Stationary-phase Pseudomonas aeruginosa Levofloxacin tolerance
The ubiquitous opportunistic pathogen, Pseudomonas aeruginosa, is highly adaptive and refractory to several different classes of antibiotics. However, we found in this study that stationary-phase P. aeruginosa cultures exhibit greater sensitivity to the fluoroquinolone Levofloxacin (Levo) than other bactericidal antibiotics, including an aminoglycoside (Tobramycin) and beta-lactam (Aztreonam). To understand the basis of this sensitivity, we conducted time-lapse fluorescence microscopy experiments of cells during Levo treatment. We discovered that stationary-phase P. aeruginosa cells die rapidly during treatment and undergo heterogenous morphological changes, including explosive lysis, filamentation, and gradual loss of membrane integrity as evidenced by propidium iodide uptake. These morphologies are reminiscent of how the model organism Escherichia coli appears when recovering from fluoroquinolone treatment, a period when activation of the DNA damage-induced SOS response is crucial. Accordingly, we monitored the morphologies and survival of P. aeruginosa recA mutants and found that the SOS response is not involved in P. aeruginosa Levo sensitivity like it is for E. coli. We hypothesized that Levo sensitivity may be due to P. aeruginosa maintaining active metabolism in stationary phase. We determined that stationary-phase P. aeruginosa cells transcribe, maintain reductase activity, and accumulate reactive metabolic species which contribute to Levo-mediated death. By elucidating how P. aeruginosa cells sustain metabolic activity during the stationary phase, we can design strategies to sensitize these persistent subpopulations to Levo and maintain the efficacy of this clinically important fluoroquinolone antibiotic.