Water quality [PDF]

Lab Exercise: Examining Water Quality: Most Probable Number & Colilert® Test Kit Lab OBJECTIVES 1. Understand the use of MPN to determine likely fecal water contamination. 2. Understand the use of MUG, ONPG and β-galactosidase in the Colilert test kit. 3. Understand the use of positive and negative controls in this experiment. 4. Determine the cleanliness of the experimental water sample. BACKGROUND Most Probable Number (MPN) is a procedure used to estimate bacterial populations in the environment (including bacterial contamination of water). This procedure is based on the application of the Theory of Probability with the following given assumptions. 1. The organisms are randomly and evenly distributed throughout the sample; 2. The organisms exist as single entities, not as chains, pairs or clusters and they do not repel one another; 3. The proper growth medium, temperature and incubation conditions have been selected to allow even a single viable cell in an inoculum to produce detectable growth; 4. The population does not contain viable, sub-lethally injured organisms that are incapable of growth in the culture medium used. Samples are diluted prior to use in order to reduce the number of positive tubes to a manageable number. The number of tubes prepared is generally based on the expected population contained within the sample. Reliable results occur when all tubes at the lower dilution are positive and all tubes at the higher dilution are negative for growth, as the dilution scheme has accurately “bracketed” the population, much as the case in dilution plating. Typically, this results in serial dilutions for MPN analysis in sets of 3, 5 or 10 MPN tube series, with more tubes increasing accuracy. MPN values are an indirect measurement of the number of viable organism in a given sample. However, MPN counts are useful for their technical ease and are particularly useful when low concentrations of organisms (