Prokaryote’s Bacteria Structure and Cell Capsule Function

Introduction

A prokaryote is any organism whose genetic information is not enclosed within a membrane-bound nucleus. Prokaryotes may have one or many cells. They are genetically simpler than eukaryotes (usually have a membrane-bound nucleus) and have no membrane-bound organelles. They reproduce through cell division and consist of a single chromosome in a circular DNA molecule.

Contrary to common belief, bacteria are not just single cells. They grow in chains or clusters consisting of thousands of genetically identical cells called a colony composed of bacterial cells enclosed within a cell capsule.

These unicellular organisms can be found in almost every place on earth, from your pet turtle to the depths of the ocean floor and from boiling springs to frozen glaciers. They are essential to plant and animal health and exist in an incredible diversity of shapes, sizes, and colors.

Bacteria are the only organisms on earth that can live in extreme environments like the bottom of a lake or at deep-sea vents under high temperature and pressure. The term extremophile refers to bacteria that live in such harsh environments.

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The Structure of Prokaryotic Cells

Figure: Prokaryote bacteria structure

The structure of a bacterium is different from that of eukaryotic cells. The bacterial cell wall is peptidoglycan, a complex polymer of sugars and amino acids that forms a callous outside layer.

The cell is surrounded by a membrane made of phospholipids (mostly phosphatidylcholine) mixed with proteins. The cell itself is filled with cytoplasm, containing thousands of different kinds of molecules.

The bacterium cell consists of prokaryotic ribosomes, along with their associated proteins and RNA polymerase. They have no true nucleus, but the genetic material lies in the nucleoid.

The DNA molecule is usually a circular molecule in bacteria. The bacterial chromosome is typically a single loop of double-stranded DNA and is relatively small.

The Bacterial Cell Wall

Figure: Gram-positive vs Gram-negative bacteria cell walls

The cell wall is mainly composed of peptidoglycan and acts as the primary defense against osmotic lysis. The peptidoglycan, which forms the basic structure of the bacterial cell wall, comprises long cross-linked chains of sugar molecules and amino acids.

The cross-linking between the sugar molecules links them into a rigid layer, which protects the cell from bursting when exposed to high salt concentrations.

In addition, the cell wall gives bacteria another form of protection against osmotic lysis. It contains anhydrous sugars that are not very active in the water. These sugars are called polyols. Polyols do not have free hydroxyl groups to react with osmotic agents like salts, and therefore, osmotic lysis is prevented.

The Plasma Membrane

Figure: Prokaryote plasma membrane

The bacterial plasma membranes are made of phospholipids with some proteins embedded on the outer surface. They serve to enclose the cell, select which molecules can pass through the membrane, and maintain the internal pH of the cell.

Phospholipids are amphiphilic molecules that contain both a nonpolar region (repelling water) and a polar region (soluble in water). The nonpolar tails form the hydrophobic core of the bilayer. The polar heads form the outer surface of the plasma membrane.

In bacteria, the phospholipid on the inner surface of the cell membrane has a basic charge (positive). The phospholipids on the outer surface have an acidic charge (negative). The negative charges on the outside repel each other and form a surface membrane with a negative charge. Bacteria cell membranes are not permeable to ions.

The Nucleoid

Figure: The location of prokaryote cell nucleoids

Because the genetic information of a bacterium is so small, it is not enough to fill the entire volume of the cell. The genetic material of bacteria, including most plasmids and conjugative elements, consists of a single closed circular DNA molecule.

However, the genetic material of some bacteria is not contained within the cell’s nucleoid in a single, continuous, circular molecule. For instance, some streptococci have linear chromosomes covalently bound at one or both ends to form a closed circle.

The linear genetic molecules are stored in the cytoplasm and are often organized into multiple, smaller rings with other DNA molecules. Bacterial plasmids and conjugative elements are also stored in this way.

