Gas production is not recognized as nitrate reduction when the organism is a known fermenter because the production of gas is not a specific indicator of nitrate reduction.
Fermentation and nitrate reduction are two different metabolic processes that can occur simultaneously in some microorganisms. Fermentation is an anaerobic process that involves the breakdown of organic compounds to produce energy, while nitrate reduction is an anaerobic respiration process that uses nitrate as a final electron acceptor.
Some microorganisms that are known fermenters can also reduce nitrate, but the presence of gas production does not specifically indicate nitrate reduction. Gas production can be a result of other metabolic processes such as the breakdown of carbohydrates during fermentation.
To confirm nitrate reduction, specific biochemical tests or assays are needed to detect the presence of nitrite or nitrogen gas, which are the end products of the nitrate reduction process. These tests are more specific and reliable than simply observing gas production. Therefore, gas production alone is not sufficient to confirm nitrate reduction, and further tests are necessary to confirm this metabolic process.