Microbial Nutrition -requirements and modes of nutrition

 

Microbial cells are structurally complex and carry out numerous functions. Nutrients are substances used in biosynthesis and energy release and therefore are required for microbial growth.In order to construct new cellular components and do cellular work, organisms must have a supply of raw materials or nutrients and a source of energy.

COMMON NUTRIENT REQUIREMENTS

Over 95% of microbial cell dry weight is made up of a few major elements: carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, calcium, magnesium, and iron. These are called macroelements or macronutrients because they are required by microorganisms in relatively large amounts. 

The first six (C, O, H, N, S, and P) are components of  carbohydrates, lipids, proteins, and nucleic acids.

The remaining four macroelements exist in the cell as cations and play a variety of roles. For example, potassium (K⁺) is required for activity by a number of enzymes, including some of those involved in protein synthesis. Calcium (Ca2⁺), among other functions, contributes to the heat resistance of bacterial endospores. Magnesium (Mg2⁺) serves as a cofactor for many enzymes, complexes with ATP, and stabilizes ribosomes and cell membranes. Iron (Fe2⁺and Fe3⁺) is a part of cytochromes and a cofactor for enzymes and electron-carrying proteins.

In addition to macroelements, all microorganisms require several nutrients in small amounts. These are called micronutrients or trace elements. The micronutrients—manganese, zinc, cobalt, molybdenum, nickel, and copper—are needed by most cells. However, cells require such small amounts that contaminants from water, glassware, and regular media components often are adequate for growth. In nature, micronutrients are ubiquitous and probably do not usually limit growth. 

Micronutrients are normally a part of enzymes and cofactors, and they aid in the catalysis of reactions and maintenance of protein structure. For example, zinc (Zn2⁺) is present at the active site of some enzymes but can also be involved in the association of regulatory and catalytic subunits.  (e.g., E. coli aspartate carbamoyl transferase). Manganese (Mn2⁺) aids many enzymes that catalyze the transfer of phosphate groups. Molybdenum (Mo2⁺) is required for nitrogen fixation, and cobalt (Co2⁺) is a component of vitamin B12.

Besides the common macroelements and trace elements, microorganisms may have particular requirements that reflect their specific morphology or environment. 

Diatoms need silicic acid (H₄SiO₄) to construct their beautiful cell walls of silica [(SiO₂)n].

Although most procaryotes do not require large amounts of sodium, many archaea growing in saline lakes and oceans depend on the presence of high concentrations of sodium ion (Na⁺).

Finally, it must be emphasized that microorganisms require a balanced mixture of nutrients. If an essential nutrient is in short supply, microbial growth will be limited regardless of the concentrations of other nutrients.

  

REQUIREMENTS FOR CARBON, HYDROGEN, OXYGEN, AND ELECTRONS

All organisms need carbon, hydrogen, oxygen, and a source of electrons. Carbon is needed for the skeletons or backbones of all the organic molecules from which organisms are built. Hydrogen and oxygen are also important elements found in organic molecules. Electrons are needed for electron transport chain and for other oxidation-reduction reactions which can provide energy for use in cellular work. Electrons are also needed to reduce molecules during biosynthesis (e.g., the reduction of CO₂ to form organic molecules).

The requirements for carbon, hydrogen, and oxygen often are satisfied together because molecules serving as carbon sources provide hydrogen and oxygen also. Heterotrophs—organisms that use reduced, preformed organic molecules as their carbon source—can also obtain hydrogen, oxygen, and electrons from the same molecules. These organic carbon sources also provide electrons to be used in electron transport as well as in other oxidation-reduction reactions, so heterotrophs use their carbon source as an energy source also. The more reduced the organic carbon source (i.e., the more electrons it carries), the higher its energy content. Thus, lipids have a higher energy content than carbohydrates.

However, one carbon source, carbon dioxide (CO₂), supplies only carbon and oxygen, so it cannot be used as a source of hydrogen, electrons, or energy. This is because CO₂ is the most oxidized form of carbon, lacks hydrogen, and is unable to donate electrons during oxidation-reduction reactions. Organisms that use CO₂ as their sole or principal source of carbon are called autotrophs. Because CO₂ cannot supply their energy needs, they must obtain energy from other sources, such as light or reduced inorganic molecules.

Heterotrophic microorganisms have great flexibility with respect to carbon sources. There is no naturally occurring organic molecule that cannot be used by some microorganism. Actinomycetes, common soil bacteria, will degrade amyl alcohol, paraffin, and even rubber. Some bacteria can use almost anything as a carbon source; for example, Burkholderia cepacia can use over 100 different carbon compounds. Microbes can degrade even relatively indigestible human-made substances such as pesticides. This is usually carried out by complex microbial communities (microbial consortia). These molecules sometimes are degraded in the presence of a growth-promoting nutrient that is metabolized at the same time—a process called cometabolism. Other microorganisms can use the products of this breakdown process as nutrients. But some microbes are extremely fastidious and catabolize only a few carbon compounds. Cultures of methylotrophic bacteria metabolize methane, methanol, carbon monoxide, formic acid, and related one-carbon molecules. Parasitic members of the genus Leptospira use only long-chain fatty acids as their major source of carbon and energy.

REQUIREMENTS FOR NITROGEN, PHOSPHORUS, AND SULFUR

Microorganisms require large quantities of nitrogen, phosphorus, and sulfur for growth. These elements may be acquired from the same nutrients/organic sources that supply carbon, or from other inorganic sources.

Nitrogen is needed for the synthesis of amino acids, purines, pyrimidines, some carbohydrates and lipids, enzyme cofactors, and other substances. Many microorganisms can use the nitrogen obtained in amino acids. Others incorporate ammonia directly through the action of enzymes such as glutamate dehydrogenase or glutamine synthetase and glutamate synthase. Most phototrophs and many chemotrophic microorganisms reduce nitrate to ammonia and incorporate the ammonia in a process known as assimilatory nitrate reduction. A variety of bacteria (e.g., many cyanobacteria and the symbiotic bacterium Rhizobium) can assimilate atmospheric nitrogen (N₂) by reducing it to ammonium (NH₄). This is called nitrogen fixation.

Phosphorus is present in nucleic acids, phospholipids, nucleotides like ATP, several cofactors, some proteins, and other cell components. Almost all microorganisms use inorganic phosphate as their phosphorus source and incorporate it directly. Low phosphate levels can limit microbial growth in many aquatic environments. Some microbes, such as Escherichia coli, can use both organic and inorganic phosphate. Some organophosphates such as hexose 6-phosphates can be taken up directly by the cell. Other organophosphates are hydrolyzed in the periplasm by the enzyme alkaline phosphatase to produce inorganic phosphate, which then is transported across the plasma membrane.

Sulfur is needed for the synthesis of substances like the amino acids cysteine and methionine, and some carbohydrates, biotin, and thiamine. Most microorganisms use sulfate as a source of sulfur and reduce it by assimilatory sulfate reduction. A few microorganisms require a reduced form of sulfur such as cysteine.

 GROWTH FACTORS

Some microorganisms have the enzymes and biochemical pathways needed to synthesize all cell components using minerals and sources of energy, carbon, nitrogen, phosphorus, and sulfur. Some lack one or more of the enzymes needed to manufacture some indispensable constituents. So, they must obtain these constituents or their precursors from the environment. Organic compounds that are essential cell components or precursors of such components but cannot be synthesized by the organism are called growth factors. There are three major classes of growth factors:

(1) amino acids, (2) purines and pyrimidines, and (3) vitamins.

Amino acids are needed for protein synthesis; purines and pyrimidines for nucleic acid synthesis. Vitamins are small organic molecules that usually make up all or part of enzyme cofactors and are needed in only very small amounts to sustain growth.

 

Functions of Some Common Vitamins in Microorganisms
Vitamin	Functions	Microorganisms Requiring Vitamin
Biotin	Carboxylation (CO2 fixation) One-carbon metabolism	Leuconostoc mesenteroides Saccharomyces cerevisiae
Cyanocobalamin (B12)	Molecular rearrangements	Lactobacillus spp.
Folic acid	One-carbon metabolism	Enterococcus faecalis
Pyridoxine (B6)	Amino acid metabolism (e.g., transamination)	Lactobacillus spp.
Niacin (nicotinic acid)	Precursor of NAD and NADP	Haemophilus influenzae
Riboflavin (B2)	Precursor of FAD and FMN	Dictyostelium spp

 

Some microorganisms require many vitamins; for example, Enterococcus faecalis needs eight different vitamins for growth.

Other growth factors required are; heme (from hemoglobin or cytochromes) is required by Haemophilus influenzae, and some mycoplasmas need cholesterol.

Understanding the growth factor requirements of microbes has significant practical applications. Microbes with known, specific requirements and those that produce large quantities of a substance (e.g., vitamins) are useful.

Microbes with a specific growth factor requirement can be used in bioassays for the factor they need. A typical assay is a growth-response assay, which allows the amount of growth factor in a solution to be determined. The amount of growth in a culture is related to the amount of growth factor present. Ideally, the amount of growth is directly proportional to the amount of growth factor; if the growth factor concentration doubles the amount of microbial growth doubles. 

For example, Lactobacillus and Streptococcus can be used in microbiological assays of most vitamins and amino acids. Microbiological assays are specific, sensitive, and simple. They still are used in the assay of substances like vitamin B₁₂ and biotin, despite advances in chemical assay techniques.

Microorganisms which are able to synthesize large quantities of vitamins can be used to manufacture these compounds for human use. Several water-soluble and fat-soluble vitamins are produced partly or completely using industrial fermentations. Good examples of such vitamins and the microorganisms that synthesize them are

·       riboflavin (Clostridium, Candida, Ashbya, Eremothecium)

·       coenzyme A (Brevibacterium)

·       vitamin B12 (Streptomyces, Propionibacterium, Pseudomonas)

·       vitamin C (Gluconobacter, Erwinia, Corynebacterium)

·       carotene (Dunaliella)

·       vitamin D (Saccharomyces).

Research focuses on improving yields and finding microorganisms that can produce large quantities of other vitamins.

Nutritional Types of Microorganisms

Depending on how the need for carbon, energy, and electrons is fulfilled microorganisms can be classified. With respect to their preferred source of carbon, there are heterotrophs or autotrophs. There are only two sources of energy available to organisms: Phototrophs use light as their energy source; chemotrophs obtain energy from the oxidation of chemical compounds (either organic or inorganic). Microorganisms also have only two sources of electrons. There are Lithotrophs (i.e., “rock-eaters”) which use reduced inorganic substances as their electron source, and organotrophs which obtain electrons from reduced organic compounds.

Sources of Carbon, Energy, and Electrons
Carbon Sources
Autotrophs	CO2 sole or principal biosynthetic carbon source
Heterotrophs	Reduced, preformed, organic molecules from other organisms
Energy Sources
Phototrophs	Light
Chemotrophs	Oxidation of organic or inorganic compounds
Electron Sources
Lithotrophs		Reduced inorganic molecules
Organotrophs		Organic molecules

 

In spite of the great metabolic diversity seen in microorganisms, most can be placed in one of five nutritional classes based on their primary sources of carbon, energy, and electrons.

Major Nutritional Types of Microorganisms
Nutritional Type	Carbon Source	Energy Source	Electron Source	Representative Microorganisms
Photolithoautotrophy (photolithotrophic autotrophy)	CO2	Light	Inorganic e- donor	Purple and green sulfur bacteria, cyanobacteria
Photoorganoheterotrophy (photoorganotrophic heterotrophy)	Organic carbon but CO2 may also be used	Light	Organic e- donor	Purple nonsulfur bacteria, green nonsulfur bacteria
Chemolithoautotrophy (chemolithotrophic autotrophy)	CO2	Inorganic chemicals	Inorganic e- donor	Sulfur-oxidizing bacteria, hydrogen-oxidizing bacteria, methanogens, nitrifying bacteria, iron-oxidizing bacteria
Chemolithoheterotrophy or mixotrophy (chemolithotrophic heterotrophy)	Organic carbon, but CO2 may also be used	Inorganic chemicals	Inorganic e-donor	Some sulfur-oxidizing bacteria (e.g., Beggiatoa)
Chemoorganoheterotrophy (chemoorganotrophic heterotrophy)	Organic carbon	Organic chemicals often same as C source	Organic e- donor often same as C source	Most non photosynthetic microbes, including most pathogens, fungi, many protists, and many archaea

 

The majority of microorganisms are either photolithotrophic autotrophs or chemoorganotrophic heterotrophs.

Photolithotrophic autotrophs (often called photoautotrophs or photolithoautotrophs) use light energy and have CO₂ as their carbon source. Photosynthetic protists and cyanobacteria employ water as the electron donor and release oxygen. Other photolithoautotrophs, such as the purple and green sulfur bacteria, cannot oxidize water but extract electrons from inorganic donors like hydrogen, hydrogen sulfide, and elemental sulphur.

Chemoorganotrophic heterotrophs (often called heterotrophs or chemoheterotrophs, or chemoorganoheterotrophs) use organic compounds as sources of energy, hydrogen, electrons, and carbon. Usually, the same organic nutrient will satisfy all these requirements. All pathogenic microorganisms are chemoheterotrophs.

The other nutritional classes are very important ecologically though they have only fewer known microorganisms. Some photosynthetic bacteria (purple and green bacteria) use organic matter as their electron donor and carbon source. These photoorganotrophic heterotrophs (photoorganoheterotrophs) are common inhabitants of polluted lakes and streams. Some of these bacteria also can grow as photoautotrophs with molecular hydrogen as an electron donor.

Chemolithotrophic autotrophs (chemolithoautotrophs), oxidize reduced inorganic compounds such as iron, nitrogen, or sulfur molecules to derive both energy and electrons for biosynthesis. Carbon dioxide is the carbon source.

Chemolithoheterotrophs, also known as mixotrophs, use reduced inorganic molecules as their energy and electron source, but derive their carbon from organic sources. Chemolithotrophs contribute greatly to the chemical transformations of elements (e.g., the conversion of ammonia to nitrate or sulfur to sulfate) that continually occur in ecosystems.

A particular species usually belongs in only one of the nutritional classes, but some show great metabolic flexibility and alter their metabolic patterns in response to environmental changes. For example, many purple nonsulfur bacteria act as photoorganotrophic heterotrophs in the absence of oxygen but oxidize organic molecules and function chemoorganotrophically at normal oxygen levels. When oxygen is low, photosynthesis and chemoorganotrophic metabolism may function simultaneously. This sort of flexibility seems complex and confusing, but it gives these microbes a definite advantage if environmental conditions frequently change.

 Conclusion

     Microorganisms require about 10 elements in large quantities for the synthesis of macromolecules. Several other elements are needed in very small amounts and are parts of enzymes and cofactors.

     All microorganisms can be placed in one of a few nutritional categories on the basis of their requirements for carbon, energy, and electrons.