Simplified picure of the Krebs cycle where you can see how pyruvate is converted to acetyl-CoA, which reacts with oxaloacetate and forms citrate (hence the alternative name the citric acid cycle). From citrate, carbon atoms are split off in the form of CO2 and after a revolution in the Krebs cycle oxaloacetate remains, which can then take care of a new molecule of acetyl-CoA.
Image: Karl-Erik Johansson (BVF, SLU). - Click on the image to enlarge it.
From one glucose molecule having 6 carbon atoms, two molecules of pyruvate having 3 carbon atoms each are formed during the glycolysis. Under aerobic conditions and if the current bacterium has aerobic metabolism, the pyruvate molecule is not fermented, but can instead be converted to acetyl Coenzyme A (acteyl-CoA), while one molecule of NAD + is reduced to NADH. The acetyl group in acetyl-CoA has two carbon atoms because carbon dioxide (CO2) has been cleaved off and can now enter the Krebs cycle (= citric acid cycle). The Krebs cycle is a prestep to the elektron transport chain (= the respiratory chain).
All enzymes required for the Krebs cycle exist in bacteria in the cytoplasm (= cytosol), as bacteria lack mitochondria. The acetyl group binds in the first step of the Krebs cycle to oxaloacetate, which has four carbon atoms, and then citrate, which has six carbon atoms, is formed. After one revolution in the Krebs cycle, 2 molecules of CO2 have been cleaved off, 3 molecules of NADH, 1 molecule of FADH2 and 1 molecule of ATP have been generated from each pyruvate molecule. What remains of the citrate molecule is again oxaloacetate, which can be re-acetylated and spin one more revolution in the cycle. In total, 8 molecules of NADH, 2 molecules of FADH2 and 2 molecules of ATP have been formed from 1 molecule of glucose. Actually, it is not ATP that is formed, but GTP. However, GTP is then converted in the cell to ATP by a reacton with ADP. NADH and FADH2 are utilized in the electron transport chain for ATP synthesis.
Other molecules than carbohydrates
Some bacteria have an enzyme (tryptophanase), which can form pyruvate from amino acids (tryptophan). Pyruvate can then be metabolized in the Krebs cycle. Other bacteria can break down cysteine into substances that can enter the Krebs cycle. In order for amino acids to be used in the Krebs cycle, they must first be deaminated, i.e. the amine group must be converted to ammonia (NH3) or rather to an ammonium ion (NH4+), which can be secreted. In addition, there are bacteria that can metabolize lipids to glycerol and fatty acids. Glycerol can be converted to pyruvate and further metabolised in the Krebs cycle and fatty acids can enter the Krebs cycle directly. Thus, there are many pathways into the Krebs cycle and bacteria with aerobic, facultative anaerobic or microaerophilic metabolism can, therefore, extract energy from many different types of substances.