Plasmids and Chromosomes in Prokaryotes

Figure: Plasmid and chromosome

Prokaryotic DNA is typically a single, circular molecule of double-stranded chromosomal DNA. In bacteria, the genetic material is usually a single ring chromosome, while in Archaea, it is a single linear molecule.

Many plasmids and conjugative elements are also single, circular molecules of double-stranded DNA. However, the plasmid or mobile element is often not part of the bacterial chromosome in these cases.

Plasmids are small, extrachromosomal DNA molecules (usually carrying genes that provide antibiotic resistance) that can replicate independently of the bacterial chromosome and move from cell to cell.

As a general rule, it is the prokaryotic chromosome that is replicated before cellular division. At the end of the cell division cycle, when DNA synthesis in prokaryotes is complete, two copies of the genetic material are present in each daughter cell.

One copy is being copied to form the chromosome of the new cells, while the second copy remains in the old cell as a plasmid.

Prokaryotic Cell Appendages

Figure: Bacterial appendages

Bacteria have a wide variety of appendages called pili that are involved with many functions.

Pili

Pili are filamentous proteinaceous appendages on the surface of certain bacteria. A pilus is a hair-like projection from the surface membrane of Gram-negative bacteria. These appendages can bind other cells and induce the formation of conjugation tubes that transfer DNA between bacteria. Bacterial pili are composed of a few different types of protein subunits, including pilin.

Pilus assembly occurs in two stages: initiation and elongation. Initiation takes place on the donor cell, and elongation occurs on the recipient cell. The pilus is assembled from several different polypeptides in a multi-step biochemical pathway with many protein-protein interactions. Once formed, the pilus is extruded as a long filament through an apparatus called the sortase.

The pilin subunits are encoded by several adjacent gene clusters that share a conserved DNA sequence motif at their 5′ end, which is necessary for initiation.

Fimbriae

Bacteria cells have many other surface appendages called fimbriae that also adhere to cells or host tissues. Fimbriae are also proteinaceous filaments that are attached to the membrane via a pilus assembly apparatus.

Gram-positive bacteria have pili and fimbriae, whereas Gram-negative bacteria have only pili. The fimbriae are so thin that they are not visible under the light microscope. They function to either attach bacteria to a specific host or to allow them to move along a substrate.

Pili in Prokaryotes: Conjugation and Transduction

One of the most notable functions of pili is allowing bacteria to transfer DNA between cells. This process occurs in three steps: attachment, exchange of DNA by diffusion, and final separation of the bacteria. This process is called conjugation and often occurs in Gram-negative bacteria. The recipient cell has one or more pili that extend outward from the cell.

The donor bacterium has pili with a pilus tip near the recipient’s pili. The pili are covered with a sticky material, which allows for attachment between the two cells. The donor then injects its DNA into the recipient cell. From here, one of two processes occurs.

If the donor and recipient cells have compatible mating types or incompatibility factors, the DNA will be transferred to the recipient and incorporated into its chromosome. If these factors are not compatible, an exchange of DNA takes place between the recipient and the donor cells.

This process is called Transformation or Conjugation and occurs in both Gram-positive and Gram-negative bacteria. The recipient cell then separates from the donor cell, taking any new DNA exchanged into its chromosome.

Transduction is a similar process that takes place between prokaryotic and eukaryotic cells. The process begins with the injection of viral DNA into a bacteria cell by a phage virus. The viral DNA is then incorporated into the bacterial chromosome, where it can travel to another cell as part of a new phage virus.

Purposes of Pili and Fimbriae

  • Pili and fimbriae help bacteria adhere to specific locations, such as specific tissues in the human body or medical devices.
  • Pili are also crucial in allowing bacteria to move along a substrate, such as a surface or inanimate object. This movement is similar to the action of an amoeba, in which individual cells migrate and then join to form a multicellular organism.
  • Another function of fimbriae is the attachment of bacteria to other entities, such as plants or rocks.

