I have tried to determine which isolates of my experiment are nitrogen fixers. I have used Norris glucose nitrogen free broth with 0.5% bromothymol blue (1.5 ml BTB in 300 ml broth). The broth was dark greenish black in colour after autoclave. After inoculation and incubation for 3 days, some of the tubes changed colour from greenish black to yellow, and others retained the same colour as negative control (uninoculated broth). How can I determine which isolate is able to fix atmospheric nitrogen from this? Is the BTB concentration too high?