species are a group of environmental bacteria that are emerging as a new source of antibiotics. One characteristic of is intrinsic resistance to multiple antibiotics, which had not been studied. To understand the resistance mechanism, we tested the effect of blocking two-component regulatory systems (TCSs) on the antibiotic resistance of, a prolific producer of antibiotics. Upon treatment with LED209, an inhibitor of the widespread TCS QseC/QseB, produced a large amount of an unknown metabolite that was barely detectable in the untreated culture. Subsequent structural elucidation by nuclear magnetic resonance (NMR) unexpectedly revealed that the metabolite was indole. Indole production was also markedly induced by adrenaline, a known modulator of QseC/QseB. Next, we identified two TCS genes, () and , in and found that mutations of and also led to a dramatic increase in indole production. We then chemically synthesized a fluorescent indole probe that could label the cells. While the (cytoplasmic response regulator) mutant was clearly labeled by the probe, the (membrane sensor) mutant was not labeled. It was reported previously that indole can enhance antibiotic resistance in bacteria. Therefore, we tested if the dramatic increase in the level of indole production in upon blocking of and would lead to enhanced antibiotic resistance. Surprisingly, we found that indole caused the intrinsically multiantibiotic-resistant bacterium to become susceptible. Point mutations at conserved amino acids in -QseC also led to antibiotic susceptibility. Because indole is known as an interspecies signal, these findings may have implications. The environmental bacterium is a new source of antibiotic compounds and exhibits intrinsic antibiotic resistance. Here, we found that the inactivation of a two-component regulatory system (TCS) by an inhibitor or by gene deletion led to a remarkable increase in the level of production of a metabolite in, and this metabolite was identified to be indole. We chemically synthesized a fluorescent indole probe and found that it could label the wild type and a mutant of the TCS cytoplasmic response regulator but not a mutant of the TCS membrane sensor. Indole treatment caused the intrinsically multidrug-resistant bacterium to be susceptible to antibiotics. Mutations of the TCS sensor also led to antibiotic susceptibility. Because indole is known as an interspecies signal between gut microbiota and mammalian hosts, the observation that indole could render intrinsically resistant susceptible to common antibiotics may have implications.

The twitching motility of bacteria is closely related to environmental adaptability and pathogenic behaviors. Lysobacter is a good genus in which to study twitching motility because of the complex social activities and distinct movement patterns of its members. Regardless, the mechanism that induces twitching motility is largely unknown. In this study, we found that the interspecies signal indole caused Lysobacter to have irregular, random twitching motility with significantly enhanced speed. Deletion of qseC or qseB from the two-component system for indole signaling perception resulted in the disappearance of rapid, random movements and significantly decreased twitching activity. Indole-induced, rapid, random twitching was achieved through upregulation of expression of gene cluster pilE1-pilY11-pilX1-pilW1-pilV1-fimT1. In addition, under conditions of extremely low bacterial density, individual Lysobacter cells grew and divided in a stable manner in situ without any movement. The intraspecies quorum-sensing signaling factor 13-methyltetradecanoic acid, designated L. enzymogenes diffusible signaling factor (LeDSF), was essential for Lysobacter to produce twitching motility through indirect regulation of gene clusters pilM-pilN-pilO-pilP-pilQ and pilS1-pilR-pilA-pilB-pilC. These results demonstrate that the motility of Lysobacter is induced and regulated by indole and LeDSF, which reveals a novel theory for future studies of the mechanisms of bacterial twitching activities. IMPORTANCE The mechanism underlying bacterial twitching motility is an important research area because it is closely related to social and pathogenic behaviors. The mechanism mediating cell-to-cell perception of twitching motility is largely unknown. Using Lysobacter as a model, we found in this study that the interspecies signal indole caused Lysobacter to exhibit irregular, random twitching motility via activation of gene cluster pilE1-pilY11-pilX1-pilW1-pilV1-fimT1. In addition, population-dependent behavior induced by 13-methyltetradecanoic acid, a quorum-sensing signaling molecule designated LeDSF, was involved in twitching motility by indirectly regulating gene clusters pilM-pilN-pilO-pilP-pilQ and pilS1-pilR-pilA-pilB-pilC. The results demonstrate that the twitching motility of Lysobacter is regulated by these two signaling molecules, offering novel clues for exploring the mechanisms of twitching motility and population-dependent behaviors of bacteria.

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