Bacterial metabolism

All cells require energy to drive life processes. The requisite energy is obtained from organic chemicals by chemoorganotrophs, from inorganic chemicals by chemolithotrophs, and from light by phototrophs. In this chapter we explore how cells conserve and use their energy and nutrients.

A cell must coordinate many different chemical reactions and organize many different molecules into specific structures. Collectively, these reactions are called metabolism. Metabolic reactions are either catabolic, which means energy releasing, or anabolic, which means energy requiring. Catabolism breaks molecular structures down, releasing energy in the process, and anabolism uses energy to build larger molecules from smaller ones.

Carbon and Nitrogen

All cells require carbon, and most prokaryotes require organic (carbon-containing) compounds as their source of carbon. Heterotrophic bacteria assimilate organic compounds and use them to make new cell material. Amino acids, fatty acids, organic acids, sugars, nitrogen bases, aromatic compounds, and countless other organic compounds can be transported and catabolized by one or another bacterium. Autotrophic microorganisms build their cellular structures from carbon dioxide (CO₂) with energy obtained from light or inorganic chemicals.

A bacterial cell is about 13% nitrogen, which is present in proteins, nucleic acids, and several other cell constituents. The bulk of nitrogen available in nature is in inorganic form as ammonia (NH₃), nitrate (NO₃), or nitrogen gas (N₂). Virtually all prokaryotes can use NH₃ as their nitrogen source, and many can also use NO₃. By contrast, N₂ can only be used by nitrogen-fixing prokaryotes. Nitrogen in organic compounds, for example, in amino acids, may also be available to microorganisms; if organic N is available and is taken up, the compound can immediately enter the monomer pool for biosynthesis or be catabolized as an energy source.

Other macronutrients

Phosphorus is a key element in nucleic acids and phospholipids and is typically supplied to a cell as phosphate (PO₄). Sulfur is present in the amino acids cysteine and methionine and also in several vitamins, including thiamine, biotin, and lipoic acid. Sulfur can be supplied to cells in several forms, including sulfide (HS) and sulfate (SO₄). Potassium (K) is required for the activity of several enzymes, whereas magnesium (Mg) functions to stabilize ribosomes, membranes, and nucleic acids and is also required for the activity of many enzymes. Calcium (Ca) is not required by all cells but can play a role in helping to stabilize microbial cell walls, and it plays a key role in the heat stability of endospores. Sodium (Na) is required by some, but not all, microorganisms, and its requirement is typically a reflection of the habitat. For example, seawater contains relatively high levels of Na, and marine microorganisms typically require Na for growth. By contrast, freshwater species are usually able to grow in the absence of Na. K, Mg, Ca, and Na are all supplied to cells as salts, typically as chloride or sulfate salts.

Micronutrients

Microorganisms require several metals as micronutrients for growth. Iron (Fe), which plays a major role in cellular respiration. Iron is a key component of cytochromes and of iron–sulfur proteins involved in electron transport reactions. Under anoxic conditions, iron is generally in the ferrous (Fe²⁺) form and soluble. However, under oxic conditions, iron is typically in the ferric (Fe³⁺) form as part of insoluble minerals. To obtain Fe³⁺ from such minerals, cells produce iron binding molecules called siderophores that function to bind Fe³⁺ and transport it into the cell. Example: Pseudomonas fluorescens can produce siderophores to scavenge ferric ions in the soil ecosystem. Please note that some organisms do not need iron for example lactic acid bacteria such as species of Lactobacillus do not contain detectable iron and grow normally in its absence. In these organisms, manganese (Mn ²⁺) often plays a role similar to that just described for iron. Many other metals are required or otherwise metabolized by microorganisms. Likewise, the other metals which are required in limited quantities are presented as table form.

Growth factors

Growth factors are organic compounds that, like trace metals, are required in only very small amounts. Growth factors are vitamins, amino acids, purines, pyrimidines, or various other organic molecules. Although most microorganisms are able to biosynthesize the growth factors they need, some must obtain one or more of them from the environment and thus must be supplied with these compounds when cultured in the laboratory. Vitamins are the most commonly required growth factors. Most vitamins function as coenzymes, which are non-protein components of enzymes. Vitamin requirements vary among microorganisms, ranging from none to several. Lactic acid bacteria, which include the genera Streptococcus, Lactobacillus, and Leuconostoc, are well-known for their many vitamin requirements, which are even more extensive than those of humans.