Is Ebola Lytic or Lysogenic?
Is Ebola lytic or lysogenic? Includes examples of lytic and lysogenic viruses - measles, rabies, and more.
Ebola, also known as Ebola virus disease (EVD), is a severe and often deadly illness caused by the Ebola virus. The virus is responsible for causing outbreaks in several African countries, with the most recent outbreak occurring in Uganda in 2022.
One of the interesting things about the Ebola virus is its ability to replicate through the lytic cycle, a mechanism of virus replication that uses the host cell to produce new copies of viral particles and destroy the host cell's DNA.
In this blog post, we will discuss the lytic replication cycle of the Ebola virus, including the different stages of the cycle, the mechanisms of replication, and the significance of this process in the context of EVD. We will also explore the impact of the lytic replication cycle on the host cells and the severity of the disease.
Key points:
- The lytic cycle involves the reproduction of viruses using a host cell to manufacture more viruses; the viruses then burst out of the cell. Ebola is a lytic virus - it reproduces due to the lytic cycle
- The lysogenic cycle involves the incorporation of the viral genome into the host cell genome, infecting it from within.
What is Ebola? How do you get it?
Ebola is a highly infectious and deadly disease caused by the Ebola virus. It is a member of the Filoviridae family of viruses, which also includes the Marburg virus. Ebola virus is characterized by long, single-stranded, and filamentous negative-sense RNA (ribonucleic acid) viruses. The genus Ebolavirus consists of six species, but only four have been known to cause human disease: Zaire ebolavirus, Sudan ebolavirus, Tai Forest ebolavirus, and Bundibugyo ebolavirus. The Zaire ebolavirus, more commonly known as the Ebola virus, was linked to severe EVD outbreaks such as the 1976 viral hemorrhagic fever outbreak in Sudan and Congo.
Ebola is primarily transmitted through direct contact with bodily fluids from infected patients or other species (e.g., gorillas and chimpanzees). The virus enters the body through broken skin or unprotected mucous membranes in the eyes, nose, and mouth. Additionally, Ebola can also be contracted through exposure to contaminated surfaces, needles or medical equipment.
Symptoms of Ebola include fever, headache, muscle pain, weakness, fatigue, diarrhea, vomiting, abdominal pain, and unexplained hemorrhage (bleeding or bruising). Symptoms may appear anywhere from 2 to 21 days after exposure to the virus. Interestingly, the bleeding associated with Ebola is thought to be caused by the rupture of cells in the lytic cycle - which is what we'll examine next.
What is the Lytic Cycle?
The lytic cycle is the main cycle of viral replication in which the viral RNA enters the host cell, transcribes itself into the host cell's messenger RNAs, and uses them to direct the ribosomes. The host cell's DNA is destroyed and the virus takes over the cell's metabolism, creating copies of itself. As the cell becomes overcrowded with viruses, the original virus releases enzymes to break the cell wall, causing the cell to burst and release new viruses. Filoviruses such as Ebola and Marburg only use the lytic cycle for replication, targeting and destroying epithelial cells, which contributes to the severity of the disease. It is important to note that the Ebola virus does not have a lysogenic replication cycle, it replicates only through the lytic cycle.
The lytic cycle of a pathogen typically includes the following phases.
Step 1: Adsorption
The pathogen attaches to specific receptors on the host cell wall. Note that in this example the pathogen is shown as a bacteriophage, which infects a bacterium. Nevertheless, the lytic cycle steps are similar for Ebola, although Ebola looks like a worm and not like a bacteriophage.
Step 2: Penetration
The pathogen injects its genome into the host cell's cytoplasm through a hole in the cell wall or through a hollow organelle such as flagella or pili.
Step 3: Replication
The pathogen genome replicates and the host cell's metabolic machinery is used to synthesize enzymes and structural components.
Step 4: Maturation
The pathogen parts assemble around the genomes.
Step 5: Release
The pathogen releases itself from the host cell by causing osmotic lysis through the action of a pathogen-coded lysozyme.
Step 6: Reinfection
The released bacteriophages can go on to infect other host bacteria.
For further reading on the steps of the Lytic process, check out this article on Libretexts.
What is the lysogenic cycle?
The lysogenic cycle is a form of viral reproduction involving the fusion of the nucleic acid of a bacteriophage with that of a host, followed by the proliferation of the resulting prophage.
