Chapter 9 - Infectious Disease

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Microorganisms are present on our bodies and can be

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beneficial

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Symbiosis and mutualism

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are relationships where the host and/or the microorganism derive benefit from the association

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Opportunistic infections

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occur when usually benign microorganisms become pathogens in the event of injury or impairment of the immune system.

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Normal flora

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are microorganisms that colonize various places in the body, and provide beneficial effects.

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Examples of normal flora

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1) Microorganisms in the intestines that aid in digestion.
2) Microorganisms on the skin and in the vagina create an environment that assist with the barrier against pathogens

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What are the stages of infection?

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1) Colonization
2) Invasion
3) Multiplication
4) Spread

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Invasion

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the organism resists or penetrates the host defenses

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Spread

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migrates locally or through the blood steam

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What are the factors that influence infection by pathogens?

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1. Mechanism of action 2. Infectivity 3. Pathogenicity 4. Virulence 5. Immunogenicity 6. Toxigenicity 7. Portal of entry

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Mechanism of action

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direct damage to cells, interference with cell metabolism

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Infectivity

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ability of the pathogens to invade and multiply in the host

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Pathogenicity

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ability of the agent to produce the decease

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Virulence

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potency of pathogens measured in terms of the number of microorganisms required to kill a host

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Immunogenicity

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ability of the pathogens to induce the immune response

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Toxigenicity

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ability to produce disease by producing soluble toxins

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Portal of entry

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the route by which pathogens infect the host

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Mechanisms by which pathogens circumvent host defenses?

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1. Surface coats
2. Surface receptors
3. Toxins
4. Antigenic variation
a) Antigenic drifts
b) Antigenic shifts
c) Gene switching

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Surface coats

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a mechanism by which a pathogen circumvent host defenses - they inhibit phagocytosis

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Surface receptors

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a mechanism by which a pathogen circumvent host defenses - bind to host cells (such as the H1 receptor in the H1N1 virus)

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Toxins

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a mechanism by which a pathogen circumvent host defenses - damage cells

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Antigenic variation

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challenges the immune system to keep up with it
1. antigenic drift
2. antigenic shift
3. gene switching

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Antigenic drift

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a mechanism by which a pathogen circumvent host defenses - is the mutations that occur in the pathogen (such as those that cause new flu strains to occur every year)

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Antigenic shift

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a mechanism by which a pathogen circumvent host defenses - are where genomes of different viruses combine to create a new virus

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Gene switching

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a mechanism by which a pathogen circumvent host defenses - is where the pathogen has many membrane antigens and turns some on and some off sporadically to appear different to the immune system

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Exogenous pyrogens

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are pyrogens produced by pathogens and cause the hypothalamus to reset the thermostat and produce a fever

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Endogenous pyrogens

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are pyrogens produced by the host and cause the hypothalamus to reset the thermostat and produce a fever

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Virulent pathogen

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is one in which very few organisms are needed to cause infection

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Virulent pathogens can

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produce endotoxins which bind with blood vessels, causing vasodilation - this decreases blood pressure and can cause septic shock

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Gram stain

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is used to differentiate bacteria - gram positive and gram negative bacteria have similar internal structures but different external structures - knowing whether an infecting bacterium is gram negative or gram positive helps in selection of antibiotic therapy

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Examples of gram positive bacteria

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Staphylococci and Streptococci

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Gram negative bacteria

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have a thicker outer membrane, which makes them more resistant to certain types of antibiotics

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Examples of gram negative bacteria

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E. coli, Enterobacter, Bacteriodes, H. Pylori, Brucella, and Legionella

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Acid-fast stain

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of tissue or body fluids can detect tuberculosis and other infections

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What is a culture for?

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is used to grow an organism to identify it

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Sensitivity test

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is used to see what kind of antibiotics the organism is sensitive or resistant to

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What do viruses do?

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they bind to a site on the host cell plasma membrane, penetrate the membrane, and use the host's metabolic machinery to replicate itself

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DNA viruses

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can become part of the host genome (the DNA transcribed to RNA to produce proteins)

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Examples of DNA viruses

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Human Papilloma Virus, Herpes Simplex Virus, Herpes Zoster, Poxvirus, Hepatitis B Virus

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RNA viruses

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directly transcribe messenger RNA which can then be translated into proteins

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Examples of RNA viruses

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Picornaviruses (colds, polio), Myxovirus (influenza), Retrovirus (AIDS), Hepatitis A Virus

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HIV

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has the enzyme reverse transcriptase, which converts the RNA to DNA - it then penetrates the host's genome

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Viruses can

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inhibit host protein synthesis, release digestive enzymes, and cause death (apoptosis) of the host cell - the released viral particles can infect adjacent cells

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Why is it difficult to create an antiviral medication

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viruses invade host cells so antivirals may kill host cells as well

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Antiviral drugs

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do not destroy the virus, but they inhibit its development and spread - many antivirals work by affecting the enzymes produced by the virus

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How long does it take for viral cultures?

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several days or weeks to show results

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Fungi

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have rigid cell walls (this allows them to be seen on KOH preparations) - they are resistant to may antibiotics - special antifungal agents must be used which can be toxic to host cells

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Examples of Fungi

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Candida albicans (thrush, vaginal yeast infections), ringworm, and athletes foot

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What do vaccinations do?

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they cause a person's adaptive immune system to produce antibodies and memory cells to prevent an infection of the pathogen

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What are the different types of vaccinations?

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Killed, attenuated, toxoid