What gas is indicated by the bubbles formed in a positive multiple tube fermentation test?

In a positive multiple tube fermentation test, the formation of bubbles is indicative of carbon dioxide. This test is commonly used to determine the ability of microorganisms to ferment specific carbohydrates, producing various by-products, one of which is gas. During fermentation, sugars are metabolized by the microorganisms, resulting in the production of acids and gases, primarily carbon dioxide.

The presence of bubbles signifies that carbon dioxide is being generated as a by-product of the fermentation process. This is particularly relevant in microbiological studies where the generation of gas can be linked to the metabolic activity of the organism being tested. The test tubes used typically contain a Durham tube—a small inverted tube that captures gas bubbles if they are produced during the fermentation process.

Carbon dioxide production is a common indicator for fermentation processes in various microorganisms, making it the correct answer in this context. Other gases like oxygen and hydrogen are not typically produced in significant quantities during this type of fermentation, nor are they primary indicators of the fermentation process in the context of this test. Nitrogen doesn't play a direct role in fermentation and is not a by-product of the metabolic activity being measured.