Question

Describe the events that occur on an agar plate containing a bacterial lawn when a single bacteriophage particle causes the formation of a bacteriophage plaque. (Section 8.4)

Short answer

A bacteriophage infects a bacterium, replicates, lyses the host, and continues infecting nearby bacteria, forming a clear plaque on the bacterial lawn.

Step-by-step solution

- Initial Infection

A single bacteriophage particle lands on the surface of the agar plate where a uniform bacterial lawn is present. This bacteriophage attaches to a susceptible bacterium and injects its viral DNA into the bacterial cell.

- Viral Replication

Inside the bacterium, the phage DNA hijacks the bacterial machinery to replicate its own genetic material and produce phage proteins. This process transforms the bacterium into a factory that creates new phage particles.

- Lysis of the Host Bacterium

After many copies of the bacteriophage are produced, the bacterial cell undergoes lysis (breaks open), releasing the new phages into the surrounding environment on the agar plate.

- Infection of Nearby Bacteria

The newly released phages infect neighboring bacteria in the immediate vicinity, repeating the cycle of infection, replication, and lysis in these nearby bacteria.

- Plaque Formation

This cycle of infection and lysis continues, leading to a clear zone known as a 'plaque' on the agar plate. Each plaque represents the area where bacterial cells have been destroyed by the virus.

Key concepts

bacterial lawn

In microbiology, a bacterial lawn refers to a uniform and continuous layer of bacteria grown on the surface of an agar plate. It is achieved by spreading bacterial culture evenly across the agar surface. This dense growth creates a suitable environment for observing the effects of bacteriophages (viruses that infect bacteria). When a bacteriophage lands on a bacterial lawn, it encounters numerous potential bacterial hosts to infect. This uniform distribution is crucial for visualizing plaque formation, where clear zones appear due to the destruction of bacterial cells by the virus.

viral replication

After the bacteriophage attaches to a bacterium and injects its DNA, viral replication begins. This involves hijacking the bacterial cell's machinery to produce new viral components. The bacterial ribosomes, enzymes, and other cellular structures are used to replicate the phage DNA and synthesize phage proteins. Eventually, new bacteriophage particles are assembled inside the bacterium. This process is rapid and efficient, enabling the production of many new phages from a single infected cell.

lysis of host bacterium

The lysis of the host bacterium is a critical step in the phage infection cycle. Once the new phage particles are fully formed inside the bacterium, an enzyme known as lysozyme is often produced. This enzyme breaks down the bacterial cell wall, causing the cell to burst open, or lyse. Lysis releases the newly formed phage particles into the surrounding environment, where they can infect neighboring bacteria. The burst size, or the number of phages released, varies depending on the type of bacteriophage and the bacterial host.

phage infection cycle

The phage infection cycle encompasses several stages, starting with the attachment of the bacteriophage to a bacterial cell. Next, the phage injects its DNA into the host bacterium. Following DNA injection, viral replication takes over, creating new phage particles. The bacterium then undergoes lysis, releasing the new phages. These released phages infect surrounding bacteria, and the cycle repeats. This ongoing cycle results in plaque formation on the bacterial lawn, visible as clear zones where bacteria have been lysed by the phage.