This transmission electron micrograph shows a T4 bacteriophage, a virus that infects only bacteria (and in this case only Escherichia coli). Phages lack any reproductive machinery and rely on the apparatus of bacteria in order to replicate. They do so by attaching to the cell wall of the bacterium with the spidery tail fibres visible here. The tail is a sheath that contracts to inject the contents of the head, the genetic material (DNA), into its host. Within 25 minutes of infection, the bacterial apparatus successfully commandeered, viral progeny fill the cell. The overcrowded bacterium bursts, releasing approximately 100 new copies of the bacteriophage.
M. Wurtz/Univ. of Basel/Science Source/Photo Researchers, Inc.
Hepatitis B Virus
The hepatitis B virus (HBV) causes inflammation of the liver. The virus is recognizable under magnification by the round, infectious “Dane particles” accompanied by tube-shaped, empty viral envelopes. Manifestations of this condition include jaundice and a flu-like illness, while chronic infection can lead to serious pathologies such as cirrhosis and cancer of the liver.
The Human Immunodeficiency Virus (HIV), which can cause acquired immune deficiency syndrome (AIDS), principally attacks T-4 lymphocytes, a vital part of the human immune system. As a result, the body’s ability to resist opportunistic viral, bacterial, fungal, protozoal, and other infection is greatly weakened. Pneumocystis carinii pneumonia is the leading cause of death among people with HIV infection, but the incidence of certain types of cancers such as B-cell lymphomas and Kaposi’s sarcoma is also increased. Neurological complications and dramatic weight loss, or wasting, are characteristic of AIDS, which is endstage HIV disease. HIV can be transmitted sexually; through contact with contaminated blood, tissue, or needles; and from mother to child during birth or breast-feeding. Full-blown symptoms of AIDS may not develop for more than 10 years after infection.
The rabies virus is usually transmitted to humans by a bite from an infected dog, but the bite of any animal (wild or domestic) is suspect in an area where rabies is present. Symptoms of the disease appear after an incubation period of ten days to one year and include fever, breathing difficulties, muscle spasms, and, in later stages, an irrational fear of water. Death almost invariably occurs within three days to three weeks of the onset of symptoms. For this reason, the emphasis of treatment is on prevention. Dogs may not be brought into the United Kingdom until a lengthy period of quarantine has elapsed, while in the United States, domestic dogs are vaccinated yearly and stray dogs are killed.
Chicken Pox Virus
The spherical varicella-zoster virus (VZV); infects most children worldwide by the age of 10. Transmitted in airborne droplets exhaled from an infected person, the virus causes a low fever and a rash of fluid-filled blisters that begin as red spots covering most of the body and the inside of the mouth. The disease is dangerous to newborn babies, to people first infected in adulthood, and to those in whom the virus remains dormant in nerve cells, erupting as the more painful and sometimes chronic zoster (shingles), later in life. VZV is a member of the herpes virus family, which also includes the causative agents of infectious mononucleosis, roseola, and oral and genital herpes.
Some bacteriophages (viruses that parasitize bacteria), left, are quite large and elaborately structured. The T4 phage pictured here consists of five proteins and as many parts: head, tail, collared neck, baseplate, and leg-like tail fibres. In contrast, an influenza virus, right, is much simpler. A lipid envelope surrounds the protein shell, or capsid, which, as in the bacteriophage, encloses coiled genetic material. Projecting from this envelope are two kinds of protein spikes that determine the infective properties of the virus. Human hosts must build new immunity each time these mutate; hence the yearly “flu season”.
Lytic and Lysogenic Cycles of a Bacteriophage
All bacteriophages (viruses that parasitize bacteria) have a lytic or infectious cycle, in which the virus, incapable of replicating itself, injects its genetic material into a bacterium. By pirating its host’s enzymes and protein-building capacities, the virus can reproduce and repackage, making about 100 new copies before it bursts from and destroys the bacterium. Some bacteriophages, however, behave differently when they infect a bacterium. The injected genetic material instead integrates itself into its host DNA, passively replicating with it to be inherited by bacterial daughter cells. In about 1 in 100,000 of these lysogenic cells, the viral DNA spontaneously activates and starts a new lytic cycle.