Antibiotics Are Selective Poisons

Antibiotics are relatively small molecules (about 20 to 100 times the size of a water molecule) that interfere with normal life processes of microbes and viruses. Human cells differ enough from pathogens for antibiotics to act selectively. For example, our cells lack walls whereas bacterial cells have them. Consequently, penicillin, which blocks cell wall synthesis, is specific to bacteria. Penicillin has adverse effects, but they arise from other properties. (Some people are allergic to the drug.)

Three general aspects of antibiotics are important when considering effectiveness. First, some antibiotics only block growth (static compounds), whereas others also kill cells (cidal or lethal compounds). Some drugs are static with one pathogen and lethal with another. For example, rifampicin kills Mycobacterium tuberculosis but blocks only the growth of Escherichia coli at concentrations usually used. The distinction is important, because static drugs allow the microbes to resume growth when the compound disappears from the body. Fortunately, the human immune system is effective at reducing the number of pathogens in an infection; consequently, static agents, such as tetracycline, can be effective treatments for some diseases. A second important feature is the molecular mechanism of antibiotic action. For example, agents that cause pathogens to break apart (lyse) cause the release of toxic microbial molecules into the patient. Those toxins can lengthen the time needed to recover from disease. A third issue is whether an antibiotic is a broad-spectrum agent or specialized for use with one pathogen species. For most treatment situations, the infecting pathogen is not identified. Treating with a broad-spectum agent allows effective treatment to begin immediately and often without the added expense of diagnostic tests. However, with diseases that require long treatment periods, such as tuberculosis, specialized agents that cause less damage to our normal bacterial flora are preferred. These narrow-spectrum agents are also less likely to select resistant mutants of other pathogens that may co-infect patients. Such benefits are not limited to tuberculosis; consequently, narrow-spectrum agents are likely to become more popular as rapid molecular diagnostic methods become more convenient.

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