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PHAGE GRAPHICS

Bacteriophage T4 on a bacterial cell surface injecting its DNA.

Bacteriophage P22 attacking a Salmonella bacterium.

Bacteriophage T4 group attacking a bacterium.

Bacteriophage T4 delicate and complex landing and injection machinery.

Phage means eat (Greek phagein). These viruses attack bacteria, literally bacteria eaters. Of course, they don't eat bacteria in the conventional sense, but they can destroy them. Phage therapy can be used to treat bacterial infections in some cases, for example when bacteria are drug-resistant.

Lambda phage illustrating lytic and lysogenic processes. The phage attaches to the bacterium and injects its DNA (red string). The free ends of the string join to make circular DNA. This DNA ring then supercoils into a clump. The viral DNA takes one of two possible routes:

LYTIC (left): The viral DNA replicates, and phage proteins are made using host cell ribosomes. These viral proteins assemble into new phage heads and tails. New viral DNA feeds into newly assembled phage heads. Once the heads are full, the new phage tails are attached. The bacterium disintegrates (lyses, from the Greek lusis, a loosening, SOED) releasing the daughter virions.

LYSOGENIC (right): The viral DNA, instead of replicating, integrates (joins) with the bacterial DNA and reproduces along with the bacterial genome, sometimes for many generations. This integrated viral genome is called a provirus (or prophage). If the survival of the host bacterium is threatened, the provirus frees itself from the host genome and replicates in a lytic cycle.