DNA damaging agents (reviewed by Michod et al., 2008 and Bernstein et al., 2012). Thus, a strong short-term selective advantage for natural competence and transformation would be its ability to promote homologous recombinational DNA repair under conditions of stress. Such stress conditions might be incurred during bacterial infection of a susceptible host. Consistent with this idea, Li et al. reported that, among different highly transformable S. pneumoniae isolates, nasal colonization fitness and virulence (lung infectivity) depends on an intact competence system. A counter argument was made based on the 1993 report of Redfield who found that single-stranded and double-stranded damage to chromosomal DNA did not induce or enhance competence or transformation in B. subtilis or H. influenzae, suggesting that selection for repair has played little or no role in the evolution of competence in these species However more recent evidence indicates that competence for transformation is, indeed, specifically induced by DNA damaging conditions. For instance, Claverys et al. in 2006 showed that the DNA damaging agents mitomycin C (a DNA cross-linking agent) and fluoroquinolone (a topoisomerase inhibitor that causes double-strand breaks) induce transformation in Streptococcus pneumoniae. In addition, Engelmoer and Rozen in 2011 demonstrated that in S. pneumoniae transformation protects against the bactericidal effect of mitomycin C. Induction of competence further protected against the antibiotics kanomycin and streptomycin. Although these aminoglycoside antibiotics were previously regarded as non-DNA damaging, recent studies in 2012 of Foti et al. showed that a substantial portion of their bactericidal activity results from release of the hydroxyl radical and induction of DNA damages, including double-strand breaks. Dorer et al., in 2010, showed that ciprofloxacin, which interacts with DNA gyrase and causes production of double-strand breaks, induces expression of competence genes in Helicobacter pylori, leading to increased transformation. In 2011 studies of Legionella pneumophila, Charpentier et al. tested 64 toxic molecules to determine which ones induce competence. Only six of these molecules, all DNA damaging agents, strongly induced competence. These molecules were norfloxacin, ofloxacin and nalidixic acid (inhibitors of DNA gyrase that produce double strand breaks), mitomycin C (which produces inter-strand cross-links), bicyclomycin (causes single- and double-strand breaks), and hydroxyurea (causes oxidation of DNA bases). Charpentier et al. also showed that UV irradiation induces competence in L. pneumophila and further suggested that competence for transformation evolved as a response to DNA damage. Horizontal gene transfer A long-term advantage may occasionally be conferred by occasional instances of horizontal