Mizanul Gowda

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1. The first step in building an RFM model is to assign Recency, Frequency and Monetary values to each customer
2. The second step is to divide the customer list into tiered groups for each of the three dimensions (R, F and M), using Excel or another tool

MPN is most commonly applied for quality testing of water i.e to ensure whether the water is safe or not in terms of bacteria present in it. A group of bacteria commonly referred to as fecal coliforms act as an indicator of fecal contamination of water. The presence of very few fecal coliform bacteria would indicate that water probably contains no disease‑causing organisms, while the presence of large numbers of fecal coliform bacteria would indicate a very high probability that the water could contain disease‑producing organisms making the water unsafe for consumption.

Water to be tested is diluted serially and inoculated in lactose broth, coliforms if present in water utilizes the lactose present in the medium to produce acid and gas. The presence of acid is indicated by the color change of the medium and the presence of gas is detected as gas bubbles collected in the inverted Durham tube present in the medium. The number of total coliforms is determined by counting the number of tubes giving positive reaction (i.e both color change and gas production) and comparing the pattern of positive results (the number of tubes showing growth at each dilution) with standard statistical tables.

MPN test is performed in 3 steps

The presumptive test is a screening test to sample water for the presence of coliform organisms.

If the presumptive test is negative, no further testing is performed, and the water source is considered microbiologically safe. If, however, any tube in the series shows acid and gas, the water is considered unsafe and the confirmed test is performed on the tube displaying a positive reaction.

The method of the presumptive test varies for treated and untreated water.

To view the full table download the PDF file from the link given in the reference.

Some microorganisms other than coliforms also produce acid and gas from lactose fermentation. In order to confirm the presence of coliform, a confirmatory test is done.

From each of the fermentation tubes with positive results transfer one loopful of medium to:

Incubate the inoculated lactose-broth fermentation tubes at 37°C and inspect gas formation after 24 ± 2 hours. If no gas production is seen, further incubate up to a maximum of  48 ±3 hours to check gas production.

The agar slants should be incubated at 37°C for 24± 2 hours and Gram-stained preparations made from the slants should be examined microscopically.

The formation of gas in lactose broth and the demonstration of Gram-negative, non-spore-forming bacilli in the corresponding agar indicates the presence of a member of the coliform group in the sample examined.

The absence of gas formation in lactose broth or the failure to demonstrate Gram-negative, non-spore-forming bacilli in the corresponding agar slant constitutes a negative test (absence of coliforms in the tested sample).

Tryptone Water Test

a. Confirmatory tests positive for indole, growth, and gas production show the presence of thermotolerant E. coli.b. Growth and gas production in the absence of indole confirm thermotolerant coliforms.

Since some of the positive results from the confirmatory test may be false, it is desirable to do completed tests. For this inoculum from each positive tube of the confirmatory test is streaked on a plate of EMB or Endo agar.

In this process, a loopful of a sample from each positive BGLB tube is streaked onto selective medium like Eosin Methylene Blue agar or Endo’s medium. One plate each is incubated at 37°C and another at 44.5± 0.2°C for 24 hours.

Following incubation, all plates are examined for the presence of typical colonies.

References and further reading