Name: prophage integration in bacteria
Uploaded: 2019-03-11T09:21:23.000Z
Duration: 43 s
Channel: JoVE

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From Molecules To Organisms: Structures And Processes

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Lysogenic Cycle of Bacteriophages

Lorsqu'un grand nombre de bactériophages est présent,un mode de réplication alternatif peut se produire. 
Il s'agit du cycle lysogénique : les phages se reproduisentsans tuer leur hôte. 
Ce cycle débute comme un cycle lytique. 
D'abord, le phage se fixe à la cellule hôteet injecte son ADN. 
Mais, une fois à l'intérieur, l'ADN du phage se recomposeet s'intègre au génome bactérien,formant un prophage. 
Le prophage n'est pas actifet n'entraîne pas la production de nouveaux phages. 
Lorsque la cellule hôte se divise,la réplication se fait avec l'ADN de l'hôte.

Castro, Vinicius Silva, Ricardo César Tavares Carvalho, Carlos Adam Conte‐Junior et Eduardo Eustáquio Souza Figuiredo. « Escherichia coli producteur de shigatoxine : pathogénicité, excrétion, méthodes de diagnostic, occurrence et épidémies d’origine alimentaire. » Revues complètes en science des aliments et en sécurité alimentaire 16, n° 6 (2017) : 1269-80. [<a href="https://doi.org/10.1111/1541-4337.12302 » target="__blank">Source</a>]

Melton-Celsa, Angela R. « Classification, structure et fonction de la toxine Shiga (Stx). » Spectre de microbiologie 2, n° 2 (2014). [<a href="https://doi.org/10.1128/microbiolspec.EHEC-0024-2013 » target="__blank">Source</a>]

cycle lysogénique des bactériophages

Contrairement au cycle lytique, les phages qui infectent les bactéries par le cycle lysogène ne tuent pas immédiatement leur cellule hôte. Au lieu de cela, ils combinent leur génome avec le génome de l’hôte, permettant aux bactéries de répliquer l’ADN du phage avec le génome bactérien. La copie incorporée du génome du phage est appelée le prophage. Certains prophages peuvent se réactiver et entrer dans le cycle lytique. Cela se produit souvent en réponse à une perturbation, comme les dommages à l’ADN, mais peut également se produire en l’absence de signaux externes.
 
 <h4>La conversion lysogène</h4>
 
 Dans certains cas, les gènes codés par des prophages peuvent altérer le phénotype de la bactérie infectée, un processus connu sous le nom de conversion lysogène. Certains phages codent des protéines ou des toxines appelées facteurs de virulence qui peuvent faciliter les infections bactériennes. Grâce à la conversion lysogène, les bactéries normalement non pathogènes peuvent devenir très virulentes via l’infection par un phage portant des facteurs de virulence.
 
 Par exemple, ces phages sont en grande partie responsables de la pathogénicité des espèces bactériennes qui causent le botulisme (Clostridium botulinum), la diphtérie (Corynebacterium diphtheriae) et le choléra (Vibrio cholerae). Sans conversion lysogène, ces bactéries ne causent généralement pas de maladie.
 
 Un exemple particulièrement bien étudié de conversion lysogène est celui de la souche Escherichia coli O157:H7. Plusieurs rappels massifs d’aliments ont résulté de la contamination par E. coli O157:H7. Cette souche d’E. coli a été infectée par un phage qui code la shigatoxine (STX), qui peut provoquer des saignements intestinaux et une insuffisance rénale. Dans le cycle lysogène, la STX n’est pas produite et les bactéries ne provoquent pas de maladie. Le phage doit réintégrer le cycle lytique pour que la STX soit produite. Malheureusement, certains antibiotiques peuvent déclencher l’induction du prophage et la production de la STX qui en résulte, ce qui rend le traitement de ces infections difficile. La recherche actuelle étudie de nouvelles thérapies qui empêchent l’initiation du prophage.

Cycle lysogénique des bactériophages

Contrairement au cycle lytique, les phages infectant les bactéries via le cycle lysogénique ne tuent pas immédiatement leur cellule hôte. Au lieu de cela, ils combinent leur...

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prophage integration in bacteria

The lysogenic cycle shows how bacteriophages can infect bacteria without killing the host right away. In this cycle, the phage genome joins the bacterial genome. The integrated phage DNA is called a prophage, and the bacterium copies it each time it replicates.

