Symbiosis and Normal Flora

• A fetus in utero is typically free of microbes under normal conditions.
• During birth, a newborn gets exposed to microbes that start colonizing the infant’s intestine.

Symbiosis

• It is a close relationship between two different species of organisms in a community.
• Mutualism: Both organisms benefit; for example, E. coli in the human GI tract.
• Commensalism: One organism benefits, and the other is neither harmed nor helped; for example, some mycobacteria in the ear.
• Parasitism: One organism benefits, while the other is harmed; for example, tuberculosis.
• 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 & E. Coli.
• Transient flora: Flora that remain for a few hours, days, or months before disappearing, for example, Bacillus Laterosporus.
• Normal flora are usually protective and don't cause disease in a healthy person in their normal habitat.
• When the balance is disturbed, normal flora can become opportunistic pathogens.

Portal of Entry

• Exogenous: From outside the body.
• Endogenous: From inside the body.
• Most pathogens have a preferred entry point; if they enter the "wrong" portal, infection may not occur.
• Some pathogens can enter through multiple portals, such as Streptococcus and Staphylococcus.

Portals of Entry

• Skin
• Mucous membranes
• Placenta:
  • Some microbes cross, leading to spontaneous abortions, birth defects, or premature births.
  • Examples include HIV, Rubivirus, Cytomegalovirus, Parvovirus B-19, Treponema pallidum, Listeria monocytogenes, Toxoplasma gondii.

Virulence

• It indicates the degree of pathogenicity or disease-provoking power of a specific microbe.

Virulence Factors

• Adhesion:
  • It is the first step of infection.
  • Achieved by pili & cell membrane proteins.
• Colonization:
  • Occurs in tissues in contact with the external environment: urogenital, digestive, respiratory tracts, and conjunctiva.
• Invasion:
  • Disrupts host cell membranes and barriers (invasins).
• Evasion of Host Defense:
  • Involves avoiding contact with phagocytes.
  • It also includes inhibiting phagocytic engulfment.
  • Also includes survival inside the phagocytes
  • Some produce products that kill or damage phagocytes before or after ingestion.
• Toxins:
  • A major virulence factor.
• Toxigenesis:
  • Refers to the organism's ability 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, either by host defense or by antibiotics.
• Less potent and less specific than exotoxins.
• Heat-stable but can be affected by 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 & proteolytic enzymes.
• Examples: enterotoxin, neurotoxin, leukocidin, hemolysin, named by their target tissue.

Portal of Exit

• The site where a pathogen leaves the infected person.
• Often the same as the portal of entry.
• Pathogens can also exit via 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: Organism remains confined to a specific tissue.
• Focal infections: Pathogen spreads from a local infection to other tissues.
• Systemic infections: Infection spreads to several sites and tissues, usually via 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 of infection like discomfort, weakness, headache, muscle pain, upset stomach.
• Convalescence: Recovery period.
• Rehabilitation: Maximizing function of diseased tissue.
• Mixed infection: Multiple infectious agents establish themselves at the same site concurrently.
• Acute infection: Appears rapidly, with severe symptoms, vanishes rapidly.
• Chronic infection: Usually less severe symptoms but persist for long periods.
• Primary infection: Initial infection.
• Secondary infection: Follows a primary infection.
• Subclinical infection: No apparent symptoms but can be of long periods of time.

Epidemiology and Public Health

• Epidemiology: The study of the distribution and cause of disease in populations. -Includes figuring out how many people are affected, where, and the outcome of the disease.

CDC

• It serves as the central source of epidemiological information.
• Releases data on morbidity, mortality & 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 period.
• Prevalence: Number of new and old or existing cases in a specific population within a specified time period.

Disease Categories

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

Reservoirs

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

Modes of Transmission

• Contact Transmission
• Direct Physical Contact:
  • Between hosts without an intermediate object.
  • Person-to-person by touching, kissing, or sexual intercourse.
  • Examples: Respiratory tract infections, staphylococcal infections, measles, scarlet fever, and sexually transmitted diseases.
• Indirect Contact:
  • Pathogen is transmitted by a fomite (nonliving object).
  • Examples: Tissues, handkerchiefs, towels, bedding, toys, clothes, diapers, eating utensils, drinking cups, medical equipment, and devices.
• Droplet Transmission:
  • Infectious agents are transmitted via respiratory droplets.
  • These organisms travel less than a meter.
• Airborne Transmission:
  • Organisms travel more than 1 meter.
• Water-borne & Food-borne Transmission:
  • Occurs through untreated/poorly treated sewage & undercooked foods or food prepared under unsanitary conditions.
  • Examples: Gastrointestinal diseases like giardiasis, amebic dysentery, cholera, shigellosis, and Hepatitis A & E.
• Bodily Fluid Transmission:
  • Transmission via blood, urine, saliva, and other bodily fluids.
• Vector Transmission:
  • Involves animals, especially arthropods.
• Biological Vectors:
  • Biting insects, including mosquitoes, ticks, lice, fleas, blood-sucking flies.
• Mechanical Vectors:
  • Passively carry agents to a new host via their feet or other bodily parts.

Healthcare-Associated (Nosocomial) Infections

• Infections acquired in a hospital or hospital-like setting.
• HAIs are among the top 10 leading causes of death in the United States.
• Annually, approximately 99,000 patients die due to nosocomial infections in the U.S. alone.

Types of Nosocomial Infections

• Exogenous HAIs: Caused by pathogens in the healthcare environment, shed by sick people.
• Endogenous HAIs: Caused by microbes in the normal flora of the patient.
• Iatrogenic HAIs: Caused by medical procedures such as catheters, invasive diagnostic procedures, and surgery.