Concentration of coliforms in samples of tap water

Coliforms are aerobic, facultative anaerobic, non-spore forming bacteria. Coliforms are gram-negative rods that ferment lactose with gas production within 48 hours of incubation at 35o C. Examples include E. coli, Klebsiella, Citrobacter and Enterobacter. Fecal coliforms are capable of gas production (or colony formation) at elevated incubation temperature (44.5o C for 24 hours) Conventionally, the presence of coliforms in a water or wastewater sample was determined using the Multiple Tube Fermentation method, also called the Most Probable Number (MPN) method. In the MPN method, a series of dilutions are made of a water or wastewater sample. These dilutions are then put into pre-prepared tubes containing a broth to culture the coliforms (or fecal coliforms). The presence coliforms in the water sample is indicated if a tube or series of tubes becomes turgid (i.e., cloudy). In addition, the formation of gas in an inverted Durham tube within each tube is also recorded. MPN is a statistical technique based on the Poisson's distribution. The Membrane Filtration Method (MFM) is a more recent, EPA-approved method to test for the presence of coliforms and fecal coliforms in water samples. A known volume of water or wastewater sample is filtered using the standard vacuum filtration apparatus, through a gridded filter, which has a sufficiently small pore-size (typically 0.45 µm or less). The filter paper that traps the coliforms (if any) from the water sample is then placed in a Petri dish containing an appropriate agar medium, which can support the growth of the coliforms or fecal coliforms. The Petri dish is covered and then placed in an incubator. After 24 to 48 hours, the Petri dishes are examined for indications of growth of colonies on the filter paper. Each colony is representative of a single coliform in the water sample. The number of coliforms should be between 20 and 80 to be statistically significant. For this purpose, several dilutions of the water or wastewater sample are prepared, filtered and cultured. Colonies may be counted with a standard colony counter or through direct visual observation. Determine the concentration of coliforms in samples of tap water and river water using the membrane filtration method, and of various surfaces in Jobst Hall.