Which bacterial reaction can lead to reversion in the KIA test?

In the context of the KIA (Kligler's Iron Agar) test, reversion refers to the process where acid production initially leads to a decrease in pH, followed by a shift back toward a more alkaline pH. This shift often occurs when certain bacteria utilize available carbohydrates, producing acids, and subsequently switch to using amino acids or proteins as a carbon source, leading to ammonia production which increases the pH.

The reaction that most clearly leads to this reversion effect is the oxidation of weaker acids. When some bacteria ferment sugars present in the KIA medium, they initially produce acids, lowering the pH and resulting in a yellow coloration. However, as fermentation progresses and the sugars become depleted, certain bacteria can begin to oxidize these organic acids, including weaker acids, leading to the production of neutral or alkaline products, such as ammonia. This process raises the pH back toward neutrality or even to alkaline levels, resulting in the reversion from yellow (acidic) back to red (alkaline) in the slant part of the test medium.

This behavior often distinguishes between different members of the Enterobacteriaceae family, as some species exhibit reversion while others do not, thereby serving as an important characteristic in microbial identification.