3; for methods see supplementary information). Thus, find protocol mutations in genes that lead to mutator phenotypes in P. aeruginosa can enhance microcolony initiation and growth during biofilm culture. This different architecture of

the biofilm formed by mutator strains has an impact on the tolerance of the biofilms to antibiotics. We found that the PAO1 ∆mutT had increased tolerance to piperacillin/tazobactam compared with the wild-type (Fig. 4). It has been shown in planktonic growth that under piperacillin/tazobactam selective pressure PAO1 ∆mutT developed a larger resistant subpopulation compared with PAO1 and that the mechanism of resistance was related to increased beta-lactamase production (Mandsberg et al., 2009). Selection of such a resistant subpopulation during treatment of the biofilm might explain the increased tolerance to piperacillin/tazobactam INCB024360 in vitro of PAO1 ∆mutT compared with PAO1. It has been shown recently that theoretically optimized PK/PD parameters failed to suppress resistance development in biofilm-grown bacteria. The antibiotic concentration that prevents the selection of resistant mutants (mutant preventive concentration) is increased in biofilms compared with planktonic growth due to the particular physiology and architecture of biofilms favouring gradual mutational

resistance development, especially in mutator strains (Macia et al., 2011). The increased tolerance to piperacillin/tazobactam of PAO1 ∆mutT might also be due to a more efficient SOS response in mutators. We have recently shown in another mutant that is unable to repair DNA oxidative

lesions that such unrepaired lesions trigger an oxidative stress response in P. aeruginosa (Mandsberg et al., 2011) that could trigger an SOS response and better survival in the presence of antibiotics. Hyperproduction of beta-lactamase (Ciofu et al., 1994; Bagge et al., 2002) and overexpresison of efflux-pumps (Jalal et al., 2000; Islam et al., 2009) are the most common mechanisms of resistance encountered in CF P. aeruginosa isolates. Due to the selective pressure exerted by maintenance antibiotic treatment, occurrence of resistant P. aeruginosa strains during chronic airway infection in CF is common, and the next most important mechanism of resistance to beta-lactam antibiotics is overproduction of the chromosomally encoded beta-lactamase (Giwercman et al., 1990; Ciofu, 2003). In biofilms of P. aeruginosa that overproduce beta-lactamase, the presence in the biofilm matrix of beta-lactamases will lead to hydrolysis of the beta-lactam antibiotics before they reach the bacterial cells. Nichols et al. (1989) predicted from mathematical models that bacteria expressing high levels of chromosomal beta-lactamase growing in biofilms would be exposed to reduced concentrations of beta-lactam antibiotics due to accumulation of the enzyme in the polysaccharide matrix.