Antimicrobial use and the emergence of antimicrobial resistance with Streptococcus pneumoniae in the United States.

The rapid emergence of resistance to antimicrobial agents by Streptococcus pneumoniae in the United States has been influenced by various factors, including the clonal nature of most resistant strains and the fact that organisms with a multiresistant phenotype have become stably endemic. The ease with which transmission occurs and the fact that humans, especially children, are often colonized asymptomatically in the upper respiratory tract have contributed to the problem. Clearly, the most important factor in the emergence of antimicrobial resistance with S. pneumoniae, however, is the selective pressure of antimicrobial agents. Potency, defined as a product of both antibacterial effect and drug delivery, is a key factor. Generally speaking, the more potent an antimicrobial agent, the less likely it is to select for resistance. This is germane to comparisons of oral agents within specific antimicrobial classes (e.g., beta-lactams, macrolides, and fluoroquinolones). Within each class, potencies differ. In view of the existence of stably endemic multidrug-resistant S. pneumoniae, given comparable cost, side-effect profile, palatability, convenience of dosing, and accessibility, use of the most potent agent(s) within a particular class is advocated.