Bacteria Classification

Figure: Gram-positive vs Gram-negative bacteria cell envelopes

Bacteria can be classified into gram-positive and gram-negative bacteria.

Gram-Positive Bacteria

Gram-positive bacteria such as staphylococci and streptococci have a thick, complex cell wall. This wall is difficult for dyes to penetrate, so the bacteria appear deep purple when stained. Their name derives from their gram staining reaction to Gram staining. It’s a long-standing laboratory procedure for the differentiation of bacterial species.

Gram-positive bacteria have a thin peptidoglycan layer but contain crystal poly-D-glucose, which makes them appear to be stained a dark blue under a microscope.

A membrane also surrounds the cytoplasm and nucleus of the gram-positive bacteria. Gram-positive bacteria can be either rods or spheres. Rhizobium, for example, is rod-shaped. Group B streptococci are spherical, with a rough wall.

Cell Structure

Figure: Gram-positive bacteria structure

Gram-positive bacteria have large amounts of peptidoglycan in their cell walls. They also have thick, complex walls surrounded by a membrane. Their cell structure has different parts to it.

Cell Envelope of Gram-positive Bacteria

The cell envelope is the sum of all the components that surround the cell wall. It protects the cells from their surroundings and gives shape to them.

Gram-positive bacteria have a very thick cell wall. It is made up of peptidoglycan and teichoic acid. The cell envelope is the cell wall and the membrane on top of the cell wall that holds everything in place.

The cell envelope contains the cytoplasm, which is a watery fluid surrounded by the membrane. The cytoplasm contains all of the things that carry out the cell’s functions, for example, enzymes and organelles

Gram-Negative Bacteria

Gram-negative bacteria have only a thin peptidoglycan layer and do not contain poly-D-glucose. Therefore, they are stained a dark blue by the counterstain and so appear ‘negative’ under the microscope.

The cell structure

Figure: Location of porins in Gram-negative bacteria

The structure of gram-negative bacteria is quite different from that of gram-positive. They have a thin cell wall, few to no capsules, no cytoplasmic membrane, and the cell wall is mainly made up of lipopolysaccharide (LPS).

These bacteria’s outer membrane contains porins. The Porins form small pores in the outer membrane. These allow low molecular weight molecules to enter and exit, preventing the bacteria from losing too much water and becoming dehydrated.

Protection against dehydration is vital in environments that are salty or have low levels of dissolved oxygen. The bacteria must take in water to survive and take in oxygen from the environment.

The outer membrane of gram-negative bacteria also contains an enzyme called colicin V, which can kill or prevent the growth of other bacteria.

Cell Envelope and Membranes

The cell envelope is a membrane-bound structure that surrounds the cytoplasm of a cell and separates its contents from the external environment. The envelope acts as a selective barrier to large or harmful molecules that might threaten the cell.

A typical gram-negative bacterial envelope comprises three layers: an outer membrane, a periplasmic space, and an inner membrane.

The inner membrane separates the cytoplasm of the cell from its nucleus. The periplasm is the area between the inner and outer membranes.

The bacterial envelope contains porins to allow small molecules to pass through the membrane, such as ions, sugars, and amino acids. The space between the inner and outer membranes is called the periplasmic space and may contain enzymes involved in cell metabolism.

This membrane is necessary to prevent the cytoplasm of the cell from being too salty. When a bacterial cell is placed in a hypotonic solution (an environment with less salt than the cell), water will rush into the cell, diluting the salts inside the cell and causing it to swell.

When the inner membrane is broken, water will rush into the cell, causing lysis. For this reason, the enzymes that protect the cell from harmful materials are all in place before lysis occurs.

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The Cell Capsule of Bacteria

Figure: Position of the slimy capsule

The cell capsule is the outside layer of the cell wall. It covers all exterior areas and protects them from damage by protecting against toxins, water loss, and so on.

The capsule also helps the cell to identify itself from other types of cells in the body. This is necessary because it would attack itself when the immune system is under attack if it did not. Or it could be like a double negative and inactivate its defenses and let itself be attacked by harmful foreign bodies.

