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Most Probable Number (MPN)

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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  1. Most Probable Number (MPN) CE 370 - Lab

  2. 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.

  3. Purpose of Experiment To learn enumeration technique of coliform bacteria

  4. 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)

  5. 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)

  6. 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

  7. 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.

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

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