The lysogenic cycle is a viral replication cycle in which the viral DNA or RNA enters a host cell and incorporates itself into the host DNA as a new set of genes known as prophage. The host cell continues to survive and reproduce, and the virus is reproduced in all of the cell's offspring. The virus remains dormant until the host conditions deteriorate, such as the depletion of nutrients. At this point, the prophages become active and initiate the reproductive cycle, resulting in the lysis of the host cell. This cycle is in contrast to the lytic cycle, which immediately results in lysing of the host cell. An example of a virus known to follow the lysogenic cycle is the phage lambda of E. coli.
Temperate viruses, such as bacteriophages, can undergo both lysogenic and lytic cycles, while virulent viruses only replicate via the lytic cycle. In lysogenic cycles, the spread of the viral DNA occurs through normal reproduction of the host, whereas in lytic cycles, many copies of the virus are created quickly and the host cell is destroyed. Some examples of lysogenic cycles in bacteria include Corynebacterium diphtheriae, Vibrio cholerae, and Clostridium botulinum.
Although the example diagram shown below refers to a bacteriophage and not Ebola, the cycles process is similar. Some kinds of bacteriophages even go through both cycles. These pathogens are called "temperate" bacteriophages. Ebola, however, only goes through the lytic cycle - not the lysogenic cycle.
What are some examples of lytic viruses?
Several viruses in the filovirus classification are lytic, including:
Filovirus Strains:
Marburg
Ebola:
Reston ebolavirus
Tai Forest ebolavirus
Bundibugyo ebolavirus
Sudan ebolavirus
Zaire ebolavirus
You can learn more about these viruses at this link. Here are some pictures to show you what these Ebola-like viruses look like:
Measles is also a lytic disease - it infects animal cells, not bacteria. Another lytic bacteriophage is T4, which infects E. coli. HSV2 (Herpes simplex virus, type 2 - sexually transmitted) is also lytic, but its counterpart HSV1 (Herpes simplex virus, type 1 - oral herpes) is lysogenic.
What are some examples of lysogenic viruses?
Rabies is lysogenic, not lytic
Rabies viruses are enveloped negative-stranded RNA Rhabdoviruses and can infect a broad range of animal hosts. After replication and assembly of new virus particles, viruses are released from host cells. Some viruses carry out this process without destroying the cell. However, some viruses destroy host cells as a means of release. This process of host cell lysis during release is more common with non-enveloped viruses, such as polioviruses. The rabies virus, however, does not cause cell lysis during release. Rabies virus particles are assembled and bud at the plasma membrane, leaving the host cell intact. Therefore, rabies is lysogenic, not lytic.
Some viruses can become lysogenic through conversion
Lysogenic conversion is a process in which a non-virulent bacteria becomes a highly virulent pathogen by incorporating virulence factors carried on a lysogenic prophage. The virulence genes can be carried within prophages as autonomous genetic elements called morons, which confers an advantage to the bacteria and indirectly benefits the virus through enhanced lysogen survival. Examples of this include:
- Corynebacterium diphtheriae, which produces the toxin of diphtheria only when infected by the phage β,
- Vibrio cholerae, which can become toxic and produce cholera toxin when infected with the phage CTXφ,
- Shigella dysenteriae, which produces dysentery toxins from the genes of lambdoid prophages,
- Streptococcus pyogenes, which produces a pyrogenic exotoxin through lysogenic conversion and causes scarlet fever, and c
- Certain strains of Clostridium botulinum
What is Ebola's latency period?
Viruses that infect plant or animal cells may also enter a dormant state where they do not actively produce viral particles for extended periods. An example of this is animal herpes viruses, such as herpes simplex viruses, which cause oral and genital herpes in humans. This dormant state is known as latency, and these viruses can exist in nerve tissue for long periods without producing new viral particles, only to reactivate periodically and cause skin lesions where replication occurs. Ebola has a short latency period of less than a few days.
Conclusion: Ebola is a lytic, not lysogenic virus
Ebola is a virus that primarily replicates through the lytic cycle. This means that once it enters a host cell, it begins using the cell's energy and resources to make copies of itself, eventually causing the host cell to burst and release new virus particles. In contrast, the lysogenic cycle allows the viral genome to integrate into the host's DNA and replicate along with it without immediately causing the host cell to lyse.
While some viruses, such as animal herpes viruses, can exist in a latent state, it is not known to be the case for Ebola. Additionally, certain bacteria can become virulent through lysogenic conversion with the virulence factors carried on the lysogenic prophage, but this is not known to occur with Ebola.