While it seems clear that drug-resistant microorganisms often have point mutation(s) in drug-target molecule genes (Walsh, 2000), no reports have yet described how and why such mutations occur in clinically important drug-resistant bacteria. We have reported that oxygen can Cyclopamine mw induce DNA damage, causing mutations in rpoB and Rif resistance, in a strict anaerobe (Takumi et al., 2008). In the environment, there are numerous mutagens capable of damaging DNA and inducing mutation. Clinical drugs, such as some used in cancer therapy, may also be mutagenic (Kunz & Mis, 1989). Cigarette smoke also

contains many mutagenic chemicals (Fujita & Kamataki, 2001; Yim & Hee, 2001). Environmental microorganisms, especially indigenous microorganisms, may frequently be exposed to mutagens. Pseudomonas aeruginosa is an indigenous bacterium and emerging drug resistance in this bacterium is a growing concern (Jalal & Wretlind, check details 1998; Mouneimne et al., 1999; Akasaka et al., 2001; Wydmuch et al., 2005). In this study, we exposed P. aeruginosa to mutagens that are known to induce point mutation. The environmental concentrations of mutagens are similar or even higher than those we have used in the present experiments. The concentration of EMS in the Viracept case was 2.3 mg mL-1. (Gerber & Toelle, 2009), that of 1,6-DNP in soot was 0.41–0.71 μg g−1 (Schauer et al., 2004), and that of BCNU was 4 μg mL−1 in human plasma and

3.3 mg mL−1 in injection fluid (Petros et al., 2002). We have set the exposure time at 24 h because indigenous bacteria may be exposed 3-oxoacyl-(acyl-carrier-protein) reductase to these mutagens continuously in

the environment. We selected Rif and CPFX, because the emergence of microorganisms resistant to Rif and to CPFX is a growing concern (Jalal & Wretlind, 1998; Wydmuch et al., 2005). In addition, both antibacterial agents have obvious target molecules and mutations related to these target molecules are known to confer drug resistance (Campbell et al., 2001; Mariam et al., 2004). Pseudomonas aeruginosa is inherently relatively resistant to Rif (Yee et al., 1996), but has been susceptible to high concentrations of Rif. At the same time, the emergence of Rif-resistant M. tuberculosis is also a growing concern (Yee et al., 1996; Murphy et al., 2006). CPFX has been highly effective in treating P. aeruginosa infections, but recently, CPFX-resistant P. aeruginosa has become a growing problem (Jalal & Wretlind, 1998). Rif- and CPFX-resistant P. aeruginosa emerged after exposure to EMS and MNU. Meanwhile, BCNU induced Rif resistance, and 1,6-DNP induced CPFX resistance. NNN did not increase Rif- or CPFX-resistant P. aeruginosa. While BP induced mutation in S. Typhimurium TA100, Rif- or CPFX-resistant P. aeruginosa did not result. Susceptibility to BP differs considerably among strains (Jemnitz et al., 2004). We supposed that the P. aeruginosa was not susceptible to the mutagenic action of BP metabolites.