General Microbiology - Schlegel H. 1987

Microbial Growth
Microbial Nutrition

Water is essential for Microbial growth. Nutrients are compounds dissolved in water that microorganisms use to build their Cellular Structures and generate energy. Different microorganisms have highly diverse requirements regarding the composition of nutrient media and other environmental conditions. Consequently, numerous recommendations have been proposed for formulating culture media for various microorganisms. In principle, nutrient media must meet the following minimum requirements: they must contain all the essential cellular building blocks in forms that microorganisms can readily assimilate.

Requirements for chemical elements. Based on their quantitative contribution to cellular composition, elements are classified into macro- and microelements (Trace Elements). The former include ten elements found in all organisms: carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, calcium, magnesium, and iron (С, О, Н, N, S, Р, К, Ca, Mg, Fe). Microelements, or trace elements, include manganese, molybdenum, zinc, copper, cobalt, nickel, vanadium, boron, chlorine, sodium, selenium, silicon, tungsten, and others, which are not required by all organisms. Most of these trace elements, needed only in minute amounts, are present as impurities in macronutrient salts, and also enter the nutrient medium from glassware and dust. Therefore, Special Methods are required to determine The Need for certain microelements.

Most elements are added to the nutrient medium in the form of salts. The composition of a simple synthetic medium is given in Table 6.1.

Class="center">Table 6.1. Example of a simple synthetic medium

Carbon and Energy Sources. Organisms that derive energy via Photosynthesis or through The oxidation of Inorganic Compounds are, for the most part, capable of utilizing СO₂ as their primary carbon source. These autotrophic organisms fix СO₂. All other organisms obtain their cellular carbon primarily from Organic compounds. The latter generally serve as sources of both energy and carbon; they are partly assimilated for Cell synthesis and partly oxidized to generate energy. Quantitatively, Polysaccharides—specifically Cellulose and starch—predominate among natural organic compounds on Earth. The structural Building Blocks of these polymers, glucose molecules, can be utilized by A wide variety of microorganisms. However, microorganisms are also capable of utilizing virtually all other naturally occurring organic compounds.

Accessory growth factors. In addition to mineral nutrients, carbon sources, and energy sources, many organisms require certain supplementary compounds known as growth factors. These substances are integral Components of the cell, yet some organisms lack The ability to synthesize them on their own. Such growth factors fall into three major groups: Amino Acids, Purines and Pyrimidines, and Vitamins. Amino acids, purines, and pyrimidines are the building blocks of Proteins and Nucleic Acids, meaning The Cell requires them in substantial quantities. Vitamins, on the other hand, function as constituents of Coenzymes or prosthetic groups and thus participate in catalytic processes; consequently, they are required only in very small amounts (Table 6.2). Organisms that require growth factors are termed auxotrophs, in contrast to prototrophs, which have no such requirements.

Sulfur and nitrogen. Both elements are present in the cell primarily in their reduced forms—as sulfhydryl groups and amino groups, respectively. Most microorganisms are capable of acquiring these elements in their oxidized forms, as sulfate and nitrate, which they subsequently reduce. The most common nitrogen source for microorganisms is ammonium salts. Some prokaryotes can fix molecular nitrogen (N₂); others require Amino acids as a nitrogen source, where nitrogen is already in a bound form. Similarly, not all microorganisms can reduce sulfate; some require hydrogen sulfide or Cysteine as a sulfur source.

Table 6.2. Vitamin solution suitable for soil and aquatic Bacteria

Oxygen. Oxygen is a constituent of water and is thus made available to Cells through it. In addition, it is present in СO₂ and numerous organic compounds. Furthermore, many organisms require molecular oxygen (O₂). The primary function of O₂ is to serve as the terminal electron acceptor in aerobic Respiration, during which O₂ is reduced to water. Oxygen atoms originating from O₂ are incorporated into cellular biomass only when methane, long-chain Hydrocarbons, or aromatic hydrocarbons serve as the carbon sources.

Based on their relationship to molecular oxygen, organisms can be divided into at least three main groups. Obligate aerobes obtain energy exclusively via respiration and therefore depend entirely on O₂. Obligate anaerobes can grow only in oxygen-free environments; O₂ is toxic to them (see the end of Section 7.4). Facultative anaerobes can grow both in the presence and absence of O₂. Within this group, two types should be distinguished: aerotolerant lactic acid bacteria can grow in the presence of atmospheric oxygen but cannot utilize it—they derive energy solely through Fermentation; whereas other facultatively anaerobic bacteria (such as Enterobacteriaceae) and many Yeasts can switch between respiration (in the presence of O₂) and fermentation (in the absence of O₂).

Many aerobic bacteria (if not the majority of them) are microaerophiles; that is, although they require oxygen for energy generation, they cannot tolerate the ambient partial pressure of O₂ found in the atmosphere (0.20 bar), requiring instead between 0.01 and 0.03 bar.