BACTERIAL METABOLISM: TYPES AND THEIR CHARACTERISTICS - BIOLOGY - 2025

The bacterial metabolism includes a series of chemical reactions necessary for life of these organisms. The metabolism is divided into degradation or catabolic reactions, and synthesis or anabolic reactions.

These organisms exhibit admirable flexibility in terms of their biochemical pathways, being able to use various sources of carbon and energy. The type of metabolism determines the ecological role of each microorganism.

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Like eukaryotic lineages, bacteria are mainly made up of water (around 80%) and the rest in dry weight, made up of proteins, nucleic acids, polysaccharides, lipids, peptidoglycan and other structures. Bacterial metabolism works to achieve the synthesis of these compounds, using the energy from catabolism.

Bacterial metabolism does not differ much from the chemical reactions present in other more complex groups of organisms. For example, there are common metabolic pathways in almost all living things, such as the glucose breakdown or glycolysis pathway.

Accurate knowledge of the nutritional conditions that bacteria require to grow is essential for creating culture media.

Types of metabolism and their characteristics

The metabolism of bacteria is extraordinarily diverse. These unicellular organisms have a variety of metabolic "lifestyles" that allow them to live in areas with or without oxygen and also vary between the source of carbon and energy they use.

This biochemical plasticity has allowed them to colonize a series of varied habitats and play diverse roles in the ecosystems they inhabit. We will describe two classifications of metabolism, the first is related to oxygen utilization and the second to the four nutritional categories.

Use of oxygen: anaerobic or aerobic

Metabolism can be classified as aerobic or anaerobic. For prokaryotes that are fully anaerobic (or obligate anaerobes), oxygen is analogous to a poison. Therefore, they must live in environments completely free of it.

Within the category of aerotolerant anaerobes, bacteria are able to tolerate oxygen environments, but are not capable of cellular respiration - oxygen is not the final electron acceptor.

Certain species may or may not use oxygen and are "facultative", since they are capable of alternating the two metabolisms. Generally, the decision is related to environmental conditions.

At the other extreme, we have the group of obligate aerobes. As the name implies, these organisms cannot develop in the absence of oxygen, as it is essential for cellular respiration.

Nutrients: essentials and trace elements

In metabolic reactions, bacteria take nutrients from their environment to extract the energy necessary for their development and maintenance. A nutrient is a substance that must be incorporated to guarantee its survival through the supply of energy.

The energy from the absorbed nutrients is used for the synthesis of the basic components of the prokaryotic cell.

Nutrients can be classified as essential or basic, which include carbon sources, nitrogen molecules, and phosphorous. Other nutrients include different ions, such as calcium, potassium and magnesium.

Trace elements are only required in trace or trace amounts. Among them is iron, copper, cobalt, among others.

Certain bacteria are not capable of synthesizing a specific amino acid or certain vitamin. These elements are called growth factors. Logically, growth factors are widely variable and largely depend on the type of organism.

Nutritional categories

Bacteria can be classified into nutritional categories taking into account the carbon source they use and where they get their energy from.

Carbon can be taken from organic or inorganic sources. The terms autotrophs or lithotrophs are used, while the other group is called heterotrophs or organotrophs.

Autotrophs can use carbon dioxide as a carbon source, and heterotrophs require organic carbon for metabolism.

On the other hand, there is a second classification related to energy intake. If the organism is capable of using the energy from the sun, we classify it in the category of phototroph. In contrast, if energy is extracted from chemical reactions, they are chemotrophic organisms.

If we combine these two classifications we will obtain the four main nutritional categories of bacteria (it also applies to other organisms): photoautotrophs, photoheterotrophs, chemoautotrophs and chemoheterotrophs. Below we will describe each of the bacterial metabolic capacities:

Photoautotrophs

These organisms carry out photosynthesis, where light is the source of energy and carbon dioxide is the source of carbon.

Like plants, this bacterial group has the pigment chlorophyll a, which allows it to produce oxygen through a flow of electrons. There is also the pigment bacteriochlorophyll, which does not release oxygen in the photosynthetic process.

Photoheterotrophs

They can use sunlight as their source of energy, but they don't turn to carbon dioxide. Instead, they use alcohols, fatty acids, organic acids, and carbohydrates. The most prominent examples are green non-sulfur and purple non-sulfur bacteria.

Chemoautotrophs

Also called chemoautotrophs. They obtain their energy through the oxidation of inorganic substances with which they fix carbon dioxide. They are common in hydroterminal respirators in the deep ocean.

Chemoheterotrophs

In the latter case, the source of carbon and energy is usually the same element, for example, glucose.

Applications

Knowledge of bacterial metabolism has made an immense contribution to the area of ​​clinical microbiology. The design of optimal culture media designed for the growth of some pathogen of interest is based on its metabolism.

In addition, there are dozens of biochemical tests that lead to the identification of some unknown bacterial organism. These protocols allow an extremely reliable taxonomic framing to be established.

For example, the catabolic profile of a bacterial culture can be recognized by applying the Hugh-Leifson oxidation / fermentation test.

This methodology includes growth in a semi-solid medium with glucose and a pH indicator. Thus, oxidative bacteria degrade glucose, a reaction that is observed thanks to the color change in the indicator.

Similarly, it is possible to establish which pathways the bacteria of interest use by testing their growth on different substrates. Some of these tests are: the assessment of the fermentation pathway of glucose, detection of catalases, reaction of cytochrome oxidases, among others.

References

1. Negroni, M. (2009). Stomatological microbiology. Panamerican Medical Ed.
2. Prats, G. (2006). Clinical microbiology. Panamerican Medical Ed.
3. Rodríguez, J. Á. G., Picazo, JJ, & de la Garza, JJP (1999). Compendium of Medical Microbiology. Elsevier Spain.
4. Sadava, D., & Purves, WH (2009). Life: The Science of Biology. Panamerican Medical Ed.
5. Tortora, GJ, Funke, BR, & Case, CL (2007). Introduction to microbiology. Panamerican Medical Ed.