SummaryIn response to sublethal concentrations of antibiotics, bacteria often induce an adaptive response that can contribute to antibiotic resistance. We report the response of Bacillus subtilis to bacitracin, an inhibitor of cell wall biosynthesis found in its natural envi-

The quorum sensing disrupter (5Z)-4-bromo-5-(bromomethylene)-3-butyl-2(5H)-furanone (furanone) of the alga Delisea pulchra was previously found by us (Environ Microbiol 3:731-736, 2001) to inhibit quorum sensing in Escherichia coli via autoinducer-2 (AI-2, produced by LuxS). In this study, DNA microarrays were used to study the genetic basis of this natural furanone inhibition of AI-2 signaling (significant values with p < 0.05 are reported). Using DNA microarrays, the AI-2 mutant Escherichia coli DH5alpha was compared with the AI-2 wild-type strain, E. coli K12, to determine how AI-2 influenced gene expression. Escherichia coli K12 was also grown with 0 and 60 microg/mL furanone to study the inhibition of quorum sensing gene expression. It was found that 166 genes were differentially expressed by AI-2 (67 were induced and 99 were repressed) and 90 genes were differentially expressed by furanone (34 were induced and 56 were repressed). Importantly, 79% (44 out of 56) of the genes repressed by furanone were induced by AI-2, which indicated that furanone inhibited AI-2 signaling and influenced the same suite of genes as a regulon. Most of these genes have functions of chemotaxis, motility, and flagellar synthesis. Interestingly, the aerotaxis genes aer and tsr were discovered to be induced by AI-2 and repressed by furanone. Representative microarray results were confirmed by RNA dot blotting. Furthermore, the E. coli air-liquid interface biofilm formation was repressed by furanone, supporting the results that taxis and flagellar genes were repressed by furanone. The autoinducer bioassay indicated that 100 microg/mL furanone decreased the extracellular concentration of AI-2 2-fold, yet luxS and pfs transcription were not significantly altered. Hence, furanone appeared to alter AI-2 signaling post-transcriptionally.