Final answer:
In the KIA slant, glucose is utilized first due to inducer exclusion and upon depletion, lactose is used, regulated by the lac operon and catabolite activator protein.
Step-by-step explanation:
The utilization of sugars such as glucose and lactose in the Kligler's Iron Agar (KIA) slant is tightly regulated by bacteria and involves various metabolic and genetic controls. Initially, bacteria prefer to consume glucose because it is a readily accessible energy source, and the presence of glucose inhibits the use of lactose through a mechanism called inducer exclusion. High intracellular glucose levels promote the binding of unphosphorylated EIIAGlc to the lactose permease enzyme, preventing lactose from entering the cell. This keeps the lac operon, a set of genes responsible for digesting lactose, repressed.
However, once glucose is depleted, cells increase the concentration of cyclic AMP (cAMP), which, in combination with the catabolite activator protein (CAP), activates the lac operon, allowing for the digestion of lactose. Thus, the switch from glucose to lactose usage as a carbon source is influenced by the phosphorylation status of EIIAGlc and the level of cAMP. In the context of the KIA test, this metabolic regulation can affect the interpretation of results which rely on the color changes in the medium due to acid production from fermentation. Aerobic respiration will turn the slant dark red, while fermentation will result in acid production only in the butt (bottom) of the tube.