Final answer:
Bacteria require the production of gas, specifically carbon dioxide, to utilize citric acid for energy as it is an integral part of the citric acid cycle and fermentation processes.
Step-by-step explanation:
To utilize citric acid for energy, bacteria must be capable of producing gas during the fermentation process. Citric acid metabolism releases carbon dioxide, a gas, as part of the citric acid cycle and during its catabolism on Simmons' citrate agar.
This gas production is essential for bacteria to grow on a medium that contains citric acid as the sole carbon source. In the citric acid cycle specifically, citric acid is broken down to release NADH and FADH2, which then undergo oxidative phosphorylation to produce ATP, a process that also involves the production of CO2.