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Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
Counting Micro Organisms
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Counting Micro Organisms

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

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