Water Problem

Problem:

Health department workers recently tested a small pond at the back of a housing development and found it to have an extremely high level of coliforms. A stream that begins at a spring approximately 0.5 mile away feeds the pond. Water enters the stream at two points: (1) a ditch which drains an area serving as the drainage field of a septic tank (2) a pipe which drains an area beneath a house. (See Figure below). Using the tests for coliforms, we will perform Presumptive, Completed and Confirmed tests on samples of water obtained at various points in the system. Use the results in the table below and the diagram to determine the source of contamination of the pond. (See the Enteric Laboratory Presentation in Module 5 and your A and P and Microbiology Study disk by Rowe for information on this topic.)

Results:

Presumptive Test Confirmed Test Completed Test

Sample Location

Gas EMB Agar - Lactose Fermentation Gram Reaction

Lactose

Fermentation

+ - + - +

Gram negative rods - + -

Spring + -

Pipe + + + +

Ditch + -

Stream + + + +

Pond + + + +

Ditch Pipe

Pond

Pond

Pond

Questions

1. Define coliforms.

2. Why do we test for coliforms instead of individual pathogens to determine if water is safe to drink?

3. In this problem, what would be the likely result of the Confirmed test if the health care workers omitted the Presumptive test and performed the Confirmed test first?

4. A student observed gas formation in all 5 of the lactose broth tubes in the Presumptive test but the EMB plates in the Confirmed test showed no growth. What could the student conclude about the organisms present in the original water sample?

5. What was the source of contamination of the pond (spring, pipe, ditch, stream or pond)?

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