Indole-induced reversion of intrinsic multiantibiotic resistance in Lysobacter enzymogenes

Lysobacter are a group of environmental bacteria that are emerging as a new source of antibiotics. One characteristic of Lysobacter is intrinsic resistance to multiple antibiotics, which had not been studied. To understand the resistant mechanism, we tested the effect of blocking two-component regulator systems (TCS) on antibiotic resistance of L. enzymogenes , a prolific producer of antibiotics. Upon treatment with LED209, an inhibitor of the widespread TCS QseC/QseB, L. enzymogenes produced a large amount of an unknown metabolite that was barely detectable in the untreated culture. Subsequent structural elucidation by NMR unexpectedly revealed the metabolite to be indole. Indole production was also markedly induced by adrenaline, a known modulator of QseC/QseB. Next, we identified two TCS genes, Le-qseC/Le-qseB , in L. enzymogenes and found that mutants of Le-qseC/Le-qseB also led to a dramatic increase of indole production. We then chemically synthesized a fluorescent indole probe that could label the cells. While mutant Le-qseB (cytoplasmic response regulator) could be clearly labeled by the probe, mutant Le-qseC (membrane sensor) was not labeled. It had been reported that indole could enhance antibiotic resistance in bacteria. Therefore, we tested if the dramatic increase of indole production in L. enzymogenes upon blocking Le-qseC/Le-qseB would lead to an enhanced antibiotic resistance. Surprisingly, we found that indole caused the intrinsically multi-antibiotic resistant L. enzymogenes to become susceptible. Point mutations at conserved amino acids in Le -QseC also led to antibiotic susceptibility. Because indole is known as an interspecies signal, the findings may have implications. IMPORTANCE The environmental bacteria Lysobacter are a new source of antibiotic compounds and exhibit intrinsic antibiotic resistance. Here, we found that inactivation of a two-component regulator system (TCS) by inhibitor or by gene deletion led to a remarkable increase of a metabolite9s production in L. enzymogenes , 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 mutant of the TCS9 cytoplasmic response regulator, but not mutant of the TCS9 membrane sensor. Indole treatment reversed the intrinsically multi-drug resistant L. enzymogenes 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 the intrinsically resistant L. enzymogenes susceptible to common antibiotics may have implications.