Chapter 5 Flashcards
Microbes (or Microorganisms)
Microscopic particles inhabiting virtually every ecological niche on earth. Most microbes do not transmit disease. Millions pass over or through the human body without causing significant ill effects.
Pathogens
Organisms that have a deleterious effect on the human host and have the potential to confer infections - damage to human tissue conferred by infectious agents.
Commensalism (in the context of microbe-human relationships)
A relationship where a microorganism derives benefit by being provided with a nutrient-rich, protected environment in the human body, with little or no effect on human health.
Mutual Benefit (in the context of microbe-human relationships)
A relationship between many types of bacteria and the human body where both the microbe and the human derive benefit.
Classes of Infectious Particles
Infectious diseases can be caused by bacteria, viruses, protists (protozoans) or fungi. More recently, prions, a protein particle, have been found to cause transmissible disease in animals and humans.
Bacteria (as a class of infectious particles)
Living cells with specific cellular features. Figure 5.3 shows a scanning electron micrograph of Streptococcus epidermidis, a bacterial species well known to humans.
Viruses (as a class of infectious particles)
Complex structures governed by the molecular mechanisms of living cells, carrying genes (DNA or RNA) and composed of biological macromolecules. They rely on host cells in order to reproduce and therefore do not fulfill all classical definitions of living cells. Figure 5.5 shows an SEM of Ebola viruses attacking a cell.
Protists (Protozoans) (as a class of infectious particles)
Living cells with specific cellular features; mentioned as one of the causes of infectious diseases.
Fungi (as a class of infectious particles)
Living cells with specific cellular features. Figure 5.4 shows Candida albicans, a common fungal infection of humans.
Prions (as a class of infectious particles)
A protein particle that causes disease in animals and is transmissible to humans. Bovine Spongiform Encephalopathy (mad cow disease) in cattle and Variant Creuztfeldt-Jacob disease in humans are examples. Represents an entirely novel class of infection.
Ecology (in the context of human infections)
The study of the interactions of living organisms with each other and with their environment. From this perspective, the human body is a system of different niches inhabited by specialist microorganisms.
Ecological Niche
The particular ecological location an organism inhabits. Various places on and in the human body feature different physical and chemical conditions to which microbes have adapted to colonize.
Resident Microflora
Microbes that normally inhabit niches on the human body. They may inhabit areas like the mucosa of the nares, the epidermis, the colon, the pharynx, or the distal end of the urinary tract.
Sterile Compartments of the Human Body (normally free of microbes)
The CNS, heart, kidneys, peritoneal cavity. Intrusion of microbes and colonization of these areas usually has particularly serious medical consequences.
True Pathogens
Microorganisms that will cause disease in almost any individual they infect.
Opportunistic Pathogens
Microorganisms that will not lead to disease in individuals with healthy immune defences. They may cause a secondary infection by taking advantage of weakened immune defences after a previous infection.
Factors Influencing Immunocompetence (and susceptibility to opportunistic infections)
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Previous infection
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Age (early childhood and advanced age)
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Presence of toxins (drugs, environmental chemicals, etc.)
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Nutritional status
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Presence of chronic disease
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Presence of existing HIV infection
True Pathogen Virulence Factors
Often more pathogenic than opportunists. For example, Clostridia release highly toxic exotoxins.
Opportunistic Pathogen Virulence Factors
Many contain endotoxins (the liposaccharide portion of the cell wall released when they die) that cause reactions in the host, including fever and pain.
Chain of Human Infection
The interaction of the infectious particle, the human host, and the environment that leads to infectious diseases. It involves:
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An infectious agent conveyed from a natural reservoir.
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A mode of transmission.
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Portal of entry into the susceptible human.
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Portal of exit from the human host for transmission to others. Understanding this chain is crucial for interrupting the spread of infection. Figure 5.8 illustrates this concept.
Reservoir of Infection
The source from which the microbe was acquired. This can be another human, another animal host, or an object or substance. Can be an exogenous source (outside the body like air, soil, water, other humans, or animals) or an endogenous source (within the body like the infected person’s intestine, urogenital tract, or skin).
Nosocomial Infections
Infections where the source of infection is the clinic setting. The concentration of immunocompromised and infected persons in clinics increases the risk of infection.
Vectors
Particular objects or other organisms that carry an infectious particle to the human. A biological vector is a living organism in which the infecting particle undergoes part of its life cycle. A mechanical vector carries an infecting microorganism passively on its surface.
Portals of Entry
The site where pathogens gain access to host tissues. Modes of transmission and their portals include:
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Direct Contact: Infected tissue or secretions contact exposed mucous membranes (e.g., sexual contact).
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Penetration: Disruption of passive surface barriers (skin or mucous membranes) through abrasions, burns, wounds, surgery, bites, etc..
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Ingestion: Oral uptake of solids or liquids into the digestive tract (through the intestinal wall). Reduced stomach acidity (achlorhydria) can increase susceptibility.
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Inhalation: Microorganisms breathed into the respiratory tract, gaining access through epithelial linings and alveoli of the lungs.
