Symbiosis and Normal Flora

• A fetus in utero is normally microbe-free.
• During birth, a newborn gets exposed to microbes, starting intestine colonization.

Symbiosis

• Symbiosis: A close relationship between different species in a community.
• Mutualism: Both organisms benefit; for example, E. coli in the human gastrointestinal tract.
• Commensalism: One organism benefits, the other is neither helped nor harmed; for example, some mycobacteria in the ear.
• Parasitism: One organism benefits, the other is harmed; for example, tuberculosis, helminths, and protozoa.
• Amensalism: One organism inhibits another's growth/survival without being affected; 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: Present for hours, days, or months before disappearing; for example, Bacillus Laterosporus.
• Normal flora are protective and usually do not cause disease in a healthy person.
• When balance is disrupted, normal flora can become opportunistic pathogens.

Portal of Entry

• Exogenous: From outside the body.
• Endogenous: From inside the body.
• Most pathogens have a preferred portal of entry.
• If a pathogen enters the "wrong" portal, infection may not occur.
• Some infectious agents enter via multiple portals, for example, Streptococcus and Staphylococcus.

Portals of Entry

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

Virulence

• Virulence: The degree of pathogenicity or disease-provoking power of a microbe.
• Virulence factors contribute to the severity of disease.
• Adhesion: The first step of infection, facilitated by pili and 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 using invasins.
• Evasion of Host Defense: Includes avoiding contact with phagocytes, inhibiting phagocytic engulfment, surviving inside phagocytes, or producing products that kill/damage phagocytes.
• Toxins: a major virulence factor, Bacterial toxins act on sites remote from the initial site of infection.
• Toxigenesis: The ability of an organism to produce toxins.

Endotoxin

• Lipopolysaccharides of gram-negative cell walls.
• Released during cell lysis and during antibiotic treatment.
• Less potent and specific than exotoxins.
• Heat-stable.
• Can be affected by certain oxidizing agents like peroxide.
• Example: Shiga toxin.

Exotoxins

• Proteins released by bacteria during exponential growth.
• More potent and specific than endotoxins.
• Heat-labile.
• Can be affected by acids and proteolytic enzymes.
• Examples: enterotoxin, neurotoxin, leukocidin, hemolysin (named according to their target tissue).

Portal of Exit

• Site where a pathogen leaves an infected person.
• Often the same as the portal of entry.
• Pathogens can leave 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 multiple 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, such as discomfort, weakness, headache, muscle pain, and 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: Appears rapidly with severe symptoms, vanishes rapidly.
• Chronic infection: Less severe symptoms but persist for long periods.
• Primary infection: Initial infection.
• Secondary infection: Follows a primary infection.
• Subclinical infection: No apparent symptoms, can be prolonged.

Epidemiology and Public Health

• Epidemiology: Study of the distribution and causes of disease in populations.
• Focus on: How many people are affected, where, and the outcome of the disease.
• CDC: Central source of epidemiological information.
• 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 cases in a specific population within a specified time period.

Disease Categories

• Endemic disease: Repeatedly present in a given population or geographical 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: Worldwide epidemic; for example, AIDS and influenza of 1918.

Reservoirs

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

Modes of Transmission

• Contact Transmission:
  • Direct Physical Contact: Between hosts without an intermediate object, like touching, kissing, or 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, and devices.
• Droplet Transmission: Infectious agents transmitted via respiratory droplets, organisms travel less than 1 meter.
• Airborne Transmission: Organisms travel more than 1 meter.
• Water-borne & Food-borne Transmission: Through untreated or poorly treated sewage, undercooked foods, or unsanitary food preparation, leads to 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 to a new host on their feet or other body parts.

Healthcare Associated (Nosocomial) Infections

• Acquired in a hospital or hospital-like setting.
• HAIs are among the top 10 leading causes of death in the United States.
• Annually, 99,000 patients die as a result of nosocomial infections in the United States 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 patient's normal flora.
• Iatrogenic HAIs: Caused by medical procedures like catheters, invasive diagnostics, and surgery.