Some prophages can switch back into the lytic cycle. This change can happen after DNA damage or even without an obvious outside trigger. When the prophage is induced, the phage begins making new virus particles again.

Prophages can also change the traits of the bacteria they infect. This effect is called lysogenic conversion. In this process, phage genes can give bacteria virulence factors, such as toxins or proteins that help infection. As a result, bacteria that were usually harmless can become much more harmful after phage infection.

This kind of conversion helps explain the disease-causing power of Clostridium botulinum, Corynebacterium diphtheriae, and Vibrio cholerae. It also plays a major role in Escherichia coli O157:H7, a strain linked to food recalls. In this strain, the phage carries the gene for Shiga-like toxin, or Stx, which can cause intestinal bleeding and kidney failure. Stx is not produced during lysogeny, and the bacteria do not cause disease until the phage enters the lytic cycle.

Treatment can be complicated because some antibiotics may trigger prophage induction and lead to Stx production. That is why current research is looking for new therapies that stop prophages from starting the lytic cycle.

Prophage Integration in Bacteria

what are viruses?

How Viruses Infect Cells and Multiply

Viruses infect cells and make new copies by using a host cell’s own machinery. A virus is a tiny infectious particle with an RNA or DNA genome inside a protective protein coat. It cannot reproduce on its own, so it must enter a cell to make more viruses. After infection, the virus sheds its protein coat and uses the host cell to transcribe and translate its genetic material. The cell then assembles many new viral particles. In many cases, these new viruses rupture the host cell and kill it.

viral structure

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Viruses have specific structures that help them infect host cells and trigger immune responses. Each virus has a core that contains either a DNA-based or RNA-based genome. That core is surrounded by a protective protein coat called the capsid. The capsid is made of smaller subunits called capsomeres. The capsid and the genome-containing core together are called the nucleocapsid. Viruses are often grouped by capsid design. Most viruses have either icosahedral or helical capsids, but some have...

lytic cycle of bacteriophages

How Phages Infect and Burst Bacteria

Bacteriophages, or phages, are viruses that infect bacteria. They have a distinctive head-tail shape. In the lytic cycle, a phage first attaches to the outside of a bacterial cell. Tail proteins bind to specific receptor proteins on the bacterium’s surface. After attachment, the phage tail injects its DNA genome into the bacterial cytoplasm. The phage then uses the bacterium’s cellular machinery to make proteins needed for replication and dispersal. When phage replication is complete, some of...

lysogenic cycle of bacteriophages

The lysogenic cycle shows how bacteriophages can infect bacteria without killing the host right away. In this cycle, the phage genome joins the bacterial genome. The integrated phage DNA is called a prophage, and the bacterium copies it each time it replicates. Some prophages can switch back into the lytic cycle. This change can happen after DNA damage or even without an obvious outside trigger. When the prophage is induced, the phage begins making new virus particles again. Prophages can...

retrovirus life cycles

How Retroviruses Replicate in Host Cells

Retroviruses replicate by copying their RNA genome into DNA inside a host cell. After entry, the viral enzyme reverse transcriptase makes a double-stranded DNA copy from the single-stranded RNA genome. The viral DNA is then carried into the nucleus and inserted into the host genome by integrase. Once integrated, the retroviral DNA is transcribed whenever the host genome is transcribed. This produces new viral RNA that can be copied and used again. The new RNA moves into the cytoplasm, where it...

viral recombination

How Viruses Swap Genes and Spread

Viral gene swapping can happen when more than one virus infects the same cell. After the viruses break open and expose their genomes for replication, matching regions can pair up and exchange sequences. This process is called recombination. Viruses with segmented genomes can also swap whole segments, which is called reassortment. These changes can help create new diseases. Some diseases can infect more than one species. Pigs can be infected by some human and bird viruses, along with the...

viral mutations

How Virus Mutations Drive Outbreaks

Virus mutations can change how quickly a virus evolves and spreads. A mutation is a change in the sequence of bases in DNA or RNA. Some mutations happen when the genome is copied. They are caused by errors made by polymerase enzymes, which replicate DNA or RNA. RNA viruses often change faster than DNA viruses because RNA polymerase does not proofread its work. That means RNA-based genomes, such as HIV, can gather mutations more quickly. Mutation and recombination give viruses the raw material...

key terms and concepts

Learn how bacteriophages integrate into bacterial DNA, form prophages, and can change bacterial traits through lysogenic conversion and toxin genes.

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Prophage Integration in Bacteria

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