Counting Micro Organisms
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Counting Micro Organisms

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Counting Micro Organisms Presentation Transcript

  • 1. Counting Micro-organisms Counting elephants is easy – counting bacteria is not quite as easy!
  • 2. Counting Micro-organisms
    • A single tiny drop of nutrient broth incubated overnight may contain 5 000 000 cells – this is a lot to count.
    • 1cm 3 may contain 10 8 cells.
    • In order to estimate numbers it is necessary to dilute the sample.
  • 3. SERIAL DILUTION 1ml into 9ml = 1:10 dilution conc. 10-1 10-2 10-3 10-4 10-5 10-6 Counting Micro-organisms
  • 4.  
  • 5. Counting Micro-organisms
    • It is possible to dilute a cell suspension in known steps in order to plate a specific range of cells. This set of plates contain a 1 to 10 dilution series.
  • 6. TOTAL COUNTS
    • Direct counts
    • The haemocytometer is a specialized microscope slide used to count cells.
    • The centre portion of the slide has etched grids with precisely spaced lines.
  • 7. TOTAL COUNTS
    • To get an accurate count there should be between 40 and 70 cells in a 1 mm square.
    • If not you dilute or concentrate the cell suspension as necessary .
  • 8. TOTAL COUNTS
  • 9. TOTAL COUNTS
    • Indirect counts
    • These depend on the effects of the organisms to estimate their numbers.
    • As organisms grow they make the nutrient broth turbid.
    • This turbidity can be measured with a colorimeter
  • 10. TOTAL COUNTS
    • The more organisms the greater the optical density of the solution.
  • 11. TOTAL COUNTS
    • Indirect count
    • The Coulter counter is a probe which measures the change in conductivity of a solution as a bacteria passes through a narrow gap.
  • 12. TOTAL COUNTS
    • Problem with INDIRECT COUNTS and DIRECT COUNTS is that they can not tell living cells apart from dead cells
    • Advantage of INDIRECT COUNT is that the process can be automated
  • 13. VIABLE COUNTS
    • Count only those cells capable of growing
    • Viable counts can be accomplished by such techniques as pour plating.
    • Assumption each viable cell gives rise to a colony.
  • 14. VIABLE COUNTS
    • Advantages:
    • The method can be made to be very sensitive.
    • One can count subsets of the population
    • Disadvantages
    • Colony-forming units may underestimate cell numbers because of clumping or chains of cells.
    • Counts require at least a few hours, usually overnight, for incubation.