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Students often have difficulty with the antibiotics, not because of any difficult concepts but because of the large number of drugs in this class. It can also be overwhelming to try to memorize the organisms that are sensitive to each drug.

Try this approach. First, make absolutely sure that you understand the general principles of therapy and some definitions. We’ll go over these in this chapter.

Second, be aware of the classes of antibiotics and the mechanism of action for the class. Note any features that are common to all drugs in the class.

Third, learn the particular adverse effects or special features of administration for the drugs in the class. Do any of the drugs cause potentially fatal side effects?

Fourth, learn the broad categories of bacterial spectrum and whether any of the drugs in the class are the drug of choice for the treatment of a particular organism. For example, are the drugs good against all of the gram-positive bacteria, but none of the gram negative? It may be useful to quickly review the most common bacteria at this point. Can you say which bacteria are gram positive and which are gram negative? A solid knowledge of this content will really help when you try to learn about antibiotics. Remember that the sensitivity of bacteria to antibiotics changes over time and in different locations.

This looks like a long list of things to learn, but it is really quite manageable. Don’t get too bogged down in trying to remember the second-line drugs for treatment of certain organisms or which drug to use in case of allergies, and so on. This can be added later to the base of knowledge that you develop now.


To be a useful antibiotic, a compound should inhibit the growth of bacteria without harming the human host.

This should be self-evident, but it is the basis for understanding most of the mechanisms of action of these drugs. The compound should affect some aspect of bacteria that is not present in mammalian cells. We’ll come back to that later.

The drug should penetrate body tissues in order to reach the bacteria.

This again should be self-evident. Again, this is the basis for knowing whether a drug is orally absorbed and whether it will cross the blood-brain barrier. For example, if the patient has a gastrointestinal (GI) infection, you would give a drug orally that is not absorbed by the GI tract. The bacteria are thus treated, and the patient has few side effects. Likewise, the drugs that are used to treat meningitis are ones that cross the blood-brain barrier. The drug that is extremely effective against Haemophilus influenzae does no good for the patient if it cannot ...

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