None of the tested isolates grown in ASM (from both treatment and control groups) displayed the hypermutable phenotype. The only hypermutable isolate detected in this study was generated following growth in Luria Bertani (LB) for 18 hours (Figure 2 see more and Table 1). Although diversification occurred with respect to only a few of the phenotypic properties tested, the proportions of the isolates exhibiting these traits varied considerably

between treatment groups (Figure 1). The proportions of these phenotypic changes accounted for the within and between-treatment group variation seen in the numbers of mutant haplotypes (Figure 1). Hierarchical analysis of variance indicated that the majority (77%) of diversity was distributed between isolates within populations, rather than the same traits systematically apportioned between replicate buy ABT-888 populations or between treatments (Table 2). Table 2 Hierarchical analysis of variance (σ 2 ) for diversity   Sigma % Variations between treatment 0.03 6.18 Variations between samples within treatment 0.09 16.42 Variations within samples 0.42 77.40 Total ROCK inhibitor variations 0.54 100.00 Discussion Although it is known that the phenotypic and genotypic characteristics of

P. aeruginosa populations within the CF lung fluctuate over time [9, 16], the factors that are responsible for this diversification are not fully understood. When P. aeruginosa LESB58 was grown in ASM with and without sub-inhibitory concentrations of antibiotics, we observed differential effects of antibiotics commonly used to treat CF patients on the diversity of LESB58 populations in the ASM model. In particular, increased levels of phenotypic diversification occurred in LESB58 populations grown in ASM when sub-inhibitory concentrations of colistin, ceftazidime and azithromycin were present. However, extensive

diversification of the P. aeruginosa populations was not seen in the presence of sub-inhibitory concentrations of meropenem. There are a number of mechanisms by which sub-inhibitory concentrations of antibiotics could potentially enhance bacterial diversification. One potential mechanism could involve the antibiotics inducing mutagenesis within bacterial populations, causing variation and/or promoting the hypermutability phenotype [31–34]. A second potential Cell press mechanism could involve the antibiotics acting as signalling molecules, altering the QS systems within bacterial populations and subsequently promoting social evolution and diversification [35, 36, 38]. Antibiotic exposure has been shown to induce mutagenesis by triggering the SOS response and thus increasing the expression of error-prone DNA polymerases, which could give rise to diversity within bacterial populations [31–34]. It is possible that ceftazidime induced mutagenesis in the LESB58 populations through the induction of the SOS response.