Shown below is a Petrifilm analysis of a water sample that was tested for fecal

Question

Shown below is a Petrifilm analysis of a water sample that was tested for fecal contamination. One milliliter of a 10-3 dilution of the water was pipetted onto the Petrifilm and, after 48 hours of incubation at 37, the Petrifilm appeared as shown below. Use the information in the Petrifilm Interpretation Guide that is posted on iLearn to answer the questions below. A. Calculate the number of E. coli/ml in the water sample. (Show your work). B. Calculate the number of coliforms in the water sample. (Show your work). C. Why do we test for coliforms rather than testing for potential pathogens?

Explanation / Answer

On the E. coli/coliform Petrifilm, blue colonies with entrapped gas marks E. coli and coliforms are marked by red colonies with gas. Colonies that appear on the foam barrier aren't counted.

Most E. coli produce beta-glucuronidase which produces a blue precipitate associated with the colony indicated by the blue or red-blue colonies. The top film traps gas produced by the lactose fermenting coliforms and E. coli. About 95% of E. coli produce gas, as indicated by colonies associated with entrapped gas (within approximately one colony diameter). Blue colonies without gas are not counted as E. coli. Other coliform colonies are red and closely associated with entrapped gas. The total coliform count consists of both the red and blue colonies associated with gas.

A) E. coli count: 45 (blue colonies with gas)

i.e. 45 * 103 cells/ml

B) Total coliform count: 95 (red and blue colonies with gas)

i.e. 95 * 103 cells/ml

C) Water pollution caused by fecal contamination can potentially cause diseases from pathogens. Often, pathogen conc. from fecal contamination is very small, and the number of different possible pathogens is relatively large. Hence, it is not practical to test for pathogens in every water sample that is collected. Instead, indirect evidence is used for detecting pathogens by testing for an "indicator" organism like coliform bacteria. Coliforms are rather easy to identify, are usually present in larger numbers than more dangerous pathogens, and respond to the environment, wastewater treatment, and water treatment similarly to many pathogens. As a result, testing for coliform bacteria can be a reasonable indication of whether other pathogenic bacteria are present.

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