I have an unknown microbe, and I think it's Proteus vulgaris, but I'm not sure that the hydrogen sulfide test is positive.

 

After performing the H2S test, I get a yellow butt, with NO black color along line of stab inoculation, but a dark precipitate at the TOP of the medium.

 

What does that mean???

 

A black color along the stab or throughout the medium is supposed to be positive, and no black is negative. Well I'm stuck somewhere in the middle.

 

Please help, I've looked everywhere for this type of result and can't figure out what it means.

H2S reaction aside, the easiest way to identify and Proteus species is to just streak a plate and look at the colony morphology. The genus Proteus have very distict "swarming" growth patterns on blood agar because of its motility. It grows out in concentric rings, or may even make a film. Also, all Proteus species are urease postive. You could also try to re-innoculate the

H2S media, and see if you get the same result. I find it best to do innoculations in duplicate if possible. I would lean more toward the positive side; if there wasn't sulfur reduction, there shouldn't be any black.