What is point of Durham tubes in a Nitrate reduction test?

[instantresults]

Lab manual is confusing me, and I'm not sure if its actually wrong, or the wording is bad, or if this is because I'm not understanding something.

It says:

"Before a broth can be tested for nitrate reductase activity it must be examined for evidence of denitrification. This is simply a visual inspection for the presence of gas in the Durham tube. If the Durham tube contains gas, the test is complete. Denitrification has taken place"

then 4 paragraphs later it says this:

"One or more of the organisms selected for this exercise are fermenters and possible producers of gas other than molecular nitrogen; therefore you will proceed with adding reagents to all tubes."

So why does the manual say that the test is complete if I see gas in the durham tube, while the next paragraph tells me to proceed adding reagents to all tubes. regardless of gas in the durham tube?

And that makes me then question what the point of using the durham tubes is; As I understand this, the gas in durham tube could be a number of different gases other than N2, but denitrification produces N2, doesn't it? So if the gas in tube is unknown, what was the point of using durham tubes? It says some organisms are known fermenters, so the durham tubes seem pointless, with respect to testing for presence of nitrogen.

Thanks for any help

 

[Bystander]

"Conditional" logic; "if this, then that." Unless it's Monday, in which case, "this" implies "something else."