Deck 29: Bacteriological Examination of Water: Multiple-Tube Fermentation and Membrane Filter Techniques

Question 1

Describe the purpose of lactose and LES Endo agar in these tests.

Answer 1

The bacteriological water analysis uses the coliforms as the indicator organisms, to predict the presence of pathogenic organisms. The special characteristics of the indicator organisms are, they are gram negative rods, non-spore forming and lactose fermenting organisms. So, the lactose is the necessary ingredient, for the assessment of the coliform presence in the water.
The LES Endo Agar (Lawrence Experimental Station Endo Agar) is a differential medium, which differentiates the coliforms (lactose fermenters) from other non-lactose fermenting bacteria. On the LES Endo Agar, the coliforms exhibits the golden metallic sheen, and red to black colonies. But, the Salmonella typhi (non-lactose fermenter) exhibits the white colonies. In addition, the Endo agar inhibits the growth of gram positive bacteria.

Question 2

What are some limitations of the membrane filter technique?

Answer 2

Limitations of Membrane Filter Technique:
1. Considerably, the pore size of the membrane filter is very much small (0.45 µm). So, most turbid waters cannot be filtered properly; because they may clog in to the filter. Due to such reason, the further dilution of test sample must be performed, prior to the membrane filtration. Note: The rate of dilution should be noted for the final calculation of the bacterial quantification.
2. The membrane filter technique provides the appropriate results for the coliform counts in between 30-300 organism per filter disc. So, the increased presence of coliforms, in the water sample requires the multiple tube fermentation test.

Question 3

Define the term coliform.

Answer 3

Coliforms:
The microbial population, which inhabit in the colon is collectively termed as Coliforms. Coliforms are generally gram negative rods and able to ferment the lactose. In addition, the coliforms are nonpathogenic and non-spore forming organisms. The coliforms are considered as an indicator organisms, in the microbiological water analysis.

Question 4

Briefly explain what is meant by presumptive, confirmed, and completed tests in water analysis.

Question 5

See Note 3, MPN determination, on page 244 for the question.

Question 6

In the United States, which bacterium is used as an indicator organism for coliforms?

Question 7

What nutritional means could be used to speed up the growth of the coliform organisms when using the membrane filter technique?

Question 8

Name an advantage of the membrane filter technique over the multiple-tube fermentation test.

Question 9

Describe two other applications of the membrane filter technique.

Question 10

What color will a colony testing positive on the Endo media exhibit?

Question 11

Why not test for pathogens such as Salmonella directly rather than using an indicator organism, such as coliform bacteria?

Question 12

Why does a positive presumptive test not necessarily indicate that the water is unsafe for drinking?

Question 13

List three organisms that usually give a positive presumptive test.