Craig C. Freudenrich
Antibiotics specifically target bacteria. They are designed to disrupt important structures in bacteria, such as the cell walls and cell membranes, or specific processes in bacteria such as DNA replication or protein synthesis. But viruses do not work like bacteria, so antibiotics do not affect them. New classes of medications called antiviral drugs have been developed to take advantage of our knowledge of the biology of viruses.
Viruses consist of a hereditary molecule (DNA or RNA) surrounded by a protein coat; in some cases, the virus is surrounded by a fatty envelope. Viruses have no organelles or metabolism and do not reproduce on their own, which means they are not living things. Instead, viruses reproduce by taking over the host cell's reproductive machinery. Unfortunately, you often provide the host cell.
There are two ways that viruses reproduce. The first way is by attaching to the host cell and entering it or injecting their DNA/RNA into the cell. The viral DNA causes the host cell to make copies of its DNA and translate that DNA to make viral proteins. The host cell assembles new viruses and releases them when the cells break apart and die, or it buds the new viruses off, which preserves the host cell. This approach is called the lytic cycle.
The second way is when the virus enters the host cell using the cell's own materials. A viral enzyme called reverse transcriptase makes a segment of DNA from its RNA using host materials. The DNA segment gets incorporated into the host cell's DNA. There, the viral DNA lies dormant and gets reproduced with the host cell. When some environmental cue happens, the viral DNA takes over, makes viral RNA and proteins, and uses the host cell machinery to assemble new viruses. The new viruses bud off. This approach is called the lysogenic cycle; these viruses are called retroviruses and include herpes viruses and HIV, which causes AIDS.
Antiviral drugs are designed to target some of the specific ways that viruses reproduce:
- Interfere in DNA synthesis -- antiviral drugs (acyclovir, retrovir) mimic DNA or RNA building blocks and stop the enzymes that make new viral DNA.
- Block reverse transcriptase -- lamivudine, efavirenz
- Block integration fusion -- of viral DNA with host DNA, such as enfuvirtide
- Block assembly of new viruses -- protease inhibitors
- Block budding of new viruses -- zanamivir, oseltamivir
- Inhibit uncoating of viruses -- the uncoating happens on entry with drugs such as amantidine and rimantadine.
Antiviral research continues. For example, new findings from the University of California, Irvine may help researchers better customize therapies for flu strains that have thus far resisted antiviral drugs. The influenza virus tends to mutate from one strain to another so that antiviral drugs eventually become ineffective. The new research, which was aided by the San Diego Supercomputing Center at UC San Diego, may pave the way to predict how packet structures on flu protein surfaces promote viral replication. When pharmaceutical manufacturers identify how the proteins evolve, they can better target antiviral therapies [source: Drug Discovery and Development].
Antibiotics work to destroy a living bacterium's ability to reproduce. That's their only job. But viruses aren't alive, and they don't reproduce by themselves, so antibiotics can't do anything to them - there's nothing "alive" for them to "kill." Viruses are just bits of DNA or RNA that inject themselves into a living cell, where the cell itself makes copies of the virus.
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