I teach undergraduate microbiology at a community college. We inoculated PR tubes for glucose, sucrose, and lactose. One of the organisms we regularly use is Pseudomonas aeruginosa. It has historically shown consistent -/- results for all three sugars. Today's results were unexpected. P. aeruginosa in the glucose tubes had about an inch of yellow on the top. At first I thought maybe contaminated, but all student groups got the same outcome. My lab techs are doing some quality control, but I thought I 'd see if anyone else has seen this, or might have an explanation.