Symbiosis and Normal Flora

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

Symbiosis

• A close relationship between two different species of organisms in a community
• Mutualism: Both members 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
• These are considered normal flora, living on secretions and dead cells
• Parasitism: One organism benefits, while the other is harmed; for example, tuberculosis, helminths & protozoa
• Amensalism: One organism hampers or prevents the growth/survival of another, without being affected by the other organism; for example, penicillium

Normal Flora (Microbiota)

• Resident flora: normal flora throughout life such as S. epidermidis & E. coli
• Transient flora: remain for a few hours, days, or months before they vanish, such as Bacillus Laterosporus
• Normal flora are usually protective and do not cause disease in their normal habitat in a healthy person
• When balance is interrupted, normal flora can become opportunistic pathogens

Portal of Entry

• Exogenous portals come from outside the body
• Endogenous portals come from inside the body
• Majority of pathogens have a preferred portal of entry
• If a pathogen enters the "wrong" portal, infection may not occur
• Some infectious agents enter via more than one portal, like Streptococcus and Staphylococcus
• Portals of entry include:
  • 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 is the degree of pathogenicity or disease-provoking power of a specific microbe
• Virulence factors include:
  • Adhesion: The first step of infection, achieved by pili & cell membrane proteins
  • Colonization: This occurs in tissues in contact with the external environment; such as the urogenital tract, digestive tract, respiratory tract, and conjunctiva
  • Invasion: Disrupts host cell membrane & barriers by invasins
  • Evasion of Host Defense: Avoids contact with phagocytes, inhibits phagocytic engulfment, survives inside phagocytes, or produces products that kill or damage phagocytes
  • Toxins (major virulence factor): Toxigenesis is the ability of an organism to produce toxins
  • Bacterial toxins can act on sites remote from the original site of infection

Endotoxin

• Lipopolysaccharides of gram-negative cell wall
• Released during cell lysis (by host defense or by action of antibiotics)
• Less potent and less specific than exotoxins
• Heat-stable
• Can be affected by certain oxidizing agents; such as peroxide
• Example: Shiga toxin

Exotoxins

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

Portal of Exit

• The site of the pathogen leaving the infected person
• Often the same as the portal of entry
• A pathogen can also leave the host by defecation, blood, nasal secretions, saliva, sputum, respiratory droplets, tears, earwax

Etiology of Infectious Disease

• The study of the cause of disease

Patterns of Infection

• Local infections: The organism enters the body and remains confined to a specific tissue
• Focal infections: A pathogen spreads from a local infection to other tissues
• Systemic infections: An 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: the time from initial contact with the infectious agent to the appearance of first symptoms
• Prodromal stage: earliest symptoms of infection, nonspecific, feeling of 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: Appears rapidly, with severe & sharp symptoms, vanishes rapidly
• Chronic infection: Usually less severe symptoms but persist for long periods
• Primary infection: initial infection
• Secondary infection: follows primary infection
• Subclinical infection: no apparent symptoms, can be of long periods

Epidemiology and Public Health

• Epidemiology: Is the study of the distribution and cause of disease in populations
• Considers the number of people affected, where, and the outcome of the disease
• CDC: Central source of epidemiological information
• CDC 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 specific time period

Disease Categories

• Endemic disease: Repeatedly present in a given population or geographical area (e.g., common cold)
• Sporadic disease: Breaks out only occasionally (e.g., typhoid fever in the US)
• Epidemic disease: Occurs with greater frequency than usual in a population of a given area (e.g., influenza)
• Pandemic disease: Worldwide epidemic, such as AIDS and influenza of 1918

Reservoirs

• Sites where pathogens are maintained & become a source of infection
  • Animal reservoir: pathogens that normally infect animals & can affect humans (zoonoses)
  • Human carries: Symptom-free
  • 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, sexual intercourse
    • Examples: Respiratory tract infections, staphylococcal infections, measles, scarlet fever, sexually transmitted diseases
  • Indirect Contact: pathogen is transmitted by a fomite (nonliving object) e.g., tissues, handkerchiefs, towels, bedding, toys, clothes, diapers, eating utensils, drinking cups, medical equipment and devices
• Droplet Transmission:
  • Infectious agents are transmitted via respiratory droplets
  • Organisms travel less than a meter
• Airborne Transmission: Organisms travel more than 1 meter
• Water-borne & Food-borne Transmission:
  • Through untreated or poorly treated sewage & undercooked foods, or food prepared under unsanitary conditions
  • Causes gastrointestinal diseases such as: giardiasis, amebic dysentery, cholera, shigellosis, Hepatitis A & E
• Bodily Fluid Transmission: Blood, urine, saliva, and other bodily fluids
• Vector Transmission: Animals, especially arthropods
  • Biological Vectors: Biting insects, including mosquitoes, ticks, lice, fleas, blood-sucking flies
  • Mechanical Vectors: Passively carry agents to a new host by their feet or other bodily 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 due to them in the US
• 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 use of medical procedures, such as the use of catheters, invasive diagnostic procedures, and surgery