Capsules vary in size, shape, and chemical composition. Capsules make it easier for the immune system to know the difference between cells and foreign things. Some of the foreign bodies include viruses and other harmful bacteria.

They are large enough to be seen in a light microscope but not in a regular microscope. Capsules are usually made up of proteins, and they do not contain peptidoglycan or any other wall building blocks.

A capsule is also not part of the cell wall, but it is still rigid, making the cell’s shape more stable. It’s usually made up of glucans and lipopolysaccharides. One example of a cell capsule is from the genus Caulobacter. The latter has a thick-walled cell and a relatively small capsule.

Functions of the Bacterial Cell Capsule

  • They are a protective barrier. The capsule is the outside layer of the cell wall, which protects the cell from damage. Without this protection, the inside of the cell will be exposed.
  • The capsule inhibits the process of osmosis which is when water enters a cell from outside. Outer membranes allow only small molecules to pass through.
  • The capsule allows the cells to be seen by phagocytes which are white blood cells that ingest damaged or foreign material.
  • The capsule allows the immune system to distinguish between foreign cells and good ones.
  • They are an identifying feature of bacteria. Capsules make it easier for the immune system to know the difference between cells and foreign things like viruses or other harmful bacteria.
  • Protective capsule on bacteria prevents dehydration and chemicals from getting inside the cell. They identify bacteria that help the immune system distinguish between its cells and other harmful bacteria.

Prokaryote bacteria versus Eukaryotes

Prokaryotic cells (bacteria and archaebacteria) can be compared to eukaryotic cells. There are five different characteristics used to classify bacteria: Cell wall, Cytoplasmic membrane, Cell shape, Gram reaction, and Motility.

  • The significant differences between prokaryotes, archaebacteria, and eukaryotes are the cell wall, structure of the nucleus, and cell production.
  • Eukaryotes have complex structures inside the cell because they have very complex organelles. Prokaryotic cells do not have all of the same structures which are in eukaryotes.
  • In prokaryote bacteria, the cell membrane is simple and less specific. This factor makes the prokaryotes more adaptable than the eukaryotes.
  • The prokaryotic bacteria have a rod-shaped cell structure, while the eukaryotic cells have different shapes since they have organelles.
  • Since the cell’s DNA is not contained inside a nucleus, prokaryotes are easier to study and work with than eukaryotes.
  • Prokaryote bacteria have no cytoplasmic membrane, and the cell wall is rigid. Eukaryotes have a flexible cell membrane and do not have a rigid cell wall.
  • Eukaryotes have an organized nucleus with different chromosomes. Prokaryotic bacteria do not have a nucleus or organized DNA

Reproduction in Prokaryotes

Figure: Binary fission

Prokaryotes reproduce through the process of binary fission. This is when half of a prokaryote cell splits to become two identical daughter cells. Prokaryotes have an extra membrane that they can use to divide, and this is how they reproduce.

Bacteria cannot double their number in one division. They can only split into two identical daughter cells called binary fission. Cells that have more DNA are caused to multiply faster than cells that have less DNA. The result is 2 Prokaryotic cells.

Prokaryotes share genes through plasmids. Plasmids are pieces of DNA that can be transferred from one cell to another, and they can pass on specific traits.

During reproduction, a double ring forms around the cell. The first ring causes the DNA to move away from the middle, and this is called replication. After replication, a second ring forms and causes the DNA to move back together. The membrane closes, the ends join up, and it reforms a normal cell wall.

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Take Away

Their lack of a nucleus defines prokaryote bacteria. They are also usually smaller than eukaryotic cells. These bacteria have cell walls, and each wall is made of peptidoglycan. Prokaryotic cells have small capsules on the outside. These capsules form a protective layer from dehydration and chemicals getting in the cell. Prokaryotes also reproduce through binary fission, meaning half the cell splits into two equal cells.

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