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How much normal flora is associated with the human body? Where is it located? When is it acquired?
- ~1014 bacteria associated with the ~1013 body cells (the majority in the large bowel)
- Viruses, fungi, and protozoa form only a minor component
- Internal organs and tissues are normally sterile
- Acquired rapidly during and shortly after birth
- Continuously changes throughout life
- Normal flora occur in parts that are exposed to the environment (skin, nose, mouth, intestinal tract, urinogenital tract)
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Why are exposed areas of the skin a bad environment for microbes?
- pH is 4-6 due to organic acids
- Low moisture content
- High salt levels
- Rapidly changing conditions due to external environment
- Moist areas (sweat glands in underarm, genitals, nipples, and umbilicus) are colonized
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Describe the two types of sweat glands
- Eccrine glands: main sweat glands, concentrated in certain areas (feet, hands), but distributed over the entire body.
- Thermoregulation
- Not many bacteria associated with these glands (dermcidin)
- dermcidin: anti-microbial peptide secreted by eccrine glands as part of innate immunity
- Apocrine glands: found primarily in the underarms, genitals, nipples, and umbilicus and WHEREVER THERE IS BODY HAIR
- Microbial growth in the secretions from these glands = reason for deodorant
- Develop during the early to mid-puberty ages
- Reason why food preparers are required to use hair nets
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What are the two types of skin normal flora?
- Transients: organisms that don't grow, but appear as contaminants
- Can be almost anything
- E. coli is one of the most common transients (hands, door knobs, faucets, etc)
- Residents: able to survive/grow on the skin
- Almost all are gram+ (Staphylococcus[aerobic] and Corynebacterium [anaerobic])
- Secretion changes during puberty result in an increase of Propionibacterium acnes
- Yeasts aren't usually found on the skin, but a number of fungi (Candida) occur on the scalp/around the nails
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What are the factors that affect skin normal flora in an individual?
- Weather (heat, humidity, sweat, etc)
- Age
- Hospitalization (exposure)
- Personal hygiene
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General information about the normal flora of nose and mouth
- Majority of bacteria in the nose and mouth are anaerobes
- Common species include Streptococci, Staphylococci, diptheroids, and gram- cocci
- Some aerobic bacteria found in normal flora are potentially pathogenic (S. aureus, S. pneumonia, S. pyogenes, Neisseria meningitidis)
- Candida is also a potential pathogen (mouth thrush)
- Mucous membranes in the mouth have ~the same microbial density of the large intestines
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Positive and negative reasons for growing bacteria in the mouth?
- Mouth is constantly getting nutrient for the bacteria
- Saliva: furnishes moisture and nutrients, but has toxic proteins
- Lysozyme- responsible for most antimicrobial action, breaks glycosidic linkages in peptidoglycan
- Lactoperoxidase- Enzyme that generates a singlet oxygen (toxic)
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How does the normal flora change as the teeth grow in?
- Before teeth: primarily Streptococci and Lactobacilli (aerotolerant anaerobes)
- After teeth: surfaces of teeth/gingival crevices are colonized by anaerobic bacteria
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What is dental plaque? Dental caries?
- A thick layer of a mixture of bacteria and polymers
- Dental caries (cavities): demineralization and destruction of the tooth hard tissues (enamel, dentin, and cementum)
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Describe dental plaque development
- Deposition of acidic glycoproteins from saliva
- Attachment of facultative S. sanguis, S. sobrinus, S. mutans, and S. mitis on those layers
- Heavy growth of the above organisms results in colonization of the dental plaque and consumes all the oxygen
- Secondary growth of filamentous fusobacterium (anaerobic bacteria that produce lactic acid from sugars) and Actinomyces (obligate anaerobes)
- As dental plaque accumulates the resident microflora produce high concentrations of lactic acid which cause decalcification of the tooth enamel (dental caries)
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Give information about S. mutans and S. sobrinus in detail
- Primarily responsible for dental caries
- Produce lactic acid by fermenting SUCROSE
- Lactic acid dissolves CaPO4 in enamel leaving the tooth open to bacterial attack
- Grow in cracks/crevices between teeth
- Produces dextran (very thick and sticky glucose polymer) only when growing on SUCROSE via dextransucrase (enzyme)
- S. mutans is not part of the oral normal flora in populations with no sucrose in the diet
- ~90% of the US population has S. mutans
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Describe the normal flora of the upper respiratory tract
- Throat, nasal passages, nasopharynx
- Most of the microbes live in the mucus linings of these areas
- The flora is highly variable
- Some can carry pathogenic bacteria as part of the normal flora such as...
- S. pyogenes: strep throat, flesh eating bacterium
- S. aureus: staph infection, boils, etc
- Corynebacterium diphtheriae: diptheria
- Streptococcus pneumoniae: bacterial pneumonia
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Describe the normal flora of the lower respiratory tract
- Trachea, bronchi, lungs
- Quite sterile due to...
- filtration in nasal passages
- Air slowing as it passes into the lungs
- Ciliated epithelium push particulate matter up/out of the lungs
- Many clinically normal individuals may have the fungus Pneumocystis jiroveci in their lungs
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What is smoker's cough?
- Paralysis of the ciliated epithelial cells of the lower respiratory tract by nicoctine and other cytotoxins in tobacco smoke
- Causes accumulation of mucus, particles, and bacteria (bronchitis)
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Describe the normal flora in the gastrointestinal tract
- Stomach harbors only transient organism (high pH)
- Gastric mucosa may be colonized by acid-tolerant lactobacilli and streptococci
- H. pylori is carried by many without gastric ulcer symptoms
- The upper small intestine is only lightly colonized
- Populations increase markedly in the ileum where streptococci, lactobacilli, enterobacteriaceae, and Bacteroides may all be present
- Bacterial numbers are very high in the large bowel (a large fermentor)
- The vast majority are obligate anaerobes
- Bacteroides, Clostridium sp, and filamentous fusiform (G- rods w/ tapering ends) are common and major components of feces
- Relatively small # of facultative anaerobes (E. coli, Enterococcus faecalis)
- Harmless protozoans can occur, but are not frequent
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Describe the bioconversions in the large intestine
- Gas: by the metabolism of fermentative and methangoenic microorganisms
- H2 and CO2 by fermentative bacteria
- CH4 from H2 and CO2 by methanogenic bacteria
- Odor products: H2S, NH3, amines, indole...
- Vitamin B12 & K: essential vitamins for humans made by intestine flora
- Steroids: produced in the liver, but modified/activated by the gut microbes, then reabsorbed
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What are the three dominant phyla of fecal microbiota
- Bacteroidetes: Gram-, nonspore-forming, anaerobic, rod-shaped
- Includes genus Bacteroides and Porphyromonas
- Not particularly harmful
- Firmicutes: Gram+, spore-forming, anaerobic/facultative/obligate anaerobic
- Includes genus Clostridium and Bacillus
- Can cause health issues
- Actinobacteria:
- The Firmicutes to Bacteroidetes ratio is of significant relevance in human micriobiota composition
- Obese mice in a study have 50% fewer bacteroidetes and more Firmicutes
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Describe the normal microbial flora in the Urogenital tract
- Bladder: normally sterile
- Urethra: lightly colonized with Staphylococcus epidermidis, Streptococcus faecalics, and Gram- enterics (E. coli and Proteus mirabilis).
- These opportunistic pathogens can cause bladder infections (cystitis)
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Describe the age-related normal flora changes in the vagina
- Before puberty: mildly alkaline, predominated by Staphyococci, Streptococci, E. coli
- Puberty+: mildy acidic, Lactobacillus aerophilus predominates secreting lactic acid while fermenting glycogen
- Prevents overgrowth by other vaginal organisms
- Menopause+: secretion of glycogen stops, flora returns to pre-puberty condition
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Describe the fungi of the vagina
- A number of fungi occur
- Candida can overgrow to cause thrush if vaginal pH rises and competing bacteria diminish
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What are the advantages of normal flora?
- Skin bacteria produce fatty acids which discourage potential pathogens
- Gut bacteria release a number of antibacterials (bacteriocins, colicins)
- Vaginal lactobacilli maintain an acidic environment, which suppresses growth of other organisms
- The sheer number of bacteria in the intestine out-compete potential pathogens for living space
- Gut bacteria produce B and K vitamins in substantial amounts if diet is deficient
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What are the disadvantages of normal flora?
- The potential for spread of microorganisms into sterile parts of the body...
- 1. Intestine is perforated
- 2. Skin is broken
- 3. Extraction of teeth
- 4. honeymoon cystitis- perinatal organisms ascent the urethra causing UTI
- Members of the normal flora often cause nosocomial infection during invasive treatments
- Overgrowth can occur when...
- 1. Composition of flora changes (after antibiotics)
- 2. Local environment changes (increases in stomach or vaginal pH)
- 3. Immune system become ineffective (AIDS, clinical immunosuppression)
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What are the types of symbiotic relationships?
- commensalism: one species gets shelter/food without harming the other
- mutualism: mutually beneficial
- Many cases are interdependent
- Parasitism: only the parasite benefits
- There are "blurred lines" and overlap for each type of symbiosis - not clearly defined
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Describe commensalism in detail w/ examples
- One species gets shelter/food without harming the other species
- Extensive commensal microbial flora on skin, in mouth, and in GI tract
- Can become harmful (parasitic) if environmental conditions change (eg - Bacteroides, E. coli, Staphylococcus aureus)
- Can benefit the host (mutual) by preventing colonization of others (intestinal flora) and producing metabolites for host (bacteria/protozoa in ruminant stomach)
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Describe mutualism in detail w/ examples
- Mutually beneficial relationship
- Bacteria/protozoa in stomachs of domestic ruminants play essential role in digestion/utilization of cellulose and receive environment and nutrition for survival
- Line between commensalism and mutualism can be hard to draw (enteric bacteria in humans are good for health, but not REQUIRED for survival)
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Describe parasitism in detail w/ examples
- Parasite benefits (environment, nutrients, metabolic needs, regulatory signals, etc) at determinant of host (All pathogens are parasites)
- Many parasites establish innocuous (commensal) associations with their natural host, but may become pathogenic if there are changes in host's health OR infect an unnatural host
- eg- rabies virus coexists harmlessly with many wild mammals, but causes fatal disease in human
- eg- Yersinia pestis (the plague)
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Describe the advantages of parasitism (to the parasite) (detailed)
- Metabolic advantages: host provides a variety of metabolic requirements so the parasite can devote its resources to replication
- Nutritional advantages: host provides nutrition to parasite, but the extent varies between groups
- Some use macromolecular material from the host and digest it with their own enzymes
- Others rely on pre-digestion of the host and take up only low MW molecules
- Reproductive advantages: evident from metabolic and nutritional advantages
- HOWEVER- development may be controlled by the host if the host loses the ability to regulate its own development (evolution)
- This disadvantage forces the parasite to have a host, and will result in death if none are available
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Describe the parasite dependency spectrum
- Viruses (complete dependence): host provides basic building blocks, AND synthetic machinery and energy required
- Non-viral parasites (partial dependence): have genetic/cellular machinery for independent metabolic activity
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