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Most Probable Number (MPN) PowerPoint PPT Presentation


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Most Probable Number (MPN). CE 370 - Lab. Introduction. Many of the Bacteria that cause serious disease, such as typhoid fever, cholera, and dysentery, are discharged along with fecal wastes. These disease-causing organisms are called pathogens and are difficult to isolate and identify.

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Most Probable Number (MPN)

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Most probable number mpn l.jpg

Most Probable Number (MPN)

CE 370 - Lab


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Introduction

  • Many of the Bacteria that cause serious disease, such as typhoid fever, cholera, and dysentery, are discharged along with fecal wastes.

  • These disease-causing organisms are called pathogens and are difficult to isolate and identify.

  • Fortunately, less harmful, easily isolated bacteria called indicator organisms can be used indirectly to detect pathogens.

  • Coliform bacteria is used as an indicator organisms. They live in the intestine of the human beings in large numbers, non disease-causing, easily detectable.

  • The presence of coliform bacteria indicates that the water has been polluted with human or worm-blooded animals waste. It is a warning signal that more dangerous bacteria may be present.


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Purpose of Experiment

To learn enumeration technique of coliform bacteria


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MPN Measurement

  • A series of serial dilutions of wastewater sample is made (dilute by factors of 10)

  • Transfer 1 mL sample from each of the serial dilutions to five fermentation test tubes containing lactose medium and inverted gas collection tube

  • Incubate tubes for 24 hours at 35 ° C in a water bath

  • Accumulation of gas (CO2) in inverted gas tubes is considered a +ve reaction (No gas –ve reaction)


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  • The results of each dilution are reported as a fraction with the # of positive tubes over the total # of tubes (I.e., 3/5 means three + tubes in a five-tube sample)

  • Test for fecal coliform is similar to total coliform assay except that the medium is different (lactose, tryptose, bile salts), tubes are incubated for 3 hr at 35 ° C and 44 for 21 h.

  • Concentration of total or fecal coliform bacteria are reported as most probable number per 100 mL (MPN/100 mL)


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  • MPN is based on the application of the Poisson distribution. MPN is a statistical estimate of concentration.

  • “Standard” method for calculation of MPN is to take 10, 1,

  • and 0.1 mL of sample and dilute to 100 mL


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Example

  • A bacterial analysis of a surface water yielded the following results for the standard confirmed test of total coliform. Determine the coliform MPN using Poisson distribution.


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Solution

From Table F-1, Wastewater Engineering, Metcalf & Eddy, MPN /100 mL = 47 coliforms in 100 mL


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