Final answer:
After 10-12 hours, a Lysine Decarboxylase test will look purple if the bacteria can decarboxylate lysine, as it causes the pH to increase and the indicator in the medium changes to reflect the alkaline condition.
Step-by-step explanation:
When conducting a Lysine Decarboxylase test, the color change after 10-12 hours of incubation represents the pH change due to the decarboxylation of lysine by the bacteria being tested. If a bacterium is capable of decarboxylating lysine, it produces alkaline end products, which will raise the pH of the medium. The indicator in the medium will then turn purple to reflect this alkalinity. If the organism is unable to decarboxylate lysine, there is no change in pH, and the medium will remain or turn yellow due to fermentation or lack of activity. A green coloration is not typical for this test and could be a result of an incorrect interpretation or a different test. An example of a test that involves a color change to green is the test for phenylketonuria, where a 10% FeCl3 solution turns fresh urine emerald green. It is also worth noting that during the Gram staining procedure, both Gram-positive and Gram-negative cells will be stained purple initially, but Gram-negative cells will decolorize and can be further differentiated with a counterstain such as safranin.