Microbial metabolism refers to the diverse biochemical processes that microorganisms use to obtain energy, synthesize cellular components, and maintain viability. Bacteria and archaea exhibit extraordinary metabolic diversity, far exceeding that of plants and animals.
Energy Sources and Classification
Microorganisms are classified by their energy and carbon sources. Phototrophs, such as cyanobacteria and purple sulfur bacteria, use light energy to perform photosynthesis. Chemotrophs obtain energy from chemical compounds: chemolithotrophs oxidize inorganic compounds (H2, H2S, NH3, Fe2+), while chemoorganotrophs oxidize organic compounds (glucose, fatty acids, amino acids). Autotrophs use CO2 as their carbon source (e.g., cyanobacteria, nitrifying bacteria), whereas heterotrophs require organic carbon sources (e.g., E. coli, Bacillus, most pathogens).
Aerobic Respiration
Glycolysis (Embden-Meyerhof-Parnas pathway) converts glucose (6C) to two pyruvate (3C) molecules, producing 2 ATP and 2 NADH per glucose. The citric acid cycle (Krebs cycle, TCA cycle) oxidizes acetyl-CoA to CO2, producing NADH, FADH2, and GTP. The electron transport chain (ETC), located in the cytoplasmic membrane, oxidizes NADH and FADH2 as electrons pass through complexes I-IV with O2 serving as the final electron acceptor; ATP is generated via oxidative phosphorylation through the proton motive force driving ATP synthase.
Anaerobic Metabolism
In anaerobic respiration, alternative electron acceptors are used such as nitrate (NO3- → NO2- → N2), sulfate (SO42- → H2S), or carbonate (CO2 → CH4), commonly found in denitrifying, sulfate-reducing, and methanogenic bacteria. Fermentation uses organic compounds as both electron donors and acceptors, with no ETC or oxidative phosphorylation; ATP is produced solely by substrate-level phosphorylation. Common fermentation pathways include homolactic (Lactobacillus, Streptococcus → lactic acid), alcoholic (yeast → ethanol + CO2), mixed acid (E. coli → lactic acid, acetic acid, ethanol, CO2, H2), and butanediol (Enterobacter). Fermentation end-products are used industrially for producing yogurt, cheese, bread, beer, wine, and biofuels.
Nitrogen Metabolism
Nitrogen fixation, the conversion of N2 to NH3 by the nitrogenase enzyme complex, is carried out by Azotobacter, Rhizobium (symbiotic with legumes), and cyanobacteria. Nitrification involves the oxidation of NH3 to NO2- (Nitrosomonas) and NO2- to NO3- (Nitrobacter). Denitrification is the reduction of NO3- to N2 gas under anaerobic conditions (Pseudomonas, Paracoccus). Assimilatory nitrate reduction converts NO3- to NH3 for biosynthesis of amino acids and nucleotides.
Metabolic Regulation
Catabolite repression ensures that glucose, as the preferred carbon source, represses genes for metabolizing alternative sugars (e.g., the lac operon in E. coli). Feedback inhibition prevents overproduction as end products of biosynthetic pathways inhibit the first enzyme in the pathway. Two-component regulatory systems (e.g., EnvZ/OmpR) sense environmental changes and adjust gene expression accordingly.
