Antibiotic-resistant bacteria are an ever-evolving medical concern. Now, Marc Prudhomme, Laetitia Attaiech, Jean-Pierre Claverys, and colleagues (CNRS, Toulouse, France) report that antibiotics increase genetic exchange and the chance for adaptation in Streptococcus pneumoniae by activating its transformation pathway. The findings highlight the danger of inappropriate antibiotic use. Figure 1 A competence gene is normally expressed at low levels (gray triangles), but is activated (pink) by antibiotics. Transformation (the uptake and genomic integration of exogenous DNA) in S. pneumoniae can only occur when the bacteria are competent. Competence is a transitory state in bacteria. Although its regulation is rather well understood, the signals that trigger it remain elusive. Recent evidence suggests that in S. pnemoniae competence is a stress response to environmental change. Claverys's team therefore wondered whether antibiotic-induced stress might trigger competence. Out of the dozen or so antibiotics that the team checked, six up-regulated the competence pathway when used at concentrations that killed approximately 50% of the bacteria. These antibiotics kill bacteria by either damaging DNA, inhibiting protein synthesis, or blocking DNA synthesis. Thus, the mechanism of action of a particular antibiotic cannot be used to predict its ability to induce competence. Approximately 40% of the human population carry S. pneumoniae asymptomatically in the nose and throat. If the bacteria invade other tissues, however, severe diseases such as pneumonia, meningitis, and osteomyelitis can develop. Two of the antibiotics found to induce competence are commonly used to treat respiratory tract infections. Although the generation of antibiotic-resistant bacteria cannot be completely prevented, choosing antibiotics that do not promote genetic exchange may help to minimize future problems. Reference: Prudhomme, M., et al. 2006. Science. 313:89–92. [PubMed]