The first test used in the multiple tube fermentation method for coliform bacteria determination is...

In the multiple tube fermentation method for determining coliform bacteria, the initial screening phase is known as the presumptive test. This test is crucial as it provides an early indication of the presence of coliforms in a water sample. During the presumptive test, a series of tubes containing a nutrient broth are inoculated with a diluted sample of water. If coliforms are present, they will ferment lactose, producing gas and resulting in a positive reaction. This step is designed to identify the potential presence of coliform bacteria before any confirmatory testing is performed.

The subsequent steps, such as the confirmed and completed tests, rely on the results obtained from the presumptive test. They help to verify and accurately identify the specific coliform bacteria present. The BOD (Biochemical Oxygen Demand) test is outside the context of coliform testing, as it measures the amount of oxygen consumed by microorganisms in a water sample and is not directly used for coliform detection. Hence, the presumptive test stands as the critical first phase in assessing the presence of coliform bacteria.