Environmental Microbiology

MPN TABLE

Written by MicroDok

MPN TABLE

The basic concept for the MPN method is to dilute the sample to be analyzed to such a degree that the microorganisms (inocula) in each of the Durham’s tubes will sometimes (but not always) contain viable organisms. Thus by replicates, and dilution series, this will result in a fairly accurate estimate of the most probable number of cells in the analyzed sample. MPN technique has three test stages which includes (1) The Presumptive Test, (2) The Confirmed Test and (3) The Completed Test. Before the Durham’s tubes are inoculated with the test bacterium, the chance is at least 95 percent that the confidence interval associated with the eventual result will enclose the actual concentration of the sample; and this signifies the meaning of the 95 percent confidence intervals in the MPN Table (Table 1).

Table 1. Most probable number (MPN) of bacteria per 100 ml of sample according to positive test results from 15 tubes at three different dilutions

No. of Tubes Giving Positive Reaction out of: MPN Index per 95% Confidence Limits:            

5 of 10 ml       5 of 1 ml            5 of  0.1 ml        100 ml:      Lower          Upper            

0                      0                      0                        <2                      –                      –

            0                      0                      1                          2                  <0.5                    7

            0                      1                      0                          2                  <0.5                    7

            0                      2                      0                          4                  <0.5                  11

            1                      0                      0                          2                  <0.5                    7

            1                      0                      1                          4                  <0.5                   11

            1                      1                      0                          4                  <0.5                   11

            1                      1                      1                          6                  <0.5                   15

            1                      2                      0                          6                  <0.5                   15

            2                      0                      0                          5                  <0.5                   13

            2                      0                      1                          7                     1                     17

            2                      1                      0                          7                     1                     17

            2                      1                      1                          9                     2                     21

            2                      2                      0                          9                     2                     21

            2                      3                      0                        12                     3                     28

            3                      0                      0                          8                     3                     24

            3                      0                      1                        11                     4                     29

            3                      1                      0                        11                     4                     29

            3                      1                      1                        14                     6                     35

            3                      2                      0                        14                     6                     35

            3                      2                      1                        17                     7                     40

            4                      0                      0                       13                     5                     38

            4                      0                      1                        17                     7                     45

            4                      1                      0                        17                     7                     46

            4                      1                      1                        21                     9                     55

            4                      1                      2                        26                    12                    63

            4                      2                      0                        22                     9                     56

            4                      2                      1                        26                     9                     78

            4                      3                      0                        27                     9                     80

            4                      3                      1                        33                   11                     93

            4                      4                      0                        34                   12                     93

            5                      0                      0                        23                     7                     70

            5                      0                      1                        31                   11                     89

            5                      0                      2                        43                   15                   110

            5                      1                      0                        33                   11                     93

            5                      1                      1                        46                   16                   120

            5                      1                      2                        63                   21                   150

            5                      2                      0                        49                   17                   130

            5                      2                      1                        70                   23                   170

            5                      2                      2                        94                   28                   220

            5                      3                      0                         79                  25                   190

            5                      3                      1                      110                   31                   250

            5                      3                      2                      140                   37                   340

            5                      3                      3                      180                   44                   500

            5                      4                      0                      130                   35                   300

            5                      4                      1                      170                   43                   490

            5                      4                      2                      220                   57                   700

            5                      4                      3                      280                   90                   850

            5                      4                      4                      350                  120                  1000

            5                      5                      0                      240                  68                    750

            5                      5                      1                      350                  120                  1000

            5                      5                      2                      540                  180                  1400

            5                      5                      3                      920                  300                  3200

            5                      5                      4                      1600                640                  5800

            5                      5                      5                      ³1600                –                           –

Most Probable Number Index and 95 % Confidence Limits for Five Tube, Three Dilution Series

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