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What are the obligatory steps for infection and their corresponding phenomenon in the body
- Attachment and/or entry into body (infection)
- Local or general spread in the body (spread)
- Multiplication (multiplication)
- Evasion of host defenses (evasion of host defenses)
- Shedding from the body or exit (transmission)
- Causing damage in the host (pathology, disease)
- *not necessary, but can be essential for shedding
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What are the four types of infection with specific examples
- 1. Microbes with specific mechanisms for attaching to, or penetrating, the body surfaces of normal healthy hosts
- eg. most viruses, and certain bacteria
- 2. Microbes introduced into normal healthy hosts by biting arthropods
- eg. malaria, plague, typhus, yellow fever
- 3. Microbes introduced into normal healthy hosts via skin wounds or animal bites
- eg. clostridia, rabies, Pasteurella multocida (avian cholera)
- Microbes able to infect a host only when surface or systemic defenses are impaired
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What variables affect the disease expression in an individual? Example of infections which give a fairly consistent disease expression. Example of infections that do not.
- Infecting dose and route
- age
- sex
- presence of other microbes
- nutritional status
- genetic background
- consistent: measles, cholera
- wide spectrum: syphilis
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List the body surfaces that can be sites of microbial infection
- Skin
- Respiratory tract
- Gastrointestinal tract
- Urogenital tract
- Oropharynx
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What tends to inactivate non normal-flora microbes on the skin?
- Fatty acids (pH ~5.5)
- antimicrobial secretions by sebaceous and other glands
- certain peptides formed by keratinocytes
- materials produced by the normal skin flora
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Give examples of microbes that can traverse unbroken skin to cause infection
- leptospira
- larvae of Ancylostoma
- larvae of Schistosoma
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What is the world's most common zoonosis? Describe the typical infection route
Leptospirosis: commonly transmitted by animal-urine infected water that contacts skin, eyes, or mucous membranes
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What are the two methods of infectious agent transmission by biting arthropods w/ description AND example
- Mechanical transmission: pathogen on insect feet or other body part
- eg houseflies can transfer typhoid fever and bacillary dystentery (shigellosis) from infected feces to food
- Biological transmission: active transmission between hosts
- eg. arthropod bites infected person, pathogen reproduces inside vector w/o harming the vector, and is transmitted to a new host during a blood feed
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What helps to prevent the conjunctiva from infection? What can compromise it?
- Continuous flushing action of tears
- Tears include lysozyme and other microbial peptides
- Physical "windshield wiper" action of eyelid
- Compromised by decreased lacrimal gland secretion, conjunctival damage, or eyelid damage
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What is trachoma?
An eye infection by Chlamydia trachomatis through direct contact of contaminated finger, fly, towel, etc to the conjunctiva
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What are the specific mechanisms that pathogens have evolved to invade a normal healthy respiratory tract w/ description AND examples
- 1. attach firmly to surface of mucosal cells by using specific adhesion molecules
- eg. Hemagglutinin on influenza virus to sialic acid on respiratory tract cells
- 2. inhibit ciliary activity
- eg. Bordatella pertussis (whooping cough) produces ciliostatic substances
- 3. avoid phagocytosis or avoid destruction after phagocytosis
- eg. Tubercle bacilli survive in the phagosome of alveolar macrophages by inhibiting phagolysosome formation
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Give examples of microbes that infect the large bowel and small intestine.
- Large bowel: Shigella
- Small intestine: most salmonellae, rotaviruses
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What are the factors involved in GI tract infection w/ explanation and examples?
- Infection depends on balance between production and removal of bacteria in the intestine (is # of attached microbes enough to infect?)
- eg. V. cholerae establishment on mucosal surface is sufficient
- Shigella flexneri requires exploitation of inflammation response for infection
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Describe infection by Shigella flexneri.
- Can only enter cells from basal surface
- 1. Uptaken by M cells and invades local macrophages
- 2. Infected macrophages induce inflammation which causes disruption of epithelial barrier
- 3. Now many Shigella can enter from intestinal lumen and invade epithelial cells from below
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What are the mechanisms in the GI to counteract microbes w/ description
- Mucus: protects GI epithelia with molecules that bind to microbial adhesins OR by secreted IgA (specific)
- acids: most bacteria are acid sensitive (prefer slightly alkaline)
- bile salts: detergents that are toxic to some bacteria
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What microbes can pass through the mucus layer in the GI tract? Why?
- V.cholerae, salmonellae, and E. coli are motile and can propel themselves through the mucus layer to reach GI epithelia
- V. cholerae secretes mucinase that degrades mucin
- Non-motile microbes rely on random/passive transport through mucus layer
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Give a detailed account of Heliobacter pylori's defense mechanism and symptoms/disease
- Attaches by adhesins to stomach wall and produces large amounts of urease which converts local urea to ammonia providing a buffer cloud
- Induces apoptosis in gastric epithelial cell walls causing inflammation, dyspepsia, and sometime an duodenal or gastric ulcer
- DO NOT invade tissues, but can stay in the stomach for years asymptomatically
- May caused development of stomach cancer
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How can some bacteria escape death from bile acids?
- Some have conjugated bile acid hydrolase (CBAH) to remove taurine AA from a cholic acid bile salt
- Some have BaiA that removes a H from the cholica acid hydroxyl group
- Both inactivate the bile acid
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Examples of organisms that can colonize vagina during reproductive life
- lactobacilli (cause acidic environment from glycogen fermentation)
- certain streptococci
- diphtheroids
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Give a specific example of an attachment mechanism of a urinary tract invader
- gonococci produce a peptide on the bacterial pili which binds to a syndecan-like proteoglycan on the urethral cell
- The cell is the induced to engulf the bacterium
- This is called parasite-directed endocytosis
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Describe oropharynx defenses from infection
- Flushing action of saliva (1L/day)
- Secretory IgA antibodies
- Antimicrobial substances (eg lysozyme)
- normal flora
- leukocytes present on mucosal surfaces/ in saliva
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Describe the mechanisms of oropharyngeal infection including example organisms
- Different streptococci make specific attachments via lipoteichoic acid molecules on the pili to the buccal (mouth) epithelium and tongue (resident Streptococcus salivarius)
- to teeth (Streptococcus mutans)
- to pharyngeal epithelium (invading Streptococcus pyogenes)
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