Final answer:
To maintain a stable pH and ensure bacterial health during the culturing process in a lab, the optimal substance is a buffer.
Step-by-step explanation:
When culturing bacteria in a lab and accumulated metabolic byproducts alter the pH, becoming either too acidic or too basic, the optimal substance to maintain a stable pH and ensure bacterial health is a buffer. A buffer is a chemical system that helps to maintain a stable pH by absorbing excess hydrogen ions or hydroxide ions, preventing a radical change in the pH of the environment. This is vital since extreme pH levels can denature proteins, disrupt the proton motive force used in ATP production, and affect the structure of other macromolecules like lipids and DNA.