Bacteria become highly tolerant to antibiotics when nutrients are limited. The inactivity of antibiotic targets caused by starvation-induced growth arrest is thought to be a key mechanism producing tolerance (1). Here we show that the antibiotic tolerance of nutrient-limited and biofilm Pseudomonas aeruginosa is mediated by active responses to starvation, rather than by the passive effects of growth arrest. The protective mechanism is controlled by the starvation-signaling stringent response (SR), and our experiments link SR–mediated tolerance to reduced levels of oxidant stress in bacterial cells. Furthermore, inactivating this protective mechanism sensitized biofilms by several orders of magnitude to four different classes of antibiotics, and markedly enhanced the efficacy of antibiotic treatment in experimental infections.

This report describes the identification and analysis of a 2-component regulator of Pseudomonas aeruginosa that is a potential aminoglycoside antibiotic combination therapy target. The regulator, AmgRS, was identified in a screen of a comprehensive, defined transposon mutant library for functions whose inactivation increased tobramycin sensitivity. AmgRS mutations enhanced aminoglycoside action against bacteria grown planktonically and in antibiotic tolerant biofilms, against genetically resistant clinical isolates, and in lethal infections of mice. Drugs targeting AmgRS would thus be expected to enhance the clinical efficacy of aminoglycosides. Unexpectedly, the loss of AmgRS reduced virulence in the absence of antibiotics, indicating that its inactivation could protect against infection directly as well as by enhancing aminoglycoside action. Transcription profiling and phenotypic analysis suggested that AmgRS controls an adaptive response to membrane stress, which can be caused by aminoglycoside-induced translational misreading. These results help validate AmgRS as a potential antibiotic combination target for P. aeruginosa and indicate that fundamental stress responses may be a valuable general source of such targets.aminoglycoside ͉ AmgRS ͉ Pseudomonas aeruginosa ͉ 2-component regulator ͉ CpxRA

The identification of organisms from positive blood cultures generally takes several days. However, recently developed rapid diagnostic methods offer the potential for organism identification within only a few hours of blood culture positivity. In this study, we evaluated the performance of three commercial methods to rapidly identify organisms directly from positive blood cultures: QuickFISH (