Fitness of P. fluorescens SBW25 and derived mutants. Fitness of SBW25 (wt) and mutants PBR826 (Δpsp, psp) and PBR827 (ΔpstC, pst) relative to SBW25-lacZ was measured with cells grown in Pi-rich (PR) and Pi-limited (PL) media. Data are means and standard errors of ~8 independent cultures. A fitness of zero indicates that the fitness of the mutant is identical to wild type (a negative value indicates a reduction in fitness relative to wild type). Bars identified by different letters are significantly different (P < 0.05) by Tukey's HSD.