Question

Does a positive presumptive test mean that the water is absolutely unsafe to drink? Explain. 2. Describe the appearan...


1. . Does a positive presumptive test mean that the water is absolutely unsafe to drink? Explain.

2. Describe the appearance of coliforms on Levine EMB agar.

3.. List five characteristics of coliform bacteria.

Answer

Presumptive tests are cheaper than the confirmatory test. These tests are conducted to ensure the quality of the sample.

  1. The presumptive test for water generally includes checking for the presence of fecal coliforms in water, however, for confirming the presence or absence of Escherichia coli ( specificity)there is a need for a confirmatory test. Thus it can be said that the positive presumptive test can indicate that the water is unsafe to drink ( water may be unsafe to drink ) but it doesn’t confirm the unsafety of the water. It is also important to note that there can be false positives because of human error that can occur. Hence there is a need for a confirmatory test for confirming the quality testing of water.
  2. Coliforms such as E.coli form blue-black colonies on the Levine EMB agar with a green metallic sheen. On the other hand, Enterobacter spp., in acidic condition form pink brown colonies. This media can be used for the detection of coliforms in the sample.
  3. The five characteristics of coliform bacteria are –
  • These bacteria are gram-negative such as E.coli.
  • These are facultatively anaerobic.
  • They are known to ferment lactose and produce acid and gas.
  • The coliform bacteria doesn’t form endospore.
  • They are also known as indicator organisms.

 

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