Bacterial Viruses are Known as Bacteriophage

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Viruses that infect bacteria are often called bacteriophage or phage for short. Phage is derived from the Greek for "eat" and refers to the way in which bacterial viruses eat holes or "plaques" in a lawn of bacteria growing on the surface of agar (Fig. 17.06). Bacterial viruses were heavily used in the early days of molecular biology to investigate the nature of the gene. Because viruses only contain DNA or RNA surrounded by a protein coat and because bacteriophage infect the simplest of all cells, bacteria, they proved highly convenient.

Bacteria have a cell wall protecting their cell membrane and so bacterial viruses cannot simply merge with the membrane, as do animal viruses. Therefore, bacterial viruses do not bother with an outer envelope layer. They just have a protein shell surrounding the DNA or RNA. After binding to the cell surface, they inject their nucleic acid into the bacterial cell and the outer protein coat of the virus particle is left behind (Fig. 17.07). Many well-known bacterial viruses have a complex capsid that resembles a miniature moon-lander. The capsid has an eicosahedral head, a tail and six-landing legs with attachment proteins at the tips. The tail contracts and injects the DNA through the bacterial cell envelope.

In 1952 Hershey and Chase performed a classic experiment using bacteriophage T4 to demonstrate that only the DNA entered the host cell, E. coli (Fig. 17.08). The protein and the DNA of the virus particles were radioactively labeled with two dif-

bacteriophage (phage) Virus that infects bacteria phage Short for bacteriophage, a virus that infects bacteria plaque (When referring to viruses) A clear zone caused by virus destruction in a layer of cultured cells or a lawn of bacteria

Bacterial Viruses are Known as Bacteriophage 459

Plaque: zone of dead and lysed bacteria

Plaque: zone of dead and lysed bacteria

FIGURE 17.06 Formation of Plaques in Lawn of Bacteria

Bacteriophage are viruses that infect bacteria. To isolate individual types of bacteriophage, plaques are made. A mixture of bacteriophage is added to a large number of bacteria, and poured onto a nutrient agar plate. The bacteria grow quickly, covering the agar with a cloudy layer of bacteria, known as a lawn (red). Wherever a bacteriophage infects a bacterial cell, it destroys the cell and produces many more bacteriophage. These spread out to infect neighboring bacteria, forming a clear zone in the lawn that is called a plaque. Each plaque contains descendents of the single original bacteriophage that landed in that region of the lawn. If needed, purified lines of bacteriophage can be isolated from individual plaques.

Phages are the Most Numerous Life Form

It is likely that there are more bacteriophages on our planet than any other life form.There are an estimated 1030 bacteria on the planet and it is estimated that there are probably about 10 phages for every living bacterial cell, which would give a total of around 1031 phage. Virus particles, including bacteriophage, are ubiquitous on earth. Examination of seawater under the electron microscope has shown typical counts of 50 x 106 virus particles per ml. It has been estimated that phages destroy up to 40% of the bacteria in the ocean every day. Remnants of these lysed bacteria add significant amounts of organic matter to the ocean water and may affect global carbon cycling. A colossal amount of novel genetic material is present in the vast number of phages present in natural habitats. Preliminary surveys have indicated that around 75% of the genes carried by phages are unrelated to anything presently in the DNA data banks.

Many of the virus particles in the environment are probably orphaned in the sense that susceptible host cells are no longer available in their habitats. In some cases the host cell may be extinct or perhaps only mutants resistant to the virus have survived. Conversely, many virus particles are inherently defective and are incapable of successfully infecting host cells, even if available. This is especially true of RNA viruses where the mutation rate is extremely high. The benefit of a high mutation rate is that the virus constantly changes and so evades recognition by the host defense systems. The downside is that most mutations are deleterious and a high percentage of defective virus genomes are made. Indeed, for some RNA viruses the majority of virus particles released are defective mutants and only a minority are infectious particles.

ferent isotopes. The labeled viruses were added to bacterial cells and the fate of the two radioactive labels was followed. The protein, labeled with 35S, was left outside and the DNA, labeled with 32P, entered the cells. Moreover, some of the 32P labeled DNA was found in the new generation of virus particles liberated when the infected cells burst. Since only the virus DNA enters the host cell and the other components are abandoned outside, this provides further evidence that nucleic acids rather than proteins carry genetic information. Historically, the Hershey and Chase experiment

FIGURE 17.07 Bacterial Viruses Inject their Nucleic Acid

To enter a bacterial cell, bacterial viruses must get their genomes through three layers, the bacterial outer membrane, the cell wall, and the inner membrane. The bacterial wall structure prevents the virus from simply merging membranes, as in animal cells. To overcome the defenses, the virus punches a hole through the three layers and injects its DNA (or RNA) into the cytoplasm.

FIGURE 17.07 Bacterial Viruses Inject their Nucleic Acid

To enter a bacterial cell, bacterial viruses must get their genomes through three layers, the bacterial outer membrane, the cell wall, and the inner membrane. The bacterial wall structure prevents the virus from simply merging membranes, as in animal cells. To overcome the defenses, the virus punches a hole through the three layers and injects its DNA (or RNA) into the cytoplasm.

demonstrated for the first time that DNA alone was the carrier of the genetic information and that the associated protein was not required. These findings prompted other researchers to investigate the structure of DNA and its role as the genetic material.

Viruses may replicate aggressively, killing the host cell. Alternatively, they may limit themselves to duplicating their genome in step with cell division.

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