    • Incidence: Number of new cases in a given population within a specified time.
    • Prevalence: Number of new and old cases in a specific population within a specified time.

Disease Categories

- Endemic disease: Repeatedly present in a given population or geographic area, such as the common cold.
- Sporadic disease: Breaks out only occasionally, for example, typhoid fever in the U.S.
- Epidemic disease: Occurs more frequently than usual in a population, such as influenza.
- Pandemic disease: A worldwide epidemic, such as AIDS and the 1918 influenza.

Reservoirs

• Sites where pathogens are maintained and become a source of infection.
  • Animal reservoir: Pathogens that infect animals and can affect humans (zoonoses).
  • Human carriers: Individuals who are symptom-free.
  • Nonliving reservoirs: Include soil, water, food, fomites.

Modes of Transmission

• Contact Transmission

  • Direct Physical Contact: Between hosts without an intermediate object.
    • Person-to-person contact like touching, kissing, and sexual intercourse.
    • Examples include respiratory tract infections, staphylococcal infections, measles, scarlet fever, and STDs.
  • Indirect Contact: Pathogen transmitted by a fomite (nonliving object) like tissues, handkerchiefs, towels, bedding, toys, clothes, diapers, eating utensils, drinking cups, medical equipment.
  • Droplet Transmission: Infectious agents are transmitted via respiratory droplets that travel less than a meter.
  • Airborne Transmission: Organisms travel more than 1 meter.

• Water-borne & Food-borne Transmission

  • Through untreated or poorly treated sewage and undercooked or unsanitarily prepared foods.
  • Causes gastrointestinal diseases like giardiasis, amebic dysentery, cholera, shigellosis, and Hepatitis A & E.

• Bodily Fluid Transmission

  • Blood, urine, saliva, and other bodily fluids.

• Vector Transmission

  • Animals, especially arthropods.
    • Biological Vectors: Biting insects, like mosquitoes, ticks, lice, fleas, and blood-sucking flies.
    • Mechanical Vectors: Passively carry agents on their feet or other body parts.

Healthcare Associated (Nosocomial) Infections

• Acquired in a hospital or similar setting, among the top 10 leading causes of death in the U.S.
  • Annually, 99,000 patients die from nosocomial infections in the U.S.
  • Exogenous HAIs: Caused by pathogens in the healthcare environment shed by sick people.
  • Endogenous HAIs: Caused by microbes in the patient's normal flora.
  • Iatrogenic HAIs: Caused by medical procedures like catheter use, invasive diagnostics, and surgery.