Tennessee Wastewater Operator Practice Exam

If a vial from an MPN test is cloudy from bacterial growth and glows blue under a black light, what does that indicate?

1. The test is negative for total coliforms

2. The test is positive for total coliforms and E. Coli

3. The test is positive for E. Coli only

4. The test is inconclusive

The correct answer is: The test is positive for total coliforms and E. Coli

A vial that is cloudy from bacterial growth indicates that there is a significant presence of bacteria in the sample. When this vial also glows blue under a black light, it suggests that specific bacteria are present that fluoresce under ultraviolet light. In the context of an MPN (Most Probable Number) test, the combination of cloudiness and blue fluorescence strongly points to the presence of both total coliforms and E. coli, particularly because E. coli is a type of coliform that is specifically tested for due to its fecal contamination significance. The fluorescence is typically associated with the presence of E. coli strains that produce certain enzymes resulting from their metabolism of the substrates present in the medium used during the test. This dual indication makes it clear that both total coliforms, which encompasses a broader group of bacteria found in the environment, and E. coli, which is a specific indicator organism of fecal contamination, are present in the sample. Thus, the result confirms positive findings for both categories, constituting a definitive result rather than a negative or inconclusive status.