Bacterial Reproduction

Salmonella
Bacterial Reproduction: This Salmonella bacterium is undergoing the process of binary fission. The cell divides resulting in the formation of two identical cells. Janice Haney Carr/CDC

Bacteria are prokaryotic organisms that reproduce asexually. Bacterial reproduction most commonly occurs by a kind of cell division called binary fission. Binary fission involves the division of a single cell, which results in the formation of two cells that are genetically identical. In order to grasp the process of binary fission, it is helpful to understand bacterial cell structure.

Bacterial Cell Structure

Bacteria have varying cell shapes.

The most common bacteria cell shapes are spherical, rod-shaped, and spiral. Bacterial cells typically contain the following structures: a cell wall, cell membrane, cytoplasm, ribosomes, plasmids, flagella, and a nucleiod region.

  • Cell Wall - Outer covering of the cell that protects the bacterial cell and gives it shape.
  • Cytoplasm - A gel-like substance composed mainly of water that also contains enzymes, salts, cell components, and various organic molecules.
  • Cell Membrane or Plasma Membrane - Surrounds the cell's cytoplasm and regulates the flow of substances in and out of the cell.
  • Flagella - Long, whip-like protrusion that aids in cellular locomotion.
  • Ribosomes - Cell structures responsible for protein production.
  • Plasmids - Gene carrying, circular DNA structures that are not involved in reproduction.
  • Nucleiod Region - Area of the cytoplasm that contains the single bacterial DNA molecule.

Binary Fission

Most bacteria, including Salmonella and E.coli, reproduce by binary fission.

During this type of asexual reproduction, the single DNA molecule replicates and both copies attach, at different points, to the cell membrane. As the cell begins to grow and elongate, the distance between the two DNA molecules increases. Once the bacterium just about doubles its original size, the cell membrane begins to pinch inward at the center.

Finally, a cell wall forms which separates the two DNA molecules and divides the original cell into two identical daughter cells.

There are a number of benefits associated with reproduction through binary fission. A single bacterium is able to reproduce in high numbers at a rapid rate. Under optimum conditions, some bacteria can double their population numbers in a matter of minutes or hours. Another benefit is that no time is wasted searching for a mate since reproduction is asexual. In addition, the daughter cells resulting from binary fission are identical to the original cell. This means that they are well suited for life in their environment.

Bacterial Recombination

Binary fission is an effective way for bacteria to reproduce, however it is not without problems. Since the cells produced through this type of reproduction are identical, they are all susceptible to the same types of threats, such as environmental changes and antibiotics. These hazards could destroy an entire colony. In order to avoid such perils, bacteria can become more genetically varied through recombination. Recombination involves the transfer of genes between cells. Bacterial recombination is accomplished through conjugation, transformation, or transduction.

Conjugation

Some bacteria are capable of transferring pieces of their genes to other bacteria that they contact. During conjugation, one bacterium connects itself to another through a protein tube structure called a pilus. Genes are transferred from one bacterium to the other through this tube.

Transformation

Some bacteria are capable of taking up DNA from their environment. These DNA remnants most commonly come from dead bacterial cells. During transformation, the bacterium binds the DNA and transports it across the bacterial cell membrane. The new DNA is then incorporated into the bacterial cell's DNA.

Transduction

Transduction is a type of recombination that involves the exchange of bacterial DNA through bacteriophages. Bacteriophages are viruses that infect bacteria. There are two types of transduction: generalized and specialized transduction.

Once a bacteriophage attaches to a bacterium, it inserts its genome into the bacterium. The viral genome, enzymes, and viral components are then replicated and assembled within the host bacterium. Once formed, the new bacteriophages lyse or split open the bacterium, releasing the replicated viruses. During the assembling process however, some of the host's bacterial DNA may become encased in the viral capsid instead of the viral genome. When this bacteriophage infects another bacterium, it injects the DNA fragment from the previously infected bacterium. This DNA fragment then becomes inserted into the DNA of the new bacterium. This type of transduction is called generalized transduction.

In specialized transduction, fragments of the host bacterium's DNA become incorporated into the viral genomes of the new bacteriophages. The DNA fragments can then be transferred to any new bacteria that these bacteriophages infect.