Which step of the multiple-tube fermentation test is conducted first?

The first step of the multiple-tube fermentation test is the presumptive test. This initial phase serves to identify potential presence of coliform bacteria in a water sample. During the presumptive test, samples are inoculated into broth media capable of indicating the fermentation of lactose, which coliforms can utilize.

The development of gas or acid in the broth signifies that coliform bacteria may be present. This stage is designed to capture a broad range of bacteria that ferment lactose, making it an essential first step in the testing process.

Once the presumptive test indicates potential coliform presence, further tests, such as the confirmed and completed tests, are performed to confirm and characterize the bacteria found. Thus, the sequence begins with the presumptive test, laying the foundation for accurate detection and analysis of possible contaminants.