Final answer:
Bacteria utilize different fermentation pathways in the absence of oxygen to produce ATP and various commercial products. Facultatively anaerobic bacteria can switch between aerobic respiration and fermentation, while obligate anaerobes rely exclusively on fermentation. Citrate utilization is a specific metabolic pathway used by some bacteria for growth and can be observed in laboratory settings via biochemical tests.
Step-by-step explanation:
The response to the student's question involves understanding the role of different types of fermentation by bacteria and how these pathways serve in ATP production or the synthesis of commercial products. When oxygen is not available, many bacteria can perform fermentation to extract energy from organic molecules.
For instance, alcoholic fermentation, conducted by yeast, produces ethyl alcohol and CO2, and is utilized in bread making and winemaking. Other bacteria can perform fermentations that produce different types of gases and organic acids, contributing to varied commercial products such as solvents, pharmaceuticals, and the distinctive flavor of certain foods like Swiss cheese through propionic acid fermentation.
Some bacteria are facultatively anaerobic, meaning they can switch between aerobic respiration and fermentation based on oxygen availability. In contrast, obligate anaerobes cannot survive in the presence of oxygen and solely rely on fermentation. The detection of gases and other biochemical by-products from these fermentation processes can help in the identification of various bacterial species in a laboratory setting.
Citrate utilization is another important pathway for some bacteria, allowing them to grow on mediums such as Simmons' citrate agar, as they can metabolize citric acid, releasing alkaline by-products that change the color of pH indicators - illustrating positive and negative results for specific biochemical tests.