Prophylaxis Preempts Disease

Sometimes, antibiotics are administered to eliminate an infection before it causes symptoms or even before an infection develops. For example, surgical patients often receive a low dose or a short course of antibiotic to prevent infection from pathogens that commonly contaminate surgical wounds. Reduced treatment is chosen to minimize toxicity, both as a direct effect of the antibiotic on the host and as an indirect effect of the antibiotic on beneficial microbes. (A full antibiotic course is not deemed necessary because the number of pathogens likely to be present is not nearly as high as in the case of a confirmed infection.)

Prophylaxis is also used with tuberculosis but only after signs of infection. When infected with the tubercle bacillus, only 10% of otherwise healthy persons develop active disease. However, we know that infection occurred in many people without disease, because they exhibit a positive reaction to a TB skin test. (The presence of the bacterium causes an immune response.) In these persons, the bacterium appears to have been forced into a dormant state. Because a subsequent loss of immune function is likely to permit the dormant bacteria to grow and cause active disease, the medical community deems it prudent to treat persons exhibiting a positive skin test, even in the absence of disease. The number of infecting M. tuberculosis cells is expected to be low, which has led to the idea that single-drug therapy would be adequate. (Tuberculosis therapy usually involves a four-drug cocktail.) Prophylatic isoniazid treatment is commonly administered for 9 months.