The bacteria quickly metabolized the glucose, producing an acid slant and an acid butt (acid over acid; A/A) in a few hours. The Emben-Meyerhof-Parnas pathway is used both aerobically (on the slant) and anerobically (in the butt) to produce ATP and pyruvate. On the slant, the pyruvate is further metabolized to CO2, H2O, and energy. After further incubation (18 hours) the glucose was consumed, and then the bacteria utilized lactose and/or sucrose, maintaining an acid slant. The results are recorded as acid over acid (A/A). If the medium were incubated longer, over 48 hours, the lactose and sucrose would be depleted, and the slant would revert to an alkaline pH due to metabolism of the peptones. In the anerobic butt, the bacteria convert pyruvate into stable acid endproducts, thus the butt remains acidic.
Bacteria commonly producing an A/A reaction with or without gas include: Enterobacter aerogenes, E. cloacae, Escherichia coli, Klebsiella oxytoca, and K. pneumoniae.
Reference: MacFaddin, J.F. (2000) Biochemical Tests for Identificaion of Medical Bacteria, 3rd. ed. Lippincott Williams & Wilkins, Philadelphia.