![]() The essence of the MPN method is to dilute the sample to such a degree that inocula in the tubes will sometimes but not always contain viable organisms. ![]() The individual tubes of the sample are independent. Every tube (or plate, etc.) whose inoculum contains even one viable organism will produce detectable growth or change. The bacteria are separate, not clustered together, and they do not repel each other. The bacteria are distributed randomly within the sample. The following assumptions are necessary to support the MPN method. If a confirmation test involves selecting colonies to test, then a statistical adjustment not discussed in this appendix should be used (see Blodgett 2005a.) ![]() For simplicity, however, this appendix will speak of these GUs or CFUs as individual bacteria. If, in the microbiologist's experience, the bacteria in the prepared sample in question can be found attached in chains that are not separated by the preparation and dilution, the MPN should be judged as an estimate of growth units (GUs) or colony-forming units (CFUs) instead of individual bacteria. Only viable organisms are enumerated by the MPN determination. McClure in the Bacteriological Analytical Manual (BAM), 7th edition. The following background observations are adapted and extended from the article on MPN by James T. The MPN is particularly useful for low concentrations of organisms (<100/g), especially in milk and water, and for those foods whose particulate matter may interfere with accurate colony counts. Serial dilution tests measure the concentration of a target microbe in a sample with an estimate called the most probable number (MPN). for 10 tubes at 10 ml inocula and link to spreadsheet
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