Have you heard of Mycobacterium tuberculosis? M. tuberculosis is a pathogenic bacterium that causes tuberculosis. It is a curved, rod-shaped bacteria containing a waxy cell acid made up of fatty acids. According to an analysis of the M. tuberculosis H37Rv genome, scientists have found that it has 786 genes for metabolism.
Interested in learning more about bacterial metabolism? Keep reading!
Growth, nutrition, and metabolism of bacteria
Bacteria growth is the increase in the number of bacterial cells through binary fission.
To show bacterial population growth, we can use a growth curve. This curve has four different phases: the lag phase, the log phase, the stationary phase, and the decline phase.
- The lag phase is where the cells adjust to the environment. In this phase, no cell division is happening.
- The cells reproduce through binary fission under optimal nutritional conditions in the log phase (or exponential phase).
- In the stationary phase, the number of cells produced by cell division equals the number of cells dying.
- In the last stage, the decline phase (or death phase), the microorganisms die quickly due to nutrient depletion and high levels of toxic end products.
Microbes can also be defined by their nutritional needs. But, one thing is certain: all bacteria require a source of carbon (C), nitrogen (N), energy to do work, electrons to perform biochemical reactions, and essential growth factors such as vitamins and minerals to be able to grow.
To successfully cultivate bacteria under laboratory conditions, it is essential to use a culture media that contains the nutritional requirements for the growth of that specific bacteria!
Let's look at some types of media that are commonly used by microbiologists.
- Nutrient broth is a type of complex media prepared by combining peptone and beef extract. Peptone serves as a nitrogen source, whereas beef extract is the source of organic carbon, vitamins, and inorganic salts.
- Yeast extract broth contains peptone, beef extract, and yeast extract. Yeast extract adds vitamin B and additional organic carbon and nitrogen compounds to the broth.
- Mannitol salt agar is a type of differential/selective media that inhibits the growth of most bacteria besides staphylococci. Staphylococci can grow on this media because the media has the carbohydrate mannitol present, which some staphylococci can ferment.
- Crystal violet agar is a selective media that selects for the growth of gram-negative bacteria. Another media that only allows for the growth of gram-negative bacteria is MacConkey agar. However, the MacConkey agar also has a differential function and differs from gram-negative bacteria based on their ability to ferment lactose.
- Phenylethyl alcohol agar is also a selective media, but it selects for the growth of gram-positive bacteria instead.
- Blood agar is a type of enriched media used to cultivate fastidious organisms such as Streptococcus spp.
Bacterial metabolism definition
Metabolism is referred to as the sum of all chemical reactions (anabolic + catabolic) happening in an organism. These series of biochemical reactions are essential to sustain life. Thus, bacterial metabolism is the sum of all chemical reactions happening in bacteria.
Catabolism is the breaking down of complex macromolecules into their basic components.
Anabolism is the formation of new products.
We can classify bacterial metabolism based on what a bacterium uses as a source of energy:
- If the bacteria use light as an energy source, they are called phototrophs.
- If the bacteria use the oxidation of organic/inorganic compounds as an energy source, we call them chemotrophs.
Now, when it comes to a carbon source, bacteria can be an autotroph or a heterotroph. Autotrophs use CO2 as their main carbon source to create their own organic matter, whereas heterotrophs use organic molecules from other organisms as a carbon source.
What about electron sources? Lithotrophs use inorganic molecules as an electron source. Organotrophs, on the other hand, use organic molecules as an electron source. Photoorganoheterophic organisms are organoheterotrophs that use organic carbon as a carbon source, light as an energy source, and inorganic molecules as an electron source.
Types of bacterial metabolism
Bacteria have different types of metabolism, such as aerobic cellular respiration, fermentation and anaerobic respiration, depending on the availability and their resistance to oxygen.
Bacterial energy metabolism
Metabolism is any chemical reaction that happens in a cell, so bacterial metabolism is the sum of chemical reactions within a bacteria cell. These chemical reactions can be divided into the reactions that produce energy and the ones that are used for other purposes, such as creating organic molecules or increasing the availability of certain elements like nitrogen. In this section we will focus on the first category.
Aerobic cellular respiration
As its name suggests, aerobic cellular respiration is defined as the process of breaking down glucose to make energy in the presence of oxygen. Glucose is a monosaccharide.
Fig. 2. Closed structure (Hawthorn projection) of alpha and beta glucose. The difference between the isomers is the position of the hydroxyl group attached to carbon 1. When several alpha-glucose molecules are joined, starch is generated. When several beta-glucose molecules are joined, cellulose is generated. Humans can digest starch, but not cellulose because the enzymes needed for each reaction are different.
In prokaryotic organisms, most stages of cellular respiration happen in the cytoplasm. The overall chemical formula for aerobic cellular respiration is shown below.
$$ C_{6}H_{12}O_{6} (glucose)+6O_{2} \longrightarrow 6CO_{2} + 6H_{2}O +energy (ATP + heat) $$
Aerobic cellular respiration in prokaryotes can be divided into four phases:
Glycolysis
Pyruvate oxidation
Citric acid cycle (Krebs cycle)
Electron transport chain and chemiosmosis (happens in the plasma membrane)
During aerobic cellular respiration, there are two ways in which ATP can be made: substrate-level phosphorylation and oxidative phosphorylation. In substrate-level phosphorylation, an enzyme and a substrate are used to transfer a phosphate group to ADP, creating small amounts of ATP in the stages of glycolysis and the citric acid cycle.
Oxidative phosphorylation, on the other hand, produces high amounts of ATP in the stages of the electron transport chain and chemiosmosis. Oxidative phosphorylation makes use of energy from redox reactions in the electron transport chain to phosphorylate ADP.
Glycolysis
Glycolysis is a type of carbohydrate metabolism that occurs in the cytoplasm of cells. It is the first step of cellular respiration. Glycolysis consists of many enzymatic reactions that break down glucose into two pyruvate molecules. Energy is also produced in the form of ATP, as glucose gets processed. The process of glycolysis does not require oxygen (O)!
$$ \text{Glucose} + 2\text{ }NAD^{+}+\text{ }2\text{ }ADP +\text{ }2\text{ }Pi \longrightarrow 2 \text{ pyruvate} + \text{2 }NADH\text{ }+\text{ }2\text{ }ATP $$
ATP is adenosine triphosphate, and NADH is nicotinamide adenine dinucleotide.
Pyruvate oxidation
After glycolysis, the pyruvate molecules are produced to enter the mitochondria and react with an enzyme called pyruvate dehydrogenase. This enzyme takes out carbon and two oxygen molecules from the pyruvate and adds coenzyme A, producing acetyl-CoA, NADH and CO2. Acetyl-CoA can also come from fatty acids and amino acids. This phase is called pyruvate oxidation.
Citric acid cycle (Krebs Cycle)
After pyruvate oxidation, we have the citric acid cycle. The citric acid cycle happens in the cytoplasm for prokaryotes, and in the mitochondria matrix for eukaryotes. Acetyl-CoA is the first intermediate of the citric acid cycle. After a molecule of glucose undergoes glycolysis, pyruvate oxidation, and the citric acid cycle, we are left with 10 molecules of NADH, 4 molecules of ATP, 2 molecules of FADH2 and 6 molecules of CO2.
Some textbooks refer to the Krebs cycle as the TCA cycle!
Electron transport chain and chemiosmosis
The fourth stage of aerobic cellular respiration consists of the electron transport chain and chemiosmosis. The electron transport chain consists of proteins embedded in the plasma membrane of prokaryotic cells.
In this stage, the prokaryotic electron transport chain gets energy from the electrons donated by NADH and FADH2. When NADH donates an electron, it becomes NAD+. Similarly, when FADH2 donates an electron, it turns into FAD. Then, the electron transport chain makes use of this energy caused by the movement of electrons to pump protons (H+) from the mitochondrial matrix, across the inner mitochondrial membrane, and into the intermembrane space of the mitochondria.
The electrons will keep through the electron transport chain until they reach the final electron acceptor, which is oxygen gas (O2). This oxygen molecule reacts with the H+ to form water (H2O).
After the electron transport chain builds the proton (H+) gradient, chemiosmosis is used to capture their energy. Chemiosmosis is referred to as the diffusion of the H+ ions across a membrane from an area of high concentration to an area of low concentration.
Chemiosmosis uses an enzyme called ATP synthase, and its role is to synthesize ATP!
Anaerobic cellular respiration
Bacteria that have the ability to survive and make ATP without needing oxygen use anaerobic respiration. In anaerobic cellular respiration, glycolysis happens normally. Then, the pyruvate molecules produced by glycolysis can undergo fermentation.
Lactic acid fermentation is a pathway that uses NADH to reduce pyruvate into lactic acid and NAD+.
Bacteria that are capable of doing this are known as lactic acid-producing bacteria and include bacteria such as Streptococcus faecalis, S. pyogenes, and some Lactococcus spp.
In alcohol fermentation, pyruvate gets reduced by NADH to form ethanol and NAD+.
Nitrogen metabolism
Some bacteria are capable of nitrogen fixation. These nitrogen-fixing bacteria are vital to the nitrogen cycle because they are responsible for converting atmospheric nitrogen (N2) into ammonia (NH3).
Examples of nitrogen-fixing bacteria include pseudomonas, nocardia, Clostridium pasteurianum, and methanobacterium.
Bacterial metabolic pathways
Bacterial metabolic pathways consist of a series of enzymatic reactions that cause the alteration of a substrate multiple times before arriving at the final product. Bacteria can have many metabolic pathways, such as the ones we saw above, and many others, such as photosynthesis, lipid metabolism, amino acid metabolism, and nucleotide metabolism.
To figure out the metabolic pathways present in bacteria, scientists use genome annotation. Some specific pathways found in this genome include glycolysis, citric acid cycle, fatty acid metabolism, oxidative phosphorylation, photosynthesis, purine metabolism, methane metabolism, nitrogen metabolism, sulfur metabolism, and even caffeine metabolism!
Iron-reducing bacteria metabolism
Lastly, let's talk about the metabolism of iron-reducing bacteria. These bacteria convert ferric ions (Fe3+) into ferrous ions (Fe2+).
Examples include Geobacter metallireducens, Ferribacterium limneticum and Pseudomonas.
Bacterial metabolism can be a little overwhelming. But, with time and patience, you will be able to tackle it!
Bacterial Metabolism - Key takeaways
- All bacteria require a source of carbon, nitrogen, energy to do work, electrons to perform biochemical reactions, and essential growth factors such as vitamins and minerals to be able to grow.
- Metabolism is defined as the sum of all chemical reactions (anabolism + catabolism) happening in an organism. These series of biochemical reactions are essential to sustain life.
References
- A. Harvey, R., Nau Cornelissen. Microbiology. Lippincott Williams & Wilkins. (2012)
- Cappuccino, J. G., & Welsh, C. Microbiology: a laboratory manual. Pearson Education South Asia Pte Ltd. (2019)
- Madigan, M. T., Bender, K. S. Brock biology of microorganisms. Pearson. (2021)
- Byung Hong Kim, & Gadd, G. M. Bacterial physiology and metabolism. Cambridge University Press. (2008)
- Moat, A. G., Foster, J. W. Microbial Physiology. John Wiley And Sons. (2004)
- PATRIC. Mycobacterium tuberculosis H37Rv Genome Overview (n.d)
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