Suppose you remove your test cultures from the incubator and notice that one of them—a known fermenter—has a gas bubble in the Durham tube. Knowing that fermenters frequently produce gas, you ignore the bubble and proceed to the next step. Adding reagents produces no change, and neither does adding zinc. Is this occurrence consistent with what you have learned about this test?

When the test culture was removed from the incubator and there was gas bubble in it.The gas bubble produced from the test tube is nitrogen. But if the addition of the zinc and other reagent in the culture and there is no response.

It suggest that the nitrates is being used by the bacteria. The bacteria inside can be aerobic and facultative anaerobic.

They can be gram negative microorganism and yes the occurrence states that the test was successful if the test was about facultative anaerobic and aerobic.

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.