Drug resistance is the inability of a drug to bring about an effect on a disease-causing agent that occurred previously in the presence of that same medication. Resistance to an antibiotic, for example, occurs when bacteria that were previously killed by one antibiotic will now grow in the presence of that same antibiotic (i.e., the bacteria have developed a way to avoid or prevent cell death). In the United States, Streptococcus pneumoniae, a common cause of pneumonia, bronchitis, ear infections, and other conditions, was universally sensitive to penicillin prior to 1990. As of June 1999, however, penicillin was either no longer effective or was required in higher than previously effective doses to treat about 25–35 percent of all S. pneumoniae isolates. This decrease in the effectiveness of penicillin is attributed to an acquired drug resistance to penicillin by the bacteria.
MEGANNE S. KANATANI
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