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Learning Goals
By the end of this reading you should be able to:
- Describe the process of binary fission in bacteria
- Define the nutritional types of microbes in terms of their sources of carbon, electrons, and energy
- Explain horizontal gene transfer and the three mechanisms by which it occurs in bacteria
- Describe how antibiotics function and explain how some bacteria manage to disarm them
Introduction
The diverse environments and ecosystems on Earth have a wide range of conditions in terms of temperature, available nutrients, acidity, salinity, and energy sources. Prokaryotes are very well equipped to make their living out of a vast array of nutrients and conditions. To live, prokaryotes need a source of energy, a source of carbon, and some additional nutrients. When conditions are favorable prokaryotic populations can grow rapidly because they are asexual. They can also maintain genetic diversity through both mutation and the process of horizontal gene transfer. As a result, prokaryotes are one of the most diverse groups of organisms on Earth.
Microbial Growth
Provided with the right conditions (food, correct temperature, etc) microbes can grow very quickly. Depending on the situation, this could be a good thing for humans (yeast growing in the wort to make beer) or a bad thing (bacteria growing in your throat causing strep throat). It’s important to have knowledge of their growth, so we can predict or control their growth under particular conditions. While growth for multicellular organisms is typically measured in terms of the increase in the size of a single organism, microbial growth is measured by the increase in population, either by measuring the increase in cell number or the increase in overall mass.
Binary fission begins with cell elongation, which requires careful enlargement of the cell membrane and the cell wall, in addition to an increase in cell volume. Once the cell reaches a certain size it will start to replicate its DNA, in preparation for having two copies of its chromosome, one for each newly formed cell.
The starting point (origin) of replication, is close to the binding site of the chromosome to the plasma membrane. Replication of the DNA is bidirectional, moving away from the origin on both strands of the loop simultaneously (Step 1 in Fig. 2). As the new double strands are formed, each origin point moves away from the cell wall attachment toward the opposite ends of the cell. As the cell elongates, the growing membrane aids in the transport of the chromosomes (Step 2 in Fig. 2). After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins. The formation of a ring composed of repeating units of a protein called FtsZ directs the partition between the nucleoids (Step 3 in Fig. 2). The formation of the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell wall materials to the site. A septum (wall) is formed between the nucleoids, extending gradually from the periphery toward the center of the cell (Step 4 in Fig. 2). When the new cell walls are in place, the daughter cells separate (Step 5 in Fig. 2). The protein FtsZ is essential for the formation of a septum, which initially manifests as a ring in the middle of the elongated cell. After the nucleoids are segregated to each end of the elongated cell, septum formation is completed, dividing the elongated cell into two equally sized daughter cells. The entire process (the cell cycle) can take as little as 20 minutes for an active culture of E. coli bacteria that is not resource-limited.
Review Question:
Gathering Energy
All microbes have a need for three things: carbon, energy, and electrons. There are specific terms associated with the source of each of these items, to help define organisms. Let us focus on carbon first. All organisms are carbon-based with macromolecules – proteins, carbohydrates, lipids, nucleic acid – having a fundamental core of carbon. On one hand, organisms can use reduced, preformed organic substances as a carbon source. These are the heterotrophs or “other eaters.” Alternatively, they can rely on carbon dioxide (CO2) as a carbon source, reducing or “fixing” this inorganic form of carbon into an organic molecule. These are the autotrophs or “self-feeders.”
For energy, there are two possibilities as well: light energy or chemical energy. Light energy comes from the sun, while chemical energy can come from either organic or inorganic chemicals. Those organisms that use light energy are called phototrophs (“light eaters”), while those that use chemical energy are called chemotrophs (“chemical eaters”). Chemical energy can come from inorganic sources or organic sources. An organism that uses inorganic sources is known as a lithotroph (“rock eater”), while an organism that uses organic sources is called an organotroph (“organic eater”). These terms can all be combined, to derive a single term that gives you an idea of what an organism is using to meet its basic needs for energy, electrons, and carbon.
Review Question:
HGT, evolution, and antibiotic resistance
Let’s talk about sex. Bacterial sex. That is going to be difficult since bacteria do not have sex. Which presents a real problem for bacteria (and archaea, too) – how do they get the genetic variability that they need? They might need a new gene to break down an unusual nutrient source or degrade an antibiotic threatening to destroy them – acquiring the gene could mean the difference between life and death. But where would these genes come from? How would the bacteria get a hold of them? We are going to explore the processes that bacteria use to acquire new genes, the mechanisms known as Horizontal Gene Transfer (HGT).
Horizontal gene transfer (HGT) is the introduction of genetic material from one species to another species by mechanisms other than the vertical transmission from the parent(s) to offspring. These transfers allow even distantly related species to share genes, influencing their phenotypes. Classically, this type of transfer has been thought to occur by three different mechanisms:
- Transformation: naked DNA is taken up by a bacteria
- Transduction: genes are transferred using a virus
- Conjugation: the use of a hollow tube called a sex pilus to transfer genes between organisms
The link between HGT and antibiotic resistance is an active area of interest for many scientists. This video brings the mechanism of HGT in bacteria together with the methods that some bacteria use to defend themselves against antibiotics.
Rise of the Superbug – Antibiotic-Resistant Bacteria: Dr. Karl Klose at TEDxSanAntonio
Review Questions:
According to the scientist in the video, how do antibiotics work to kill bacteria?
The mechanism of HGT has been shown to be quite common in the prokaryotic domains of Bacteria and Archaea, significantly changing the way their evolution is viewed. The majority of evolutionary models, such as in the Endosymbiont Theory, propose that eukaryotes descended from multiple prokaryotes, which makes HGT all the more important to understanding the phylogenetic relationships of all extant and extinct species.
Summary
Prokaryotic organisms are intimately connected with every aspect of their environments. Their small size and rapid reproduction by binary fission (or other asexual means) allow them to take quick advantage of changing resources in the environment. It is in the prokaryotes that we can begin to separate organisms based on where they get their energy (light or chemicals), where they obtained their electrons (organic or inorganic sources), and where they obtain the carbon building blocks for growth (organic molecules or carbon dioxide). When the ability to take up genes from the environment and exchange genetic material between different bacteria is added to the mix the potential for diversity within the prokaryotic domains is expansive.
End of Section Review Questions:
2) Reproduction in prokaryotes is _______ and usually takes place by ________.
3) The overprescription of antibiotics has been proposed to have lead to an increase in antibiotic resistance in bacteria. How might this increased resistance have evolved?
References
Microbiology. Linda Bruslind http://library.open.oregonstate.edu/microbiology/
Image Attribution:
Figure 1. Image courtesy of CDC/ Bette Jensen. Public Health Image Library (Public Domain).
Figure 2. Image courtesy of CNX OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons
Figure 3. Image created and provided by D. Jennings
1/A, 2/B, 3/C
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