In the multiple-tube fermentation test, what indicates a positive result?

In a multiple-tube fermentation test, the presence of gas in Durham tubes indicates a positive result. This specific test is designed to detect fermentative activity by observing whether microorganisms can convert sugars into acids and gases.

During the process, if bacteria ferment the sugars present in the medium, they may produce carbon dioxide or other gases as byproducts. These gases accumulate in the inverted Durham tubes placed in the culture medium. When bubbles are observed in these tubes, it signifies that fermentation has occurred, confirming that the specific bacteria being tested are capable of metabolizing the sugars present.

While color changes, cloudiness, or changes in pH level can also indicate microbial activity or metabolic processes, the definitive sign of fermentation in this test is specifically the detection of gas in the Durham tubes, making it the most direct and reliable indicator of a positive result in assessing fermentation.