DEAR DR. ROACH: In a recent column, you made a point about antibiotic overuse that I hadn't heard before: "The antibiotics will be more effective when you do choose to use them." I thought the argument against overuse was strictly altruistic, that overuse increases the prevalence of drug resistant disease strains in the general population. I'd like to know more about how an antibiotic would be more effective if someone hasn't used it before. If a pair of individuals infected by the same strain are given the same antibiotic, but one has used it before, why would it be less effective for that person? — J.G.

ANSWER: Bacteria cells are older, more adaptable, grow more rapidly and make far more spontaneous mutations than the cells of animals. As a result, they can develop resistance to antibiotics rapidly, and even spread the mechanism for resistance among themselves via genetic transfer. The more frequently and longer that bacteria are exposed to antibiotics, the more likely resistance is to develop, unless literally all the bacteria are killed, which is difficult.

A person who frequently takes antibiotics is more likely to have resistant bacteria colonizing their body, and under the right circumstances, bacteria that colonize the body may become invasive. If those bacteria have developed resistance to antibiotics, that will make treatment for the individual more difficult.

Over time, bacteria with antibiotic resistance tend to be outcompeted by the nonresistant bacteria (called the "wild type" in genetics), so for an individual, taking antibiotics infrequently does not greatly increase likelihood of harboring resistant bacteria. It is, as you say, the general population that is critical. If there is a lot of antibiotic use in the community, then everyone is at increased risk. Hence, there is both an individual benefit and a larger, societal benefit for minimizing antibiotic use to where it is clearly indicated.

We have long supposed that resistance is an issue if people take an incomplete course of antibiotics, but the main problem with too short a course is that it is ineffective: The infection may come back, but isn't likely to be resistant. We are coming to realize that too-long courses of antibiotics may be more important in developing resistance, so the ideal duration of antibiotic courses is undergoing review. It is likely that shorter courses of antibiotics than currently prescribed may reduce overall resistance. For now, patients should take their antibiotics as prescribed, and only when necessary.