There are millions of habitats on earth, of both natural and human origin. In these settings, microorganisms are exposed to a tremendous variety of conditions that affect their survival. Environmental factors with the greatest impact on microorganisms are nutrient and energy sources, temperature, gas content, water, salt, pH, radiation, and other organisms. Microbes survive in their habitats through the process of gradual adjustment of anatomy and physiology, a process called adaptation. It is this adaptability that allows microbes to inhabit all parts of the biosphere. The process is also a major force behind the evolution of species, in that those species with greater adaptability will survive and pass on favorable characteristics to their offspring.
Microbial nutrition is a complex activity through which microbes acquire chemical compounds from the environment in order to sustain life. These compounds, called nutrients, are absorbed, assimilated, and subsequently used for cellular metabolism, growth, energy, and many other functions. With respect to their nutrition, microbes are not really so different from humans and other macroorganisms. Although bacteria living deep in a swamp on a diet of inorganic sulfur, or protozoa digesting wood in a termite’s intestine, may seem to show radical adaptations, even these organisms require a constant influx of certain specific substances from their habitat.
In general, all living things have an absolute need for the bioelements, traditionally listed as carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, iron, sodium, chlorine, magnesium, and a few other elements. Beyond these basic requirements, microbes show significant differences in the source, chemical form, and amount of the elements they use. Any substance, whether an element or compound, that an organism must get from a source outside its cells is called an essential nutrient. Once absorbed, these nutrients are processed and transformed into the chemicals of the cell.
Two categories of essential nutrients are macronutrients and micronutrients. Macronutrients are required in relatively large quantities and play principal roles in cell structure and metabolism. Examples of macronutrients are compounds such as sugars and amino acids that contain carbon, hydrogen, and oxygen. Micronutrients, or trace elements, such as manganese, zinc, and nickel, are present in much smaller amounts and are involved in enzyme function and maintenance of protein structure. What constitutes a micronutrient can vary from one microbe to another and often must be determined in the laboratory. This determination is made by deliberately omitting the substance in question from a growth medium to see if the microbe can grow in its absence.
Another way to categorize nutrients is according to their carbon content. Most organic nutrients are molecules that contain a basic framework of carbon and hydrogen. Natural organic molecules are nearly always the products of living things. They range from the simplest organic molecule, methane (CH4), to large polymers (carbohydrates, lipids, proteins, and nucleic acids).
In contrast, an inorganic nutrient is composed of an element or elements lacking some combination of both carbon and hydrogen. The natural reservoirs of many inorganic compounds are mineral deposits in the soil, bodies of water, and the atmosphere. Examples include metals and their salts (magnesium sulfate, ferric nitrate, sodium phosphate), gases (oxygen, carbon dioxide), and water.
Chemical Analysis of Cell Contents
To gain insight into a cell’s nutritional makeup, it can be useful to analyze its chemical composition. The following is a brief summary of what one would find in an Escherichia coli cell (figure 1), a fair representative of any cell on earth:
● Water is the greatest component, making up 70% of the cell
● Proteins make up about 15%
● Nucleic acids constitute 7% (DNA 1%, RNA 6%)
● Carbohydrates are roughly 3%
● Lipids make up 2%
Fig1. Microbial nutrition. The best way to understand the nutritional needs of a cell like Escherichia coli is to analyze the contents of a culture of those cells (SEM 7000×). Eric Erbe, Chris Pooley/ARS/USDA
Those doing the math realize that this adds up to 97%. The remaining 3% consists of countless micronutrients and minerals, from calcium to zinc. It should also be noted that all of the molecules listed above are composed of only six elements—sulfur, phosphorous, oxygen, nitrogen, carbon, and hydrogen—often remembered by the acronym SPONCH. Although a typical cell like E. coli contains about 5,000 different compounds, nothing happens without SPONCH.
Forms, Sources, and Functions of Essential Nutrients
The elements that comprise nutrients ultimately exist in an environmental inorganic reservoir of some type, such as carbon in atmospheric carbon dioxide. These reservoirs not only serve as a source of these elements but can be replenished by the activities of organisms. Thus, elements cycle in a pattern from an inorganic form in an environmental reservoir to an organic form in organisms. Organisms may, in turn, serve as a continuing source of that element for other organisms. Eventually the element is recycled to the inorganic form, usually by the actions of microorganisms. All in all, a tremendous variety of microorganisms are involved in processing the elements. From a broader perspective, the overall cycle of life on this planet depends upon the combined action of several interacting nutritional schemes, each performing a necessary step. In fact, as we shall see in a later chapter, the framework of microbial nutrition underlies the nutrient cycles of all life on earth.
The source of nutrients is extremely varied: Microbes such as photosynthetic bacteria obtain their nutrients entirely in inorganic form from the environment. Others require a combination of organic and inorganic nutrients. For example, parasites that invade and live on the human body derive all essential nutrients from host tissues, tissue fluids, secretions, and wastes.
Refer to table1 to see an overview of the major bioelements, compounds, their sources, and their importance to microorganisms.
Table1. Sources and Biological Functions of Essential Elements and Nutrients
Carbon-Based Nutritional Types
The element carbon is so key to the structure and metabolism of all life forms that the source of carbon defines two basic nutritional groups:
● A heterotroph is an organism that must obtain its carbon in an organic form. Because most organic carbon originates from organisms, heterotrophs are nutritionally dependent on other life forms. Among the common organic molecules that can satisfy this requirement are proteins, carbohydrates, lipids, and nucleic acids. In most cases, these nutrients provide several other elements as well. Some organic nutrients (such as monosaccharides and amino acids) already exist in a form that is simple enough for absorption, but many larger molecules must be digested by the cell before absorption. Not all heterotrophs can use the same organic carbon sources. Some are restricted to a few substrates, whereas others (certain Pseudomonas bacteria, for example) are so versatile that they can metabolize hundreds of different substrates.
● An autotroph is an organism that uses inorganic CO2 as its carbon source. Because autotrophs have the special capacity to convert CO2 into organic compounds, they are not nutritionally dependent on other living things. We later enlarge on the topic of nutritional types based on carbon and energy sources.
Growth Factors: Essential Organic Nutrients
Many fastidious bacteria are not as versatile as E. coli and lack the genetic and metabolic mechanisms to synthesize every organic compound they need for survival. An essential nutrient such as an amino acid, nitrogenous base, or vitamin that cannot be synthesized by an organism and must be provided as a nutrient is considered a growth factor. For example, all cells require 20 different amino acids for proper assembly of proteins, but many cells cannot synthesize all of them. Those that must be obtained from food are called essential amino acids. A notable example of the need for growth factors occurs in Haemophilus influenzae, a bacterium that causes meningitis and respiratory infections in hu mans. It can grow only when hemin (factor X), NAD+ (factor V), thiamine and pantothenic acid (vitamins), uracil, and cysteine are provided by another organism or a growth medium.
