Micro Lab Exercises 17-20

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Micro Lab Exercises 17-20
2015-10-07 23:49:20
Microlab Book Exercises
Microlab practical 1
Reading and questions
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  1. What was the primary outcome of creating pour-plate techniques?
    To obtain an isolated colony
  2. What must be done to the original sample during pour-plate technique to achieve isolated colonies? How many cells are adequate to be counted?
    • It must be diluted several times to reduce the  the microbial population sufficiently. 
    • 25-250 cells
  3. What is CFU?
    Colony-forming Units: the # of microorganisms that form colonies when cultured using spread plates or pour plates.
  4. How can pure cultures be obtained from spread or pour plates?
    Isolated colonies are grown on media once they've been diluted enough to see individual colonies on plates. These colonies growing on the surface can be inoculated into fresh medium
  5. How do the results of the pour-plate method compare with those obtained using the streak-plate and spread-plate methods?
    Results favored towards the streak-plate and spread-plate methods.
  6. What is the main advantage of the pour-plate method over other methods of bacterial colony isolation? What is a problem that can occur using this method?
    The bacteria samples are diluted several times to ensure decreasee # of microbes. Problems: getting solution to microbes too diluted and not mixed properly.
  7. Why are the surface colonies on a pour plate larger than those within the medium?
    With aerobic bacteria, there would be a problem growing within the media with lack of oxygen.
  8. Why doesn't the 48 to 50 degrees celcius temp of the melted agar kill most of the bacteria?
    Most bacteria can grow in this range
  9. Why is it important to invert the petri plates during incubation?
    To avoid condensation
  10. For what bacteria would you use the TSI test?
    Enteric bacteria (bacteria of the intestines)
  11. After 18-24 hours of incubation, what are you looking for in the TSI test?
    Detect the presence of sugar fermentation, gas production, and H2S production.
  12. T or F: Most E. Coli are faculatively anaerobic. What does this mean?
    True: having both a respiratory and a fermentative type of metabolism
  13. This bacteria will grow only in the presence of O2.
    Obligate aerobe
  14. This bacteria will grow either aerobically or in the absence of O2, but better in its presence.
    Facultative anaerobe
  15. This bacteria will grow only in the absence of O2 and are actually harmed by its presence
    Obligate anaerobe
  16. This bacteria are ones that cannot use O2 but are not harmed by it.
    Aerotolerant anaerobe
  17. This bacteria require a small amount of O2 but are inhibited by normal atmospheric tension.
  18. What are the two most widely used methods for determining bacterial numbers?
    Standard platecount method and spectrophotometry.
  19. Explain the plate count method and spectrophotometry
    • Plate count: An indirect measurement of cell density and reveals information related only to live bacteria.
    • Spectrophotometry: based on turbidity and indirectly measures all bacteria, dead or alive
  20. When dliluting for standard plate count, what is the ideal range of colonies to be counted in the final plates?
    25-250 colonies
  21. What is CFU?
    Colony-forming units: for counting cells, given that each viable bacterial cell is seperate from all others and will develop into a single discrete colony.
  22. What is the measurement used in spectrophotometery?
    Absorbance (optical density)
  23. What is the difference between %T and absorbance?
    %T is the amount of light that is trying to pass through and absorbance is what the light is trying to pass through.
  24. Why is the viable plate count technique considered to be an indirect measurement of cell density, whereas turbidy method is not a "count" at all?
    Plate count method approximates the number of live cells on a diluted sample, while turbidity counts both dead and live cells in a medium.
  25. Why is absorbance used in constructing a calibration curve instead of percent transmittance?
    Absorbance is more useful than the transmittance because it is directly proportional to concentration.
  26. What is the purpose of constructing a calibration curve?
    A calibration curve establishes the relationship between the signal generated by a measurement instrument and the concentration of the substance being measured. Different chemical compounds and elements give different signals. When an unknown sample is measured, the signal from the unknown is converted into concentration using the calibration curve.
  27. Why is it necessary to perform a plate count in conjunction with the turbidimetry procedure?
    Because plate count will be more accurate when it comes to live cell value
  28. How would you define biomass?
    Cell #