Microbiology Help-Mannitol fermentation (phenol red)

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Im currently in 3rd semester of nursing school and I had to take all of my general ed classes with it. Micro is fun but info is limited. I did a mannitol fermentation with phenol red in a test tube. Also had a Durham tube in it. I used a positive control that came back yellow/positive. My unknown came back negative. Turns out my unknown was bacillus megaterium. Im wondering how mannitol was negative. I inoculated the mannitol on Friday but did not put it in the incubator because the lab is closed on weekends and I had to get the results on Monday. Lab assistant recommended keeping in drawer... Any idea what caused the negative result? Could it have turned yellow then went back to red? Luckily my professor said I could redo the mannitol and if it is negative, I will receive the other 5 points. Any ideas?, Im super curious!

*I encourage all nursing students to talk to their professors after class or during office hours. It shows you are interested and if they know your trying they are more likely to help you. Now this doesn't help with nursing because we our points are only test grades.

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