Symbiosis and Normal Flora

• A fetus in utero is normally free of microbes
• A newborn gets exposed to microbes during birth, initiating intestine colonization

Symbiosis

• It's a close relationship between two different species in a community

• Mutualism: Both organisms benefit; for example, E. coli in the human GI tract

• Commensalism: One organism benefits, the other is unaffected; certain mycobacteria in the ear that live on secretions and dead cells, are an example

• Parasitism: One organism benefits, the other is harmed; examples include tuberculosis, helminths, and protozoa

• Amensalism: One organism hinders another's growth without being affected itself; for example, penicillium

Normal Flora (Microbiota)

• Resident flora are normal flora throughout life, like S. epidermidis and E. coli
• Transient flora are temporary, lasting hours, days, or months, like Bacillus Laterosporus
• Normal flora is typically protective and doesn't cause disease in healthy individuals in their normal habitat
• When balance is disrupted, normal flora can become opportunistic pathogens

Portal of Entry

• Exogenous entry comes from outside the body
• Endogenous entry comes from within the body
• Most pathogens prefer a specific entry point
• If a pathogen enters the "wrong" portal, infection may not occur
• Some pathogens can enter through multiple portals; for example, Streptococcus and Staphylococcus

Entry Points

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

Virulence

• Virulence indicates the degree of pathogenicity or disease-provoking power of a microbe

• Virulence factors aid in causing disease

• Adhesion: the initial step of infection, aided by pili and cell membrane proteins

• Colonization: occurs in tissues exposed to the external environment like the urogenital, digestive, and respiratory tracts, as well as the conjunctiva

• Invasion: disrupts host cell membranes and barriers via invasins

• Evasion of Host Defense: involves avoiding phagocytes, inhibiting phagocytic engulfment, surviving inside phagocytes or producing substances that harm phagocytes

• Toxins: A main virulence factor

• Toxigenesis: An organism's ability to produce toxins

• Bacterial toxins: can act far 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 specific than exotoxins
• Heat-stable
• Can be affected by certain oxidizing agents like peroxide
• Shiga toxin is an example

Exotoxins

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

Portal of Exit

• It's where a pathogen leaves the infected person
• Often the same as the entry point
• 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: confined to a specific tissue
• Focal infections: spread from a local infection to other tissues
• Systemic infections: spread to multiple sites and tissues, often via the circulatory system - Bacteremia: presence of bacteria in the blood - Septicemia: systemic infection caused by pathogen multiplication 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 first symptoms
• Prodromal stage: the earliest, nonspecific symptoms of infection
• Convalescence: the recovery period
• Rehabilitation: maximizing function of diseased tissue
• Mixed infection: multiple infectious agents at the same site
• Acute infection: rapid onset with severe symptoms, vanishing quickly
• Chronic infection: less severe symptoms over a long period
• Primary infection: the initial infection
• Secondary infection: occurs after a primary infection
• Subclinical infection: no apparent symptoms, can be lengthy

Epidemiology and Public Health

• Epidemiology: studies the distribution and causes of disease in populations like the number of affected people, locations, and disease outcomes

CDC

• central source of epidemiological information that releases data on morbidity, mortality, and incidence of specific notifiable diseases - Morbidity: number of patients with a disease within a group - Mortality: number of deaths from a particular disease - Incidence: number of new cases in a population within a time frame - Prevalence: number of new and existing cases in a population within a time frame

Disease Categories

• Endemic disease: consistently present in a population or area, such as the common cold
• Sporadic disease: occurs occasionally, such as typhoid fever in the U.S.
• Epidemic disease: occurs more frequently than usual in a population or area, such as influenza
• Pandemic disease: a worldwide epidemic, for example, AIDS and the 1918 influenza

Reservoirs

• Sites where pathogens are maintained and become sources of infection - Animal reservoirs: pathogens that infect animals and can affect humans (zoonoses) - Human carriers: symptom-free individuals - Nonliving reservoirs: includes soil, water, food, and fomites

Modes of Transmission

• Contact Transmission - Direct physical contact: between hosts without an intermediate object, such as touching, kissing, or sexual intercourse (respiratory tract infections, staphylococcal infections, measles, scarlet fever, STDs) - Indirect contact: via a fomite (nonliving object) like tissues, handkerchiefs, towels, bedding, toys, clothes, diapers, eating utensils, drinking cups, or medical equipment
• Droplet Transmission: infectious agents transmitted via respiratory droplets traveling less than a meter
• Airborne Transmission: organisms travel more than 1 meter
• Water-borne and Food-borne Transmission: through untreated sewage or undercooked/unsanitary food that can cause gastrointestinal diseases like giardiasis, amebic dysentery, cholera, shigellosis, and Hepatitis A & E
• Bodily Fluid Transmission: via blood, urine, saliva, and other bodily fluids
• Vector Transmission: via 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 via their feet or other body parts

Healthcare Associated (Nosocomial) Infections

• Acquired in a hospital or hospital-like setting

• Among the top 10 leading causes of death in the United States

• Annually result to 99,000 deaths in the United States alone

• Exogenous HAIs: caused by pathogens in the healthcare environment, shed by sick individuals

• Endogenous HAIs: caused by microbes within the patient's normal flora

• Iatrogenic HAIs: caused by medical procedures like catheters, invasive diagnostics